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By whose direction is the nutriment so regularly united to a human body in the same laws of condistributed into the respective parts, and how are nexion : and a mind of human capacities would they kept to their specifick uniformities? Glanville. make another species, if united to a different body in
He intendeth the care of species or common natures, different laws of connexion. Bentley's Sermons. but letteth loose the guard of individuals or single He must allow that bodies were endowed with the existencies.
Browne. same affections then as ever since; and that, if an Thou nam'st a race which must proceed from me, axe-head be supposed to float upon water, which is Yet my whole species in myself I see. Dryden. specifically lighter, it had been supernatural. It is a most certain rule, how much any body hath
Bentley. of colour, so much hath it of opacity, and by so If she would drink a good decoction of sarsa, with much the more unfit it is to transmit the species. the usual specificks, she might enjoy a good health. Ray on the Creation.
Wiseman. Human reason doth not only gradually, but speci A special idea is called by the schools a species. fically, differ from the fantastick reason of brutes,
Watts. which have no conceit of truth, as an aggregate of Specifick difference is that primary attribute which divers simple conceits, nor of any other universal. distinguishes each species from one another, while
Grew. they stand ranked under the same general nature or As all things were formed according to these spe- genus. Though wine differs from other liquids, in cifical platforms, so their truth must be measured that it is the juice of a certain fruit, yet this is but a from their conformity to them.
Norris. general or generick difference ? for it does not distin. Though our charity should be universal, yet, as it gnish wine from cyder or perry: the specifick differcannot be actually exercised but on particular times, ence of wine therefore is its pressure from the grape; so it should be chiefly on special opportunities. as cyder is pressed from apples, and perry from
Id. St. Peter doth not specify what these waters were. This specification or limitation of the question
Burnet. hinders the disputers from wandering away from the The understanding, as to the exercise of this precise point of enquiry.
Id. power, is subject to the command of the will, though, He has there given us an exact geography of as to the specifick nature of its acts, it is determined Greece, where the countries, and the uses of their by the object. South. soils are specified.
Pope. His faith must be not only living, but lively too;
Specialty, specialitas, in English law, a bond, it must be put into a posture by a particular exercise bill, or such like instrument: of those several virtues that are specifically requisite deed, under the hand and seal of the parties.
a writing or to a due performance of this duty. Id. Sermons. Milton's subject was still greater than Homer's or
Littleton. These are looked upon as the next class Virgil's ; it does not determine the fate of single of debts after those of record; being confirmed persons or nations, but of a whole species. Addison. by special evidence under seal. 2 Comm.c. 30,
The constitution here speaks generally, without p. 465. the specification of any place.
place. Ayliffe's Parergon. Species, in algebra, are the letters, symbols, Specifick gravity is the appropriate and peculiar marks, or characters, which represent the quantigravity or weight which any species of natural bo- ties in any operation or equation. dies have, and by which they are plainly distinguish Species, in commerce, the several pieces of able from all other bodies of different kinds.
gold, silver, copper, &c., which, having passed
Quincy. The species of the letters illuminated with blue, were
their full preparation and coinage, are current in nearer to the lens than those illuminated with deep
public. See Money. red, by about three inches, or three and a quarter ;
Species, in logic, a relative term, expressing but the species of the letters illuminated with indigo an idea which is comprised under some general and violet appeared so confused and indistinct, that one called a genus. See Logic. I could not read them.
Newton's Opticks. Species, in optics, the image painted on the These principles I consider not as occult qualities, retina by the rays of light reflected from the sevesupposed to result from the specifick forms of things, ral points of the surface of an object, received but as general laws of nature, by which the things in by the pupil, and collected in their passage themselves are formed; their truth appearing to us through the crystalline, &c. by phænomena, though their causes be not yet dis
SPECIFIC, in philosophy, that which is pecucovered.
Id. He cannot but confess that it is a thing the most
liar to any thing, and distinguishes it from all
others. desirable to man, and most agreeable to the goodness
Specific DIFFERENCES OF PLANTS. See Boof God, that he should send forth his light and his truth by a special revelation of his will. Rogers. Our Saviour is represented every where in scrip
Specific GRAVITY is a term much employed ture as the special patron of the poor and the afflicted, in the discussions of modern physics. It exand as laying their interest to heart more nearly than presses the weight of any particular kind of matthose of any other of his members. Atterbury. ter, as compared with the weight of the same bulk
As there was in the splendour of the Roman em of some other body of which the weight is suppire, a less quantity of current species in Europe than posed to be familiarly known, and is therefore there is now, Rome possessed a much greater propor- taken for the standard of comparison. The tion of the circulating species of its time than any body generally made use of for this purpose is European city.
See HYDROSTATICS. The specifick qualities of plants reside in their native spirit, oil, and essential salt: for the water, fixt
The specific gravity of bodies is a very intersalt, and earth, appear to be the same in all plants.
esting question both to the philosopher and to
Id. the man of business. The philosopher considers A mind of superior or meaner capacities than hu- the weights of bodies as measures of the number man, would constitute a different species, though of material atoms, or the quantity of matter
which they contain. This he does on the suppo- weight of the standard as unity, and then the sition that every atom of matter is of the same number expressing the specific gravity is the weight, whatever may be its sensible form. This number of times that the weight of the standard supposition, however, is made by him with cau- is contained in that of the other substance. This tion, and he has recourse to specific gravity for comparison is most easily and accurately made in ascertaining its truth in various ways. The man fluids. We have only to make a vessel of known of business entertains no doubt of the matter, and dimensions equal to that of the standard which we proceeds on it as a sure guide in his most interest- employ, and to weigh it when empty, and then ing transactions. We measure commodities of when filled with the fluid. Nay, the most ditovarious kinds by tons, pounds, and ounces in the cult part of the process, the making a vessel of the same manner as we measure them by yards, feet, precise dimensions of the standard, may be and inches, or by bushels, gallons, and pints; avoided, by using some fuid substance for a stannay, we do this with much greater confidence, dard. Any vessel will then do; and we may and prefer this measurement to all others when ensure very great accuracy by using a vessel ever we are much interested to know the exact with a slender neck, such as a phial or matrass; proportions of matter that bodies contain. The for, when this is filled to a certain mark in the weight of a quantity of grain is allowed to inform neck, any error in the estimation by the eye will us much more exactly of its real quantity of use bear a very small proportion to the whole. The ful malter than the most accurate measure of its weight of ihe standard fluid, which fills it to this bulk. We see many circumstances which can vary mark being carefully ascertained, is kept in rethe bulk of a quantity of matter, and these are membrance. The specific gravity of any other frequently such as we cannot regulate or prevent; fluid is had by weighing the contents of this but we know very few that can make any sensible vessel when filled with it, and dividing the weight change in this weight without the addition or by the weight of the standard. The quotient is abstraction of other matter. Even taking it to the the specific gravity of the fluid. But in all other suimit of a high mountain, or from the equator cases this is a very difficult problem: it requires to the polar region, will make no change in its very nice hands, and an accurate eye, to make weight as it is ascertained by the balance, because two bodies of the same bulk. An error of the there is the same real diminution of weight in the part in the linear dimensions of a solid body pounds and ounces used in the examination. Not- makes an error of one-thirtieth part in its buik; withstanding the unavoidable change which heat and bodies of irregular shapes and friable sub and cold make in the bulk of bodies, and the per- stance, such as the ores of metals, cannot be manent varieties of the same kind of matter which brought into convenient and exact dimensions for are caused by different circumstances of growth, measurement. From all these inconveniences and texture, &c., most kinds of matter have a certain difficulties we are freed by the celebrated Archiconsistency in the density of their particles, and medes, who, from the principles of hydrostatics therefore in the weight of a given bulk. Thus the discovered or established by him, deduced the purity of gold, and its degree of adulteration, may accurate and easy method which is now univerbe inferred from its weight, it being purer in pro- sally practised for discovering the specific gravity portion as it is more dense. The density, there and density of bodies. See ARCHIMEDES. Infore, of different kinds of tangible matter becomes stead of measuring the bulk of the body by that characteristic of the kind, and a test of its purity; of the displaced Auid (which would have been it marks a particular appearance in which matter impossible for Archimedes to do with any thing exists, and may therefore be called, with propriety, like the necessary precision), we have only to obspecific. But this density cannot be directly serve the loss of weight sustained by the solid. observed. It is not by comparing the distances This can be done with great ease and exactness. between the atoms of matter in gold and in water Whatever may be the bulk of the body, this loss that we say the first is nineteen times denser than of weight is the weight of an equal bulk of the the last, and that an inch of gold contains nine- fluid; and we obtain the specific gravity of the teen times as many material atoms as an inch of body by simply dividing its whole weight by water; we reckon on the equal gravitation of the weight lost: the quotient is the specific graevery atom of matter, whether of gold or of water; vity when this fluid is taken for the standard, therefore the weight of any body becomes the in even though we should not know the absolute dication of its material density, and the weight weight of any given bulk of this standard. It of a given bulk becomes specific of that kind of also gives us an easy and accurate method of asmatter, marking its kind, and even ascertaining certaining even this fundamental point. We its purity in this form. To make this comparison have only to form any solid body into an exact of general use, the standard must be familiarly cube, sphere, or prism, or known dimensions, known, and must be very uniform in its density, and observe what weight it loses when immersed and the comparison of bulk and density must he in this standard fluid. This is the weight of the easy and accurate. The most obvious method same bulk of the standard to be kept in rernemwould be to form, with all nicety, a piece of the brance; and thus we obtain, by the by, a most standard matter of some convenient bulk, and to easy and accurate method for measuring the bulk weigh it very exactly, and keep a note of its or solid contents of any body, however irregular weight: then, to make the comparison of any its shape may be. We have only to see how other substance, it must be made into a mass of much weight it loses in the standard fuid; we the same precise bulk, and weighed with equal can compute what quantity of the standard Auid care ; and the most convenient way of expressing will have this weight. Thus should we find that the specific gravity would be to consider the a quantity of sand, or a furze bush, loses 250 oz.,
when immersed in pure water, we learn by this tion, that we meet with is by the celebrated that the solid measure of every grain of the sand, Snellius about 1615, and related in his Eratosor of every twig and prickle of the furze, when thenes Batavus. He weighed a Rhinland cubic added into one sum, amounts to the fourth part foot of distilled water, and found it 62.79 Amof a cubic foot, or to 432 cubic inches.
sterdam pounds. If this was the ordinary weight To all these advantages of the Archimedean of the shops, containing 7626 English troy grains, method of ascertaining the specific gravity of the English cubic foot must be sixty-two pounds bodies, derived from his hydrostatical doctrines nine ounces, only one ounce more than by Eveand discoveries, we may add, that the immediate raru's experiment. If it was the mint pound the standard of comparison, namely, water, is, of all weight was sixty-two pounds six ounces. The the substances that we know, the fittest for the only other trials which can come into compepurpose of a universal standard of reference. tion with Mr. Everard's are some made by the In its ordinary natural state it is sufficiently con- Academy of Sciences at Paris. Picart, in 1691, stant and uniform in its weight for every exami- found the Paris cubic-foot of the water of the nation where the utmost mathematical accuracy fountain d'Arcueil to weigh 69.588 lbs. puids is not wanted; all its variations arise from impu- de Paris. Du Hamel obtained the very same rities, from which it may at all times be sepa- result; but Mr. Monge, in 1783, says that filrated by the simple process of distillation : and tered rain-water of the temperature of 12° (Reauwe have every reason to think that, when pure, mur) weighs 69-3792. Both these measures are its density, when of the same temperature, is in- considerably below Mr. Everard's, which is variable. Water is therefore universally taken 62.5; the former giving 62·053, and the latte for the unit of that scale on which we measure 61.868. M. Lavoisier states the Paris cubic the specific gravity of bodies, and its weight is foot at seventy pounds, which makes the English called 1. The specific gravity of any other body foot 62.47. But there is an inconsistency among is the real weight in pounds and ounces, when of them which makes the comparison impossible. the bulk of one pound or one ounce of water. It Some changes were made in 1688, by royal auis, therefore, of the first importance, in all dis- thority, in the national standards both of weight cussions respecting the specific gravity of bodies, and length; and the academicians are exceedto have the precise weight of some known bulk ingly puzzled to this day in reconciling the difof pure water. For this purpose we shall reduce ferences, and cannot even ascertain with perfect all to the English cubic foot and avoirdupois assurance the lineal measures which were emounce of the exchequer standard, on account of ployed in their most boasted geodetical operaa very convenient circumstance peculiar to this tions. Such variations in the measurements made unit, viz. that a cubic foot contains almost pre- by persons of reputation for judgment and accucisely 1000 ozs. of pure water, so that the specific racy engaged the writer of this article some years gravity of bodies expresses the number of such ago to attempt another. A vessel was made of a ounces contained in a cubic foot. We begin cylindrical form, as being more easily executed with a trial made before the house of commons with accuracy, whose height and diameter were in 1696 by Mr. Everard. He weighed 2145.6 six inches, taken from a most accurate copy of cubic inches of water by a balance, which turned the exchequer standard. It was weighed in dissensibly with six grains, when there were thirty tilled water of the temperature of 55° several pounds in each scale. The weights employed times without varying two grains, and it lost were the troy weights, in the deposit of the court 42895 grains. This gives for the cubic foot of exchequer, which are still preserved, and have 998.74 ounces, deficient from Mr. Everard's an been most scrupulously examined and compared ounce and a quarter; a difference which may be with each other. The weight was 1131 ozs.fourteen expected, since Mr. Everard used the New River pennyweights. This wants just eleven grains of water without distillation. These observations 1000 avoirdupois ounces for 1728 cubic inches, or cannot be thought superfluous in a matter of a cubic foot; and it would have amounted to that such continual reference in the most interesting weight had it been a degree or two colder. The questions both to the philosopher and the man temperature indeed is not mentioned; but, as the of business. Let us, therefore, take water for trial was made in a comfortable room, we may the standard, and suppose that, when of the orpresume the temperature to have been about 550 dinary temperature of summer and in its state of of Fahrenheit's thermometer. The dimensions greatest natural purity, viz. in clean rain or snow, of the vessel were as accurate as the nice hand an English cụbic foot of it weighs 1000 ounces of Mr. Abraham Sharp, Mr. Flamsteed's as- avoirdupois, of 437.5 troy grains each. Divide sistant at Greenwich, could execute, and it was the weight of any body by the weight of an equal made by the exchequer standard of length. This bulk of water; the quotient is the specific grais confided in by the naturalists of Europe, as a vity of that body; and, if the three first figures very accurate standard experiment, and is con- of the decimal be accounted integers, the quotient firmed by many others, both private and public. is the number of avoirdupois ounces in a cubic The standards of weight and capacity employed foot of the body. Thus the specific gravity of in the experiment are still in existence, and pub- the very finest gold which the refiner can prolickly known, by the report of the Royal Society duce is 19.365, and a cubic foot of it weighs to parliament in 1742, and by the report of a 19,365 ounces. But an important remark must committee of the house of commons in 1758. 'be made here. All bodies of homogeneous or This gives it a superiority over all the measures unorganised texture expand by heat and contract which have come to our knowledge.
by cooling. The expansion and contraction by The first experimerrt, made with proper atten- the same change of temperature is very different
in different bodies. Thus water, when heated bles, we should err exceedingly if we imagined it from 60° to 100°, increases its volume nearly as small as it appears at first. We believe that thy of its bulk, and mercury only ols, and many in most plants it is at least as great as water, for substances much less. Hence it follows that an after long maceration they sink in it. experiment determines the specific gravity only The nicest and most sensible balances are nein that very temperature in which the bodies are cessary for this examination, Balances are even examined.' It will therefore be proper always to constructed on purpose, and fitted with several note this temperature, and it will be convenient pieces of apparatus which make the examination to adopt some very useful temperature for such easy and neat. We have described (see BALANCE) trials in general; perhaps about 60° of Fahren- the most convenient. Mr. Gravesande's contriheit's thermometer is as convenient as any. It vance for observipg the fractions of a gra'n is may always be procured in these climates with- extremely ingenious and expeditious, especially vut inconvenience. A temperature near to freez- for detecting the effect of viscidity. The hydro ng would have some advantages, because water meter, or aræometer, is another instrument for changes its bulk verv little between the tempe- ascertaining the specific gravity of fluids. This rature 32° and 45°. But this temperature cannot very pretty instrument is the invention of Hypealways be obtained. It will much conduce to tia, a lady of Alexandria, as eminent for intellecthe facility of the comparison to know the varia- tual accomplishments as she was admired for her tion which heat produces on pure water. The beauty. She wrote commentaries op Appolonius following table, taken from the observations of and Diaphantus, and composed Astronomical Dr. Blagen and Mr. Gilpin (Phil. Trans. 1792), Tables; all of which are lost. See HYPATIA. will answer this purpose :
We have described some of the most approved
of these instruments under the article HYDRO| Temperature Bulk of Specific
METER, and shall here make a few observations of Water. Water. Gravity.
on the principles of their construction, not as
they are usually made accommodated to the exa30
mination of particular liquors, but as indicators 35 99910 1.00090
of pure specific gravity. And we must say that 40 99070 1.00094
this would, for many reasons, be the best way of 45 99914 1.00086
constructing them. The very ingenious contri50 99932 1.00068
vances for accommodating them to particular 99962 1.00038
purposes are unavoidably attended with many 60 100000 1.00000
sources of error, both in their adjustment by the 65 100050 0.99950
maker and in their use; and all that is gained 70 100106 0.99894
by a very expensive instrument is the saving the 75 100171 0.99830
trouble of inspecting a table. A simple scale of 80 100242 0.99759
specific gravity would expose to no error in con85 100320 0.99681
struction, because all the weights but one, or all 90 100404 0.99598
the points of the scale but one, are to be obtained 100501 0 99502
by calculation, which is incomparably more exact 100 100602 0.99402
than any manual operation, and the table can
always be more exact than any complex observaThese gentlemen observed the expansion of wa tion. But a still greater advantage is that the ter to be very anomalous between 32° and 45°. instruments would thus be fitted for examining This is distinctly seen during the gradual cooling all liquors whatever, whereas at present they are of water to the point of freezing. It contracts almost useless for any but the one for which they for a while and then suddenly expands. But we are constructed. seldom have occasion to measure specific gravi Unless the hydrometer is of a considerable ties in such temperature.
size, it can hardly be made so as to extend from In examining either solids or fluids we must the lightest to the heaviest fluid which we may be careful to free their surface, or that of the have occasion to examine, even though we exvessel in which the fluid is to be weighed, from cept mercury. Some of the mineral acids are air, which frequently adheres to it in a peculiar considerably more than twice the weight of ether. manner, and, by forming a bubble, increases the When there is such a load at top, the hydromeler apparent bulk of the solid, or diminishes the is very apt to overset, and inclines with the capacity of the vessel. The greatest part of what smallest want of equilibrium. Great size is inappears on those occasions seems to have existed convenient even to the philosopher, because it is in the fluid in a state of chemical union, and to not always in his power to operate on a quantity be set at liberty by the superior attraction of the of Auid sufficient io float the instrument. Therefluid for the contiguous solid body. These air fore two, or perhaps three, are necessary for gebubbles must be carefully brushed off by hand. neral examination. One may reach from ether All greasy matters must be cleared off for the to water; another may serve for all liquors of a same reason; they prevent the fluid from coming specific gravity between 1 and 14; and the into contact. We must be no less careful that third, for ihe mineral acids, may reach from this no water is imbibed by the solid, which would to 2. If each of these be about two solid inches increase its weight without increasing its bulk. in capacity, we may easily and expeditiously deIn some cases, however, a very long maceration termine the specific gravity within Tooth part of and imbibition is necessary. Thus, in examining the truth; and this is precision enough for most the specific gravity of the fibrous part of vegeta- purposes of science or business. The chief
questions are, 1. To ascertain the specific gravity lowest division, a weight may be put on the top of an unknown fluid. This needs no farther ex of the stem, which will again sink it to the top. planation. 2. To ascertain the proportion of This weight must evidently be 0.073, or oneiwo fluids which are known to be in a mixture. tenth of the weight of the fluid displaced by the This is done by discovering the specific gravity unloaded instrument. The hydrometer, thus loadof the mixture by means of the hydrometer, and ed, indicates the same specific gravity, by the top then deducing the proportion from a comparison of the stem, that the unloaded instrument indicates of this with the specific gravities of the ingredie by the lowest division. Therefore, when loaded, it
In this mode of examination the bulk is will indicate another series of specific gravities, always the same; for the hydrometer is immerg- from 0-803 to 0·833 (= 0·803 + 0.0803), and ed in the different fluids to the same depth. will noat in a liquor of the specific gravity 0-8833 Now, if an inch, for example, of this bulk is with the whole stem above the surface. In like made up of the heaviest fluid, there is an inch manner, if we take off this weight and put on 1 wanting of the lightest; and the change made in = 0.0803, it will sink the hydrometer to the the weight of the mixture is the difference be- top of the stem; and with this new weight it will tween the weight of an inch of the heaviest and indicate another series of specific gravities from of an inch of the lightest ingredients. The num- 0.8833 to 0.97163 (0.8833 + 0.08633). And, ber of inches therefore of the heaviest fluid is in the same manner, a third weight = 0·8833 proportional to the addition made to the weight will again sink it to the top of the stem, and fit of the mixture. Therefore let B and b be the it for another series of specific ravities up to bulks of the heaviest and lightest Auids in the 1.068793. And thus, with three weights, we bulk ß of the mixture; and let D, d, and d be have procured a hydrometer fitted for all liquors, the densities, or the weights, or the specific gra- from ether to a wort for a malt liquor of two vities (for they are in one ratio) of the heavy barrels per quarter. Another weight, in the same fluid, and the light fluid, and mixture their bulk progression, will extend the instrument to the being that of the hydrometer). We have ß = strongest wort that is brewed. This is a very B + b. The addition which would have been commodious form of the instrument, and is now made to the bulk B, if the lightest fluid were in very general use for examining spirituous lichanged entirely for the heaviest, would be D quors, worts, ales, brines, and many such artid; and the change which is really made is omd.cles of commerce. But the divisions of the - Therefore ß: b=D-8:0-d. For similar scale are generally adapted to the questions which reasons we should have ß : B=D-d:D-d; naturally occur in the business. Thus, in the comor, in words, the difference between the specific merce of strong liquors, it is usual to estimate the gravities of the two fuids, is to the difference article by the quantity of spirit of a certain strength between the specific gravities of the mixture and which the liquor contains.—This we have been of the lightest Auid, as the bulk of the whole to accustomed to call proof spirit, and it is such the bulk of the heaviest contained in the mixture;' that a wine gallon weighs seven pounds twelve and the difference of the specific gravities of the ounces; and it is by this strength that the excise two fluids, is to the difference of the specific duties are levied. Therefore the divisions on gravities of the mixture and of the heaviest Auids, the scale, and the weights which connect the as the bulk of the whole to that of the lightest successive repetitions of the scale, are made to contained in the mixture.' This is the form in express at once the number of gallons, or parts which the ordinary business of life requires the of a gallon, of proof spirits contained in a gallon answer to be expressed, because we generally of the liquor. Such instruments save all trouble reckon the quantity of liquors by bulk, in gal- of calculation to the exciseran or dealer; but lons, pints, quarts. But it would have been they limit the use of a very delicate and expenequally easy to have obtained the answer in sive instrument to a very narrow employment. pounds and ounces; or it may be had from their It would be much better to adhere to the expresbulks, since we know their specific gravities. sion either of specific gravity or of bulk; and
To avoid the inconveniences of a hydrometer then a very small table, which could be comwith a very long and slender stem, or the neces- prised in the smallest case for the instrument, sity of having a series of them, a third sort has might render it applicable to every kind of Auid. been contrived, in which the principle of both The reader cannoi but have observed that the are combined. Suppose a hydrometer with a successive weights, by which the short scale of stem, whose bulk is one-tenth of that of the ball, the instrument is extended to a great range of and that it sinks in ether to the top of the stem; specific gravities, do not increase by equal quanit is evident that in a fuid which is one-tenth tities. Each difference is the weight of the liquor heavier the whole stem will emerge; for the bulk displaced by the graduated stem of the instruof the displaced Auid is now one-tenth of the ment when it is sunk to the top of the scale. It whole less, and the weight is the same as before, is a determined aliquot part of the whole weight and therefore the specific gravity is one-tenth of the instrument so loaded (in our example it is greater. Thus we have obtained a hydrometer always one-eleventh of'it). It increases therefore which will indicate, by means of divisions mark- in the same proportion with the preceding weight ed on the stem, all specific gravities from 0.73 of the loaded instrument. In short, both the to 0.803; for 0.803 is one-tenth greater than successive additions, and the whole weights of 0-73. These divisions must be made in harmo- the loaded instrument, are quantities in geomenic progressions, as before directed for an entire trical progression; and in like manner the diviscale, placing 0.73 at the top of the stem and sions on the scale, if they correspond to equal 0-803 at the bottom. When it floats at the differences of specific gravity, must also be un