ta be Theophraf THEOPHRASTA, in botany; a genus of plants longing to the clafs of rentandria and order of monogynia. Theofo- The corolla is campanulated, with divifions and fegments phifts. obtufe; the capfule unilocular, globular, very large, and many feeded. There is only one ipecies, the americana. THEOPHRASTUS, the philofopher, was born about 371 years before Chrift, and was fucceffively the difciple of Plato and of Ariftotle. He fucceeded Ariftotle in the Peripatetic fchool, and conducted the charge with fuch high reputation that he had about 2005 fcholars. He is highly celebrated for his induftry, learning, and eloquence; and for his generofity and public fpirit. He is faid to have twice freed his country from the oppreffion of tyrants. He contributed liberally towards defraying the expence attending the public meetings of philof phers; which were held, not for the fake of thew, but for learned and in renious converfation. In the public fchools he commonly appeared, as Ariftotle had done, in an elegant drefs, and was very attentive to the graces of elocution. He lived to the advanced age of 35: Some fay of 107. Towards the close of his life, he grew exceedingly infirm, and was carried to the school on a couch. He expreffed great regret on account of the fhortnels of life; and complained that nature had given long life to flags and crows, to whom it is of fo little value, and had denied it to man, who, in a longer duration, might have been able to attain the fummit of fcience; but now, as foon as he arrives within fight of it, is taken away. His lat advice to his difciples was, that, fince it is the lot of man to die as foon as he begins to live, they would take more pains to enjoy life as it paffes, than to acquire pofthumous fame. His funeral was attended by a large body of Athenians. He wrote many valuable works, of which all that remain are, several treatises on the Natural Hiftory of Flants and Foffils; Of Winds, Of Fire, &c. a rhetorical work intitled "Characters," and a few Metaphy. fical Fragments. Enfield's Hillory of Philofopky. Ibia. To Theophraftus we are indebted for preserving the works of Ariftotle. See ARISTOTLE. THEOPOMPUS, a celebrated Greek orator and hiftorian, was born in the ifland Chios, and flourished in the reign of Alexander the Great. He was one of the moft famous of all the difciples of Ifocrates, and won the prize from all the panegyrifts whom Artemifia invited to praife Maufolus. He wrote feveral works, which are lost. THEOREM, a propofition which terminates in theory, and which confiders the properties of things already made or done; or it is a fpeculative propofition de 'uced from comparing together feveral definitions. A theorem is fomething to be proved, and a problem fomething to be done. THEORETIC, fomething relating to theory, or that terminates in fpeculation. THEORY, in general, denotes any doctrine which terminates in fpeculation, without confidering the practical ufes or application thereof. THEOSOPHISTS, a fect of men who pretend to derive all their knowledge from divine illumination. They boast that, by means of this celeftial light, they are not only admitted to the intimate knowledge of God and of all divine truth, but have access to the moft fublime fecrets of nature. They afcribe it to the fingular manifeftation of divine benevolence, that they are able to make fuch a ule of the element of fire, in the chemical art, as enables them to difcover the effential principles of bodies, and to disclofe Atupendous myfteries in the phyfical world. They even pretend to an acquaintance with thofe celeftial beings which form the medium of intercourfe between God and man, and to a power of obtaining from them, by the aid of magic, aftro. logy, and other fimilar arts, various kinds of information and affiftance. Thern:o To this clafs belonged Paracelfus, Robert Fludd, Jacob Therapeu Boehmen, Van Helmont, Peter Poiret, and the Roficrucians. They are allo called FIRE-Philofophers, which fee. THERAPEUTÆ, a term applied to thofe that are wholly in the fervice of religion. This general term has been applied to particular fects of men, concerning whom there have been great difputes among the learned. THERAPEUTICS, that part of medicine which acquaints us with the rules that are to be observed, and the medicines to be employed, in the cure of difeafes. THERIACA ANDROMACHI, a compound medicine made in the form of an electuary. See PHARMACY, no 605. THERMÆ, hot baths or bagnios. Luxury and extrava rance were in nothing carried to fuch heights as in the therma of the Roman emperors. Ammian complains, that they were built to fuch an extent as to equal whole provinces; from which Valefius would abate, by reading pifeina inftead of provincia. And yet after all, the remains of fome ftill ftanding are fufficient teftimonies for Aminian's cenfure; and the accounts tranfmitted of their ornaments and furniture, fuch as being laid with precious ftones (Seneca), fet round with feats of folid filver (Pliny), with pipes and ciferns of the fame metal (Statius), add to, rather than take from, the cenfure. The molt remarkable bagnios were thofe of Diocletian and Caracalla at Rome, great part of which remains at this day; the lorty arches, ftately pillars, variety of foreign marble, curious vaulting of the roofs, great number of fpacious apartments, all attract the curiofity of the traveller. They had alfo their fummer and winter baths. THERMOMETER, an inftrument for measuring the degree of heat or cold in any body. meter. I mometer. The thermometer was invented about the beginning of Invention the 17th century; but, like many other ufeful inventions, of the the it has been found impoffible to afcertain to whom the ho Martine's nour of it belongs. Boerhaave * afcribes it to Cornelius Ejjays. Drebbel of Alcmar, his own countryman. Fulgenzio † at- *em. f. tributes it to his mafter Paul Sarpi, the great oracle of the P. 15156. Venetian republic; and Viviani gives the honour of it to + Life F. Paul, p. Galilæo. But all these are pofthumous claims. Sanctorio ‡ 158. claims this honour to himfer; and his affertion is corrobo-it. Ga rated by Borelli § and Malpighi * of the Florentine aca-lil. p. 67. demy, whofe partiality is not to be fufpected in favour Corn.in of a member of the Patavinian school. Galen. p. 736-842. Perhaps the best way to reconcile thefe different claims De M would be, to fuppofe that the thermometer was really in- animal, ik vented by different perfons about the fame time. We know pop. 175. * Opera that there are certain periods in the progress of the arts Pojib. P. 30. when the ftream of human genius runs in the fame direction, and moves towards the fame object. That part of the current which reaches the object firft may poffefs the title; but the other parts follow fo rapidly and arrive fo foon after, that it is impoffible for a fpectator to decide which is firft in point of time. T'he first form of this inftrument for measuring the de- The air grees of heat and cold, was the air-thermometer. It is a thermome well known fact that air expands with heat fo as to occupy ter defcrimore space than it does when cold, and that it is condensed bed. by cold fo as to occupy lefs fpace than when warmed, and that this expanfion and condenfation is greater or lefs according to the degree of heat or cold applied. The principle then on which the air-thermometer was conftructed is very fimple. The air was confined in a tube by means of fome coloured liquor; the liquor rofe or fell according as the air became expanded or condenfed. What the first form of the tube was, cannot now perhaps be well known; but the following defcription of the air-thermometer will fully explain its nature. The air-thermometer confifts of a glafs tube BE, con- Plate DVI, nected fig. 1. meter. jufted to the great funfhine heats of Florence, which are Thermo- [ 493 1 Thermo- nected at one end with a large glafs ball A, and at the other end immerfed in an open veffel, or terminating in a ball DE, with a narrow orifice at D; which veffel, or ball, contains any coloured liquor that will not eafily freeze. Aquafortis tinged of a fine blue colour with a folution of vitriol or copper, or fpirit of wine tinged with cochineal, will aufwer this purpofe. But the ball A muft be firt moderately warmed, fo that a part of the air contained in it may be expelled through the orifice D; and then the liquor preffed by the weight of the atmofphere will enter the ball DE, and rife, for example, to the middle of the tube at C, at a mean temperature of the weather; and in this state the liquor by its weight, and the air included in the ball A, &c. by its elafticity, will counterbalance the weight of the atmosphere. As the furrounding air becomes warmer, the air in the ball and upper part of the tube, expanding by heat, will drive the liquor into the lower ball, and confequently its furface will defcend; on the contrary, as the ambient air becomes colder, that in the ball is condenfed, and the liquor preffed by the weight of the atmosphere will afcend: fo that the liquor in the tube will afcend or defcend more or lefs according to the ftate of the air contiguous to the inftrument. To the tube is affixed a fcale of the fame length, divided upwards and downwards from the middle C into 100 equal parts, by means of which the afcent and defcent of the liquor in the tube, and confequently the va riations in the cold or heat of the atmosphere, may be obferved. 3 Its defects. 4 of wine This inftrument was extremely defective; for the air in the tube was not only affected by the heat and cold of the atmofphere, but alfo by its weight. The ipirit The air being found improper for measuring with accuracy the variations of heat and cold according to the form thermome- of the thermometer which was firft adopted, another fluid was proposed about the middle of the 17th century by the Florentine academy, This fluid was fpirit of wine, or alcohol, as it is now penerally named. The alcohol being coloured, was inclofed in a very fine cylindrical gla's tube previously exhausted of its air, having a hollow ball at one end A, and hermetically fealed at the other end D. The tall and tube are filled with rectified fpirit of wine to a convenient height. as to C, when the weather is of a mean temperature, which may be done by inverting the tube into a veffel of flagnant coloured fpirit, under a receiver of the air-pump, or in any other way. When the thermometer is properly filled, the end D is heated red hot by a lamp, and the hermetically fealed, leaving the included air of about of its natural denfity, to prevent the air which is in the fpirit frorm dividing it in its expanfion. To the tube is applied a fcal e, divided from the middle, into 100 equal parts, up. wards and downwards. Its defects. As fpirit of wine is capable of a very confiderable degree of rarefaction and coudenfation by heat and cold, when the heat of the atmosphere increases the spirit dilates, and confequently rifes in the tube; and when the heat decreases, the fpirit.defcends, and the degree or quantity of the motion is fhown by a scale. The fpirit of wine thermometer was not fubject to fome of the inconveniences which attended the air thermometer. In particular, it was not affected by variations in the weight of the atmosphere: accordingly it foon came into general Martine's ufe among philofophers. It was, at an early period, introEjaya. duced into Britain by Mr Boyle. To this inftrument, as then used, there are, however, many objections. The li quor was of different degrees of ftrength, and therefore ditferent tubes filled with it, when expofed to the fame degree of heat, would not correfpond. There was also another defect: The fcale which was adjusted to the thermometer did not commence at any fixed point. The highest term was ad 6 To obtain fome fixed unalterable point by which a deter- Different mined fcale might be difcovered, to which all thermometers fixed points might be accurately adjusted, was the subject which next propofed by drew the attention of philofophers. Mr Boyle, who feems Philofo at an early period to have ftudied this fubject with much phers. anxiety, propofed the freezing of the effential oil of annifeeds as a convenient point for graduating thermometers; but this opinion he foon laid aside. Dr Halley next propofed that thermometers fhould be graduated in a deep pit under ground, where the temperature both in winter and fummer is pretty uniform; and that the point to which the spirit of wine fhould rife in fuch a fubterraneous place fhould be the point from which the scale fhould commence. But this propofal was evidently attended with fuch inconveniences that it was foon abandoned. He made experiments on the boiling point of water, of mercury, and of fpirit of wine; and he feems rather to give a preference to the spirit of wine *: * Phil. He objected to the freezing of water as a fixed point, be- Tranf. Abr II. 34. caufe he thought that it admitted considerable latitude. 7 It feems to have been referved to the all-conquering ge- Sir Ifaac nius of Sir Ifaac Newton to determine this important point, Newton's on which the accuracy and value of the thermometer de-oil thermo pends. He chofe, as fixed, thofe points at which water meter. freezes and boils; the very points which the experiments of fucceeding philofophers have determined to be the most fixed and convenient. Senfible of the difadvantages of fpirit of wine, he tried another liquor which was homogeneous enough, capable of a confiderable rarefaction, about 15 times greater than fpirit of wine. This was linfeed oil. It has not been obferved to freeze even in very great colds, and it bears a heat about four times that of water before it boils. With thefe advantages it was made ufe of by Sir Ifaac Newton, who difcovered by it the comparative degree of heat for boiling water, melting wax, boiling fpirit of wine, and melting tin; beyond which it does not appear that this thermometer was applied. The method he ufed for adjusting the fcale of this oil thermometer was as follows: Suppofing the bulb, when immerged in thawing fnow, to contain 10,000 parts, he found the oil expand by the heat of the human body fo as to take up th more fpace, or 10,256 fuch parts; and by the heat of water boiling trongly 10,725; and by the heat of melting tin 11,516. So that reckoning the freezing point as a common limit between heat and cold, he began his fcale there, marking it O, and the heat of the human body he made 12?; and confequently, the degrees of heat being proportional to the de- Phil. grecs of rarefaction, or 256: 725 :: 12:34, this number 34 270.05 Abr. will exprefs the heat of boiling water; and by the fame vol. iv. part rule, 72 that of melting tin ‡. This thermometer was con-2. ftructed in 1701. Tranf. no 8 To the application of oil as a meafure of heat and cold, Its imper there are insuperable objections. It is fo vifcid, that it ad fections. heres too ftrongly to the fides of the tube. On this account it afcends and defcends too flowly in cafe of a fudden heat or cold. In a fudden cold, fo great a portion remains adhering to the fides of the tube after the rest has fubfided, that the furface appears lower than the corresponding temperature of the air requires. An oil thermometer is theretore not a proper measure of heat and cold. All the thermometers hitherto propofed were liable to Reaumur's many inconveniences, and could not be confidered as exact spirit of ftandards for pointing out the various degrees of tempera- wine ther ture. This led Reaumur to attempt a new one, an ac count of which was published in the year 1730 in the Me mometer. meter, That we may judge the more accurately of the proprie. Thermo. ty of employing mercury, we will compare its qualities with thofe of the fluids already mentioned, air, alcohol, and oil. Thermo- moirs of the Academy of Sciences. This thermometer was made with fpirit of wine. He took a large ball and tube, the dimenfions and capacities of which were known; he then graduated the tube, fo that the fpace from one divifion to Martine's another might contain 1000th part of the liquor; the liFays on quor containing 1000 parts when it flood at the freezing the Contruc point. He adjusted the thera.ometer to the freezing point tion of Therby an artificial congelation of water: then putting the ball of his thermometer and part of the tube into boiling water, he obferved whether it rofe 80 divifions: if it exceeded thefe, he changed his liquor, and by adding water lowered it, till upon trial it fhould just rife 80 divifions; or if the liquor, being too low, fell fhort of 8c divifions, he railed it by add. ing rectified fpirit to it. The liquor thus prepared fuited his purpose, and ferved for making a thermometer of any fize, whofe fcale would agree with his ftandard. mometers. 10 Its defects. ters. This thermometer was far from being perfect. As the bulbs were three or four inches in diameter, the furrounding ice would be melted before its temperature could be propagated to the whole fpirits in the bulb, and confequently the freezing point would be marked higher than it should be. Dr Martine accordingly found, that inftead of coinciding with the 32d degree of Fahrenheit, it correfponded with the 34th, or a point a little above it. Reaumur committed a mistake also refpecting the boiling point; for he thought that the fpirit of wine, whether weak or ftron, when immerged in boiling water, received the fame degree of heat with the boiling water. But it is well known that highly rectiñed fpirit of wine cannot be heated much beyond the 175th degree of Fahrenheit, while boiling water raifes the quickfilver 37 degrees higher. There is another thermometer that goes by the name of Reaumur's, which fhall be afterwards defcribed. Mercurial At length a different fluid was propofed, by which ther thermome- mometers could be made free from most of the defects hitherto mentioned. This fluid was mercury, and feems firft to have occurred to Dr Halley in the last century; but was not adopted by him on account of its having a fmaller degree of expanfibility than the other fluids used at that time*. Boerhaave fays that the mercurial thermometer Tranf. vol. was first constructed by Olaus Roemer; but the honour of xvii. or this invention is generally given to Fahrenheit of AmfterAbr. vol. ii. dam, who presented an account of it to the Royal Society of London in 1724. Phil. meter. 12 of air, al cohol, and Air is the mot expanfible fluid, but it does not receive Properties nor part with its heat fo quickly as mercury. Alcohol does not expand much by heat. In its ordinary itate it does not oil. bear a much greater heat than 175° of Fahrenheit; but when highly rectified it can bear a greater degree of cold than any other liquor hitherto employed as a measure of temperature. At Hudfon's Bay, Mr Macnab, by a mixture of vitriolic acid and faow, made it to defcend to 69 below of Fahrenheit. There is an inconvenience, however, attending the ule of this liquor; it is not poffible to get it always of the fame degree of ftrength. As to oil, its expanfion is about 15 times greater than that of alcohol; it fu fains a heat of 600°, and its freezing point is fo low that it has not been determined; but its vifcofity renders it uielefs. any 13 of mercury, fur les Mod. Mercury is far fuperior to alcohol and oil, and is much more Thermo. manageable than air. 1. As far as the experiments already metrical made can determine, it is of all the fluids hitherto employed? properties in the conftruction of thermometers, that which measures moft exactly equal differences of heat by equal differences of its bulk: its dilatations are in fact very nearly proportional to the augmentations of heat applied to it (a). 2. Of all liquids Recherches it is the moft eafily freed from air. 3. It is fitted to inea- de l'Atwofure high degrees of heat and cold. It fuftains a heat of jphere. 600° of Fahrenheit's fcale, and does not congeal till it fall 39 or 40 degrees below o. 4. It is the most fenfible of fluid to heat and cold, even air not excepted. Sir Benja-Phil. min Thompfon, now Count Rumford, found that mercury Tran, for was heated from the freezing to the boiling point in 58 fe- 1786. conds, while water took two minutes 13 feconds, and common air 10 minutes and 17 feconds. 5. Mercury is a homogeneous fluid, and every portion of it is equally dilated or contracted by equal variations of heat. Any one ther mometer made of pure mercury is, cateris paribus, possesfed of the fame properties with every other thermometer made of pure mercury. Its power of expansion is indeed about fix times lefs than that of spirit of wine, but it is great enough to antwer most of the purposes for which a thermometer is wanted. 14 The fixed points which are now univerfally chofen for Fixed adjusting points. Cromfiett's Mineralogy (A) We have affirmed that the expanfions of the bulk of quickfilver by heat are nearly (for they are not strictly fo) in a regular arithmetical progreffion, according to the quantity of heat it is expofed to; and fuch feems to be the cate according to the Table publifhed by Mr de Luc, at page 309. of his firft volume on the Modifications of the Atmosphere. The following extract of this table hows thefe variations: and the first and fecond differences are added, in order to render thefe irregularities more fenfible. They are fuch as can hardly be conceived from the nature of any fub- vol. ii. ftance, without the influence of extraneous and accidental caufes, which may have efcaped the attention of the obferver; neither have they been found exactly true by Dr Crawford. Mr de Luc fuppofes the whole heat from melting ice to that of boiling water to be divided into 80 parts; by the fractional fubdivifions of which he expreffes the abfolute quantities of heat, antwering to each 5, or 10 degrees of Reaumur's thermometer (22,5 of Fahrenheit's fcale); so that the whole fum of these fractions amounts exactly to the affumed number 80. They are as follow: merer. third column of differences of the equations. See Phil. Thermo Tranf. Ixiv, art. 30.; alfo Dr Mafkelyne's Paper, vol. Ixiv. art. 20. meter. 17 Thermo- adjufting thermometers to a feale, and to one another, are In the following table we have the refult of 15 different Sir George obfervations made by Sir George Schuckburgh compared Schuckwith the refult of M. de Luc's rules. ་་ Lule for ers to bulb of the tube be plunged in the water when it boils vio- As artists may be often obliged to adjust thermometers Phil. Tran. for #775 and -778. where b denotes the height of a thermometer plunged in Height of the Inch. 26,498 27,241 Ivation. Boiling Poin Height Rules. neter. Inch. 207,07 208,54 208,64 208,84 27,954 209,87 210,03 28,377 210,50 210,81 28,699 211,27 211,34 30.847 214,83 214,79 28,898 211,50 211,67 39,957 214,96 214,96 28,999 211,65 211,85 The Royal Society, fully apprized of the importance of Obfervaadjufting the fixed points of thermometers, appointed a by a comcommittee of leven gentlemen to confider of the best me- mittee of thod for this purpofe; and their report is published in the the Royal Phil. Tranf vol. Ixvii. part ii. art. 37. Society for They obferved, that though the boiling point be placed adjufti & the fixed fo much higher on fome of the thermometers now made than points. on others, yet this does not produce any confiderable error in the obfervations of the weather, at leaft in this climate; for an error of 1° in the pofition of the boiling point, will make an error only of half a degree in the pofition of 920, and of not more than a quarter of a degree in the point of 62°. It is only in nice experiments, or in trying the heat of hot liquors, that this error in the boiling point can be of much importance. In adjusting the freezing as well as the boiling point, the quickfilver in the tube ought to be kept of the fame heat as that in the ball. When the freezing point is placed at a confiderable distance from the ball. the pounded ice should be piled to fuch a height above the ball, that the error which can arife from the quickfilver in the remaining part of the tube not being heated equally with that in the ball, shall be very small, or the obferved point must be corrected on that account according to the following table: Heat ( 20 filver in the tube ought to be heated The correction in this table is expressed in 1000th parts the distance between the freezing point and the furface of the ice: e. g. if the treezing point ftands feven inches above the furface of the ice, and the heat of the room is 62, the point of 3 fhould be placed 7 X 00261, or ,018 of an inch lower than the obferved point. A diagonal scale will facilitate this correction. The quick. The committee obferve, that in trying the heat of liquors, care fhould be taken that the quickfilver in the tube of the thermometer be heated to the fame degree as that in the ball; or if this cannot be done conveniently, the obferved to the fame heat fhould be corrected on that account; for the manner degree as of doing which, and a table calculated tor this purpoie, we that in the muft refer to their excellent report in the Phil. Tranf. vol. ball. Ixvii. part ii. art. 37. 21 'The tubes P. With regard to the choice of tubes, they ought to be ex. ought to actly cylindrical. But though the diameter fhould vary a be cylindri-little, it is eafy to manage that matter in the manner procal and ca- pofed by the Abbé Nollet, by making a small portion of pillary. the quickfilver, e. g. as much as fills up an inch or half an Lecons de inch, flide backward and forward in the tube; and thus Phyf. Exp. tom. iv. to find the proportions of all its inequalities, and from 376. thence to adjust the divifions to a fcale of the most perfect equality. The capillary tubes are preferable to others, becaufe they require fmaller bulbs, and they are alfo more fenfible, and lefs brittle. The moft convenient fize for common experiments has the internal diameter about the 40th or 50th of an inch, about 9 inches long, and made of thin glafs, that the rife and fall of the mercury may be better feen. 22 The Qum grees into which the ded. The next thing to be confidered, is of what number of ber of de- degrees or divifions the fcale ought to confift, and from what point it ought to commence. As the number of the fcale ought divifions of the fcale is an arbitrary matter, the fcales which to be divi- have been employed differ much from one another in this circumftance. Fahrenheit has made 180 degrees between the freezing and boiling water point. Amonton's made 73, and Sir Ifaac Newton only 34. There is, however, one general maxim, which ought to be observed: That fuch an arithmetical number should be chofen as can easily be divided and fubdivided, and that the number of divifions fhould be fo great that there fhall feldom be occofion for fractions. The number 80 chofen by Reaumur anfwers extremely well in this refpect, because it can be divided by feveral figures without leaving a remainder; but it is too fmall a number: the confequence of which is, that the degrees are placed at too great a distance from one another, and fractions must therefore be often employed. We think, therefore, that 160 would have been a more convenient number. Fahrenheit's number 180 is large enough, but when divided its quotient foon becomes an odd number. 23 At what As to the point at which the scale ought to commence, point the various opinions have been entertained. If we knew the fcale ought beginning or loweft degree of heat, all philofophers would agree, that the loweft point of the thermometer ought to be fixed there; but we know neither the loweft nor the highest degrees of heat; we obferve only the intermediate parts. All that we can do, then, is to begin it at fome invariable to comLuence. point, to which thermometers made in different places may Therine. eafily be adjusted. If poffible too, it ought to be a point at meter. which a natural well-known body receives fome remarkable change from the effects of heat or cold. Fahrenheit began his fcale at the point at which fnow and falt congeal. Kirwan propofes the freezing point of mercury. Sir Ifaac Newton, Hales, and Reaumur adopted the freezing point of water. The objection to Fahrenheit's lowest point is, that it commences at an artificial cold never known in nature, and to which we cannot refer our feelings, for it is what few can ever experience. There would be several great advantages gained, we allow, by adopting the freezing point of mercury. It is the lowest degree of cold to which mercury can be applied as a measure; and it would render unneceffary the ufe of the figns plus and minus, and the extenfion of the fcale below o. But we object to it, that it is not a point well known; for few, comparatively speaking, who use thermometers, can have an opportunity of feeing mercury congealed. As to the other advantage to be gained by adopting the freezing point of mercury, namely, the abolition of negative numbers, we do not think it would counterbalance the advantage to be enjoyed by ufing a wellknown point. Besides, it may be asked, Is there not a propriety in afing negative numbers to exprefs the degree of cold, which is a negative thing? Heat and cold we can only judge of by our feelings: the point then at which the scale fhould commence, ought to be a point which can form to us a ftandard of heat and cold; a point familiar to us from being one of the most remarkable that occurs in nature, and therefore a point to which we can with most clearnefs and precifion refer to in our minds on all occafions. This is the freezing point of water chosen by Sir Ifaac Newton, which of all the general changes produced in nature by cold is the most remarkable. It is therefore the most convenient point for the thermometers to be used in the temperate and frigid zones; we may fay over the globe, for even in the hottest countries of the torrid zone many of the mountains are perpetually covered with fnow. 24 thermome ter-gene Having now explained the principles of the thermometer le four as fully as appears necefTary, in order to make it properly understood, we will now fubjoin an account of thofe thermo rally used. meters which are at prefent in molt general use. Thete are Fahrenheit's, De l'Ifle's, Reaumur's, and Celfius's. Fahren-` heit's is ufed in Britain, De l'Ifle's in Ruffia, Reaumur's in France, and Celfius's in Sweden. They are all mercurial thermometers. 15 Fahrenheit's thermometer confifts of a lender cylindrical Fahren tube and a fall longitudinal bulb. To the side of the tubet's de is annexed a scale which Fahrenheit divided into 600 parts, fcribed. beginning with that of the fevere cold which he had obferved in Iceland in 1709, or that produced by furrounding the bulb of the thermometer with a mixture of inow or beaten ice and fal ammoniac or fea falt. This he apprehended to be the greatest degree of cold, and accordingly he marked it, as the beginning of his fcale, with o; the point at which mercury begins to boil, he conceived to show the greatest degree of heat, and this he made the limit of his fcale. The distance between these two points he divided into 600 equal parts or degrees; and by trials, he found that the mercury flood at 32 of thefe divifions, when wa ter juft begins to freeze, or fnow or ice juit begins to thaw; it was therefore called the degree of the freez ing point When the tube was immerfed in boiling water, the mercury rofe to 212, which therefore is the boiling point, and is juft 180 degrees above the former or freezing point. But the prefent method of making the fcale of thefe thermometers, which is the fort in moit common ufe, is first to immerge the bulb of the thermometer in ice or |