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DOUBLE USE OF THE TERM ACCELERATION
TO THE EDITOR OF SCIENCE: Dr. Hering's letter in the issue of October 24 raises a question of scientific terminology of a kind not altogether unusual. He contends that the term “acceleration " is used in one sense by the engineer, namely, to signify the rate of change of speed; and indiscriminately, in two different senses by the physicist, one of these meanings coinciding with the engineering usage, and the other conflicting with it. This second use of the word is to denote the vector rate of change of another vector, the velocity. As I understand his letter, he proposes that the physicist abandon this second meaning in
in favor of the first.
Dr. Hering is an eminent engineer, and I leave it to other engineers to question, if they choose, his right to speak for them. I must protest however, against his version of the views of physicists. The term acceleration, in its strict sense, is now used by physicists only with the second of the two above meanings, and then applies, when used without any qualifying word, only to the motion of a point or particle. The word is sometimes used, in order to avoid circumlocutions, to denote merely the scalar magnitude of the vector. The need of a new word to express this second notion, in the manner in which we now customarily distinguish between velocity and speed, has long been felt. This somewhat loose usage is however quite different from the definition recommended by Dr. Hering, which would give it the meaning "tangential component of the acceleration." It would be rash to assert that the term is never used in this sense by physicists, for carelessness of language is hard to avoid, but few would be found to defend the usage.
Dr. Hering chooses as an illustration of the divergents of physicist and engineer: ... the revolution of a fly-wheel at a constant speed, the rim of which to the physicist is being constantly accelerated while to the engineer there is no acceleration, as the speed is constant.
The physicist argues, and quite correctly, that a moving body represents a vector quantity, as it has both speed and direction. The same external
force applied to such a moving body will change either the speed or the direction, depending upon the relative directions of that force and of the moving body. But as force is defined as mass X acceleration, the physicist, apparently forgetting the difference between pure and applied mathematics, methodically divides this force by the mass and calls the quotient acceleration. It simplifies his mathematics.
I have quoted these remarkable sentences at length, because I should not dare to attempt any summarizing paraphrase. Assuming that the the physicist is "arguing correctly ” when he makes a “moving body represent a vector quantity," the offense seems to consist in qua “apparently forgetting the difference between pure and applied mathematics.” What is this difference? It is that the "pure” mathematics is applicable to dynamical problems, whereas Dr. Hering's “ applied ” mathematics is not.
The case of the revolving fly-wheel offers no real difficulty either of treatment or of terminology. The “acceleration ” of the flywheel as a whole is either a term without meaning, or applies to a translatory movement. Any point or particle of the wheel is accelerated toward the axis, from which we infer the existence of a force in this direction acting on the particle. Of the fly-wheel as a whole, we speak of the “angular acceleration,” which is zero when the angular speed is constant and the direction of the axis invariable. From the vanishing of this vector we infer, not the absence of an external force, but the absence of an external torque or couple.
Take the case of a falling particle, describing a parabolic trajectory, and compare the two statements :
(a) The acceleration is g, vertically down. ward.
(6) The acceleration is g cos 0, where is the angle between the velocity and the downward vertical.
The second of these statements is in conformity with engineering usage, if I understand Dr. Hering correctly. The first statement describes the motion in such a way that if we know the velocity at any time we can find the velocity at any subsequent time, by adding the vector gt vertically downward to the original rector velocity. The second statement assumes, so far as I can see, a knowledge of one of the quantities we need to calculate. I should very much like to see Dr. Hering's “applied” mathematics applied to this simple problem. So far as I can see, though the first statement simplifies the mathematics, the second abolishes it.
Scientific terminology is like a sharp knife used for the dissection of a problem, and unequaled for its intended purpose. It is an odd coincidence that the very number of SCIENCE which contains Dr. Hering's letter contains also an address by Dr. Gray of Edinburgh, in which the sharpness of this particular knife, the term acceleration in its strict sense, is specially noted. Dr. Hering's proposal is as if one should say, “I find your razor good for sharpening pencils, please shave with something else.”
Surely the ends, neither of science nor engineering will be furthered by any such change as Dr. Hering recommends. The question is not one of simplified mathematics, but of clearness of thought.
C. M. SPARROW Rouss PHYSICAL LABORATORY,
UNIVERSITY OF VIRGINIA,
of pilot balloons which tallied remarkably well with the formula derived from theoretical considerations. This formula is
V = 71(l/L2/3) 5/8 = 71(18/L2).208, in which V represents velocity, 1 the actual lifting power of the balloon (“free-lift”), i. e., the weight it will support, and L the total lift (free-lift plus weight of balloon). Surprising as it is, pilot balloons ascend at a nearly constant rate, once they are above the more or less turbulent surface layer of air. Thus, single theodolite observations of angular altitude and azimuth of a balloon once a minute, when used in conjunction with the computed ascensional rate will yield reliable information as to the actual positions of the balloon, and, therefore, of the direction and velocities of the wind at all levels from the surface to the height at which it becomes lost to view. At the Aberdeen Proving Ground, temperatures for computing the densities of the air in the several altitude zones have been obtained by daily airplane ascents to a height of 10,000 feet. The score of pilot balloon stations in the United States east of the Rockies telegraph free-air wind data to the Weather Bureau in Washington twice daily, where they are used not only for aeronautical forecasting, but also as an auxiliary in making surface weather forecasts.
Meteorological kite flights are now being made at six stations daily (except when winds are light) for recording winds, relatively humidities, and temperatures aloft. The results are telegraphed to Washington daily, and later are published in Monthly Weather Review Supplements, where they become available for detailed investigation.2
The movements of dust, smoke and clouds are useful as well as balloons and kites for determining the movements of the free air. Dustfalls which occasionally occur in the northeastern United States have been traced
2 See, for instance, V. E. Jakl, “Some observations on temperatures and winds at moderate elevations above the ground,” Mo. Weather Rev., June, 1919, pp. 367–373. Separates of these are still available: apply to Chief, U. 8. Weather Bureau, Washington, D. C.
NOTES ON METEOROLOGY AND
AEROLOGICAL WORK-WINDS AFTER the signing of the armistice had liberated much information that had been held as confidential, it became possible to assemble a group of papers on aerological work describing the pilot balloon methods used by the Weather Bureau, Signal Corps, and Navy for observing winds at various levels, and presenting the results of various lines of research. The use of thousands of two-theodolite pilot balloon runs established an empirical formula for the ascensional rate
1 Mo. Weather Rev., April, 1919, Vol. 47, pp. 205–231. Separates of these are still available: apply to Chief, U. S. Weather Bureau, Washing ton, D. C.
back to the arid portions of the southern Great Plains. Observations of forest-fire smoke also give reliable information of air movements over long distances, as in October, 1918, when Minnesota smoke was observed throughout the eastern half of the United States except along the gulf and south Atlantic coasts. Observations on clouds may be complementary to those on pilot balloons, for the usefulness of pilot balloons decreases as the cloudiness increases. How cloud move ments may be used in local weather forecast ing has been discussed by A. H. Palmer for San Francisco, M. L. Fuller for Peoria, Ill., and H. H. Martin for Columbus, 0.5
through such regions. Clouds, rain and fog all contribute to the discomfort and danger of flying.
Perhaps the most interesting are the experiences in the thunderstorms and the up-and-down winds which accompany such storms. As the driving wedge of cold air at the surface advances ahead of the storm, the air into which the storm is moving is forced upward. The maximum turbulence is found in the region of the squall cloud, but the force of the rising air ahead of the storm is suffi. cient to carry up an airplane considerably, in spite of the efforts of the pilots to keep the nose of the plane down. The dangers from lightning and bail, are also quite as important as those from the capricious winds.
There is an annotated bibliography at the end. This article bound with two on ballooning and with reviews of Y. Henderson's “ Physiology of the aviator," and H. Luckeish's "High lights of air travel,” may be had on application to the Chief, U. S. Weather Bureau.
CHARLES F. BROOKS
AIRPLANES AND THE WEATHER An article on the “Effect of winds and other weather conditions on the flight of airplanes " is a rather extensive, though by no means complete, compilation and discussion of aviators' meteorological experiences. To quote from the synopsis :
The disturbances of the air due to daytime convection are one of the prime sources of bumpiness. Especially on hot summer days do strong, rapidly rising currents of air penetrate to great altitudes and, where encountered, jolt the airplane. Whe
be cooler air is descending, the effect is similar to that of falling into a “hole.” The height to which the effects of surface roughness extend when the wind is blowing depends upon the speed of the surface wind and the height of the obstruction,
In the free air, aviators' observations show how the layers of air flow over one another, the interface sometimes being marked by clouds and sometimes entirely invisible. At such levels are encountered billows or waves, and considerable difficulty is sometimes experienced in flying
3 See Winchell and Miller, Mo. Weather Rev., November, 1918, Vol. 46, pp. 502–506.
4 Mo. Weather Rev., November, 1918, pp. 506– 509.
5 Mo. Weather Rev., September, 1918, pp. 407413; July, 1919, pp. 473-474, and August, 1919, pp. 567-570. A limited supply of separates is held by each of the authors named: address, “Weather Bureau Office" at cities named.
6 Mo. Weather Rev., August, 1919, pp. 523-532, 10 figs.
CEREALS IN THE NORTHWEST DURING the first half of May, 1918, the Station Entomologist was called to Olympia, Wash., to consult with the farmers and county agent concerning an outbreak of aphis on wheat. He found that the aphids were not responsible for the whole trouble and submitted samples of wheat from the unthrifty fields to the station plant pathologist for diagnosis. Subsequently specimens showing the same disease were submitted from this and other localities in western Washington through the county agents of the respective counties.
Among the first lot of plants were some showing lesions at the base of the stem. These lesions were elliptical, light-centered, penetrating the leaf-sheath and the surface of the stem. Plants with these lesions and otherg with a general blackening of the lower nodes showed death of the roots at the first node, the plant attempting to make good this loss by putting out roots at the second node. In some plants two sets of roots had been successively killed and roots had been put out at the third
node. Affected plants were sickly in growth, culm it was found that this was due to a yellowish in color and showed little or no very compact surface growth of dark brown stooling.
hyphæ, approaching in some cases almost to a Later reports, especially from Cowlitz sclerotial formation. County, showed that the disease was respon. No fruiting stage of the fungus has as yet sible for uneven stand in the field with con- been connected with the sterile stage on the siderable lodging, the stems breaking over base of the culms. Until this is done we can near the surface of the ground. As reported only compare symptoms, and vegetative charthe disease showed no relation to type of soil acters of the fungus with published descripor system of culture. The disease also ap tions of foot-rot diseases of cereals. There peared on oats and barley but in less severe seems to be a very close similarity between form and showed more of a general blacken- the disease as it occurs in Washington and ing of the base of the stem and death of roots. the foot-rot of cereals caused by Ophiobolus
The disease was reported from Cowlitz, graminis Sacc., as described by McAlpine Snohomish and Thurston counties on wheat and others. with the greatest severity in Thurston county, Ophiobolus graminis and other foot-rot while it was reported from Cowlitz county as fungi are known to produce an ascigerous causing most injury to maturing grain. Re- stage on old stubble, so it seems probable that ports of the disease on barley were received the fungus causing the foot-rot of cereals in only from Pierce county. On oats the disease western Washington will be found to have a occurred in Clarke, Pierce and Snohomish perfect or ascigerous overwintering stage on counties. The wheat crop in certain localities the stubble of affected plants. There is also of Cowlitz and Thurston counties suffered a possibility of the disease occurring on the material injury while no data are at hand to native grasses. It is on these wild hosts that show that the disease caused any material some of the foot-rot fungi are known to be damage to the other cereals.
carried through a crop rotation. It has not In 1902 Cordley? reported a foot-rot of been possible to make a careful field study cereals in Oregon but gave only a brief de for the discovery of the ascigerous stage. In scription of the disease and did not determine the limited work undertaken a species of the causal organism. The disease he mentions Pleospora is the only perithecial form that has is undoubtedly identical with the disease been found in the old wheat culms. It is not which appeared during 1918 in western Wash- yet possible to say whether the disease in ington. No other occurrences of the disease Washington is identical with any of similar in the united States are recorded. The dis- European or Australian diseases. ease is either a newcomer or has escaped Reports indicate that the disease is already general notice up to the present time. rather widely distributed in western Washing
A very careful microscopic study of the ing and Cordley's account of the disease fungus found in the stem lesions was made would indicate that it may have been present in the attempt to determine the fungus. The for a considerable time. Time alone will mycelium was sterile, dark brown in color. determine whether the disease will become as with constrictions at the origin of side
serious as the foot-rot diseases of cereals in branches. The mycelium agreed fairly well s. The mycelium agreed fairly well Europe and Australia have been.
B. F. DANA, with Rhizoctonia solani Kuhn., except in the
Assistant Plant Pathologist diameter of the hyphe which were only about
AGRICULTURAL Exp. STATION, half as large. In case of plants showing a
PULLMAN, WASH. very pronounced blackening at the base of the
2 McAlpine, D., “Take-All and White Heads in 1 Cordley, A. B., Ann. Rpt.Ore. Agr. Expt. Sta., Wheat," Bul. Dept. Agr. of Victoria, 9: 1-120, 1912, pp. 66-67.
THE NEW HAVEN MEETING OF THE NATIONAL ACADEMY OF SCIENCES
THE sessions of the autumn meeting of the academy were held in the Osborn Zoological Laboratory, Yale University, on November 10 and 11. The program of scientific papers was as follows:
MONDAY, NOVEMBER 10
Morning Session Four cliff islands in the coral seas: W. M. DAVIS.
Some new theorems in the dynamics of a par. ticle: EDWARD KASNER.
The relative physiological efficiency of spectral lights of equal radiant energy content (by invitation): HENRY LAURENS and HENRY D. HOOKER.
A study in synthetic paleontology (by invitation): RICHARD S. LULL.
The adjustment to the barometer of the hæmatorespiratory functions in man (by invitation) : YANDELL HENDERSON.
Development of connective tissue in the amphibian embryo (by invitation): GEORGE A. BAITSELL.
Afternoon Session A new method for determining the solar constant of radiation : C. G. ABBOT.
A kinematio interpretation of electromagnetism (by invitation): LEIGH PAGE.
Defects found in drafted men: CHARLES B. DAVENPORT and ALBERT G. LOVE.
The effect of physical agents on the resistance of mice to cancer (by invitation): JAMES B. MURPHY,
Some restorations of extinct vertebrates. The great sponge colonies of the Devonian; their origin, rise and appearance : JOHN M. CLARKE.
On the mechanism of fever reduction by drugs (by invitation): HENRY G. BARBOUR and J. B. HERRMANN.
The Thompson effect from the point of view of dual electric conductivity: EDWIN H. HALL.
effect upon an atom of the passage of an alpha ray through it: R. A. MILLIKAN.
Leptospira icteroides and yellow fever (by invitation): HDDEYO NOGUCHI.
Calculating ancestral influence in man (by invi. tation): H. H. LAUGHLIN.
Calcium and magnesium metabolism in certain diseases (by invitation): FRANK P. UNDERHILL, JAMES A. HONEIJ and L. JEAN BOGERT.
Reconstruction of the skeleton of the sauropod dinosaur Camarasaurus Cope (Morosaurus Marsh): HENRY FAIRFIELD OSBORN and CHARLES CRAIG MOOK.
Restoration of camarasaurus and life model (communicated by H. F. Osborn): WILLIAM K. GREGORY.
Plato's Atlantis in paleogeography (communicated by H. F. Osborn): WILLIAM DILLER MATTHEW.
Afternoon Session Lethargic encephalitis and poliomyelitis: SIMON FLEXNER.
The history of the coral reefs of Tutuila, Samoa. Biographical notice of Dr. Samuel Hubbard Scudder, 1837-1911 (read by title): A. G. MAYOR.
Changes of land and ocean levels (by invitation): R. A. DALY.
On hyperplasia of nerve centers resulting from excessive loading (by invitation): SAMUEL RANDALL DETWILER.
Concentration of the water soluble vitamine of yeast: T. B. OSBORNE. ,
The manner of infection of the white pine by the blister rust, with demonstrations at the Con. necticut Agricultural Experiment Station: (by invitation): G. P. CLINTON.
Certain chemical properties of foods and their relation to nutrition, with demonstrations at the Connecticut Agricultural Experiment Station: T. B. OSBORNE and LAFAYETTE B. MENDEL.
Studies upon the life cycles of the bacteria (in. troduced by Raymond Pearl) (read by title): F. LOHNIS.
Lower California and its natural resources (in troduced by C. D. Walcott) (read by title): EDWARD W. NELSON.
A recalculation of the atomic weights (read by title): F. W. CLARKE.
Biographical memoir of Richmond Mayo-Smith (read by title): EDWIN R. A. SELIGMAN.
Biographical memoir of Samuel Wendell Williston (read by title): R. S. LULL.
Biographical memoir of Charles R. Van Hise (read by title): THOMAS CHOWDER CHAMBERLIN.
TUESDAY, NOVEMBER 11 A statistical method for studying the radiations from radioactive substances and the X-rays (by invitation). ALOIS F. KOVARIK.
Anatomical changes in the respiratory tract as sociated with acid insufflation (by invitation): M. C. WINTERNITZ.
Experimental pneumonia in monkeys (by invitation): FRANCIS G. BLAKE.
The extension of the ultra-violet spectrum. The