Septembeb 14, 1906.] 



SCIENCE. 



341 



quired ten minutes of time, and if we had 

 exact measurements of angles and times we 

 should undoubtedly from these curves be able 

 to deduce the rate of rotation of the meteor, 

 and possibly its mass. 



Most of the aerolites that come to the earth 

 show, by their pitted surfaces, that the 

 meteoric material is being split off or ejected 

 quite uniformly from the whole Surface; but 

 some meteors, like some comets, may have 

 only a very few regions on the surface from 

 which material is ejected with any special 

 force. In Mr. Davis's meteor of October 13 

 we seem to have a case in which some one 

 spot on the side of the meteor, namely some- 

 where between its head and its tail, ejected its 

 material freely and with considerable force, in 

 a direction at right angles to the axis of rota- 

 tion, or the line joining head and tail: it 

 represents the rare case of a symmetrical 

 revolving meteor. Cleveland Abbe. 



Washington, D. C, 

 August 15, 1906. 



SOME ' DEFINITIONS ' OF THE DYNE, 



It would seem comparatively easy for any 

 one whose mind has dwelt comprehendingly 

 upon the relation F = ma to define correctly 

 the unit of force in terms of mass and acceler- 

 ation. But such is evidently not the case. 

 Of the text-books of physics immediately at 

 hand, four give incorrect definitions of the 

 dyne. In each case the author is a man of 

 high and wide reputation as a writer and 

 teacher. In quoting the definitions in ques- 

 tion, I have taken the liberty of italicizing 

 the words to be omitted with advantage. 



(a) Force: dyne. 1 g. given unit acceleration 

 in 1 sec. 



(b) The practical unit of force is the dyne. 

 * * * It produces unit acceleration of unit mass 

 in unit time. 



(c) The absolute unit of force (in the C. G. S. 

 system) is called a dyne, and is that force which 

 in one second is capable of giving to a gram-mass 

 an acceleration of one centimeter per second. 



(d) The absolute unit of force is the force that, 

 acting for unit of time upon unit of mass, will 

 produce unit of acceleration. * * * The centi- 

 meter-gram-second (C. G. S.) unit of force is the 

 force that, acting for one second upon a mass 



of one gram, produces an acceleration of one 

 centimeter per second. It is called a dyne. 



The first two books are intended for univer- 

 sities and colleges, the latter two for prepara- 

 tory schools, and all four, I believe, have been 

 widely used. 



It is perhaps proper to state that each of 

 these authors gives a correct definition in 

 terms of mass, time and change of velocity, 

 but each seems implicitly to ignore the fact 

 that acceleration is not change of velocity, 

 but is rate of change of velocity. To con- 

 clude, the dyne is the force that, acting on a 

 mass of one gram, gives to it C. G. S. unit 

 acceleration (for which there is no name), 

 irrespective of the time during which the 

 force acts upon the mass, whether it be the 

 millionth part of one second, or one million 

 eons. F = ma, and when m = 1, and a = 1, 

 then F = l. 



Similar errors are of course committed in 

 defining the poundal. The text from which 

 the quotation (d) above is taken gives : 



The foot-pound-second (F. P. S.) unit of force 

 is the force that, applied to one pound of matter 

 for one second, will produce an acceleration of 

 one foot per second. It is called a poundal. 



And a fifth book, also for preparatory 

 schools, states : 



In the F. P. S. system the unit is the poundal, 

 which is the force that on being applied to 1 lb. 

 of matter for 1 sec. will give to it an acceleration 

 of 1 ft. 



In this last case, the same mistake is made 

 in the general definition of the absolute unit 

 of force, but the dyne, specifically, escapes 

 with a defective definition in terms of mass, 

 time and change of velocity. 



Harold C. Barker. 

 The Randal Morgan Laboratory 

 OF Physics, University of 

 Pennsylvania. 



SPECIAL ARTICLES. 

 A PECULIAR MUTATION OF THE PINE MARTEN. 



A trapper's skin, without skull, of a pine 

 marten (Mustela am,ericana actuosa Osgood) 

 recently offered for sale to the National Mu- 

 seum by Mr. James Aitchison, Nulato, Alaska, 



