June 4, 1915] 



SCIENCE 



827 



Also y =. a/3/2 = 16.0870. The coefficient 8 is 

 unity, which is a little unfortunate since it 

 might lead, to the erroneous impression that 

 ■we were defining unit force as that force which 

 gives unit acceleration to unit mass. Our 

 choice of unit acceleration has probably been 

 injudicious. 



Enough has been given to illustrate the prin- 

 ciple which we feel sure ought to commend 

 itself to every one who once grasps the funda- 

 mental independence of all dynamical con- 

 cepts and the strictly proportional nature of 

 the dynamical equations, all of which are 

 merely the algebraic formulation of experi- 

 mental evidence. In extenuation of our intro- 

 duction of a new set of numerical constants to 

 be memorized we can only point out that there 

 were many " J's " " 4ir's," etc., there already 

 and that we entirely do away with the trouble- 

 some and useless subject of dimensions. 



The new system is not fully developed as 

 yet, however, and until it is we have found 

 ourselves compelled to make the best of the 

 old one. We dodge the ambiguity in the 

 " ambiguous words ' weight ' and ' mass ' " by 

 the artifice of defining them. We adopt and 

 we teach the convention that " mass " shall be 

 an exact equivalent for " quantity of matter," 

 and that " weight " means the gravitational 

 force upon a mass. We teach that the measure 

 of a force (wherever the concept of force may 

 originate) may conveniently be defined by the 

 equation F = ma. We teach that it is a re- 

 markable law of nature, determined only by 

 experiment, and not to be suspected a "priori, 

 that the " body factor " in this equation is 

 strictly proportional to the weight for all 

 bodies in the same uniform gravitational field. 

 We point out that pounds-mass and pounds- 

 weight {i. e., pounds-force) are totally differ- 

 ent things, and that there are 32.2- of the units 

 of force defined by the equation F^ma in a 

 pound-weight and that therefore all forces 

 deduced in dynamical equations must be 

 divided by 32.2 if we wish to express them in 

 terms of pounds-weight, much as one would 

 reduce centimeters to feet. Conversely, all 

 forces given in pounds-weight must be multi- 

 plied by 32.2 before they can be used in 

 dynamical equations. We teach that the fun- 



damental idea of the gravitational con- 

 stant g is force per unit mass and that it 

 is also of the nature of an acceleration 

 in virtue of the relation F/m = a. We 

 hold that dynamics may be developed without 

 the introduction of arbitrary constants by the 

 assumption of three fundamental units and 

 defining all the others in terms of these three. 

 We object to Professor Kent's description of 

 a system with four fundamental units as a 

 " foot-pound-second " system instead of a 

 " foot-pound-second-pound " system, and to his 

 ridicule of the " gee-pound " or " slug " in the 

 same letter in which he says, " It has been 

 found convenient to use the letter m instead 

 of W/g." What is the unit of m if not the 

 "slug"? We frankly talk about a unit of 

 force called a poundal as a matter of conven- 

 ience, and we measure it by a defining equation 

 much as we measure a unit of velocity or of 

 work. We consider this term preferable to 

 the " pound-f oot-per-second-square," and ven- 

 ture to hope that there may some day be intro- 

 duced shorter names for the " f oot-per-second- 

 per-second" of acceleration and the "pound- 

 foot-square " of moment of inertia. 



T. L. POETEK, 

 E. C. GOWDY 



The Univeesitt of Cincinnati 



another state park needed 

 To THE Editor of Science: Two or three 

 notes of interest have appeared in Science 

 regarding the new state reservation at James- 

 ville, Onondaga County, New York, which in- 

 cludes the glacial lake, sometimes known as 

 West Green Lake. This little lake is of espe- 

 cial interest owing to its history as the site of 

 the plunge basin of a great glacial waterfall, 

 and also because in its environs is found the 

 hart's tongue fern {Phyllitis Scolopendrium,) 

 which probably ranks as the most interesting 

 and rarest fern in the United States. 



Now it is proposed to acquire another lake 

 of identical geological history. East Green 

 Lake (also known as Blue Pond, and Scolo- 

 pendrium Pond), which lies about a mile east 

 of the west lake above mentioned. The pro- 



