394 



SCIENCE 



[N. S. Vol. L. No. 1295 



second per second. The physicists, however, 

 who use this term in the same sense, also use 

 it indiscriminately in an entirely different 

 sense, namely, to express a change of direction 

 of a moving body, without any regard as to 

 whether there is any change in speed or not. 

 Thus the physicist will refer to the existence 

 of acceleration when to the engineer there is 

 none. A case in point is the revolution of a 

 fly wheel at a constant speed, the rim of which 

 to the physicist is being constantly acceler- 

 ated while to the engineer there is no acceler- 

 ation, as the speed is constant. 



The physicist argues, and quite correctly, 

 that a moving body represents a vector quan- 

 tity, 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 direc- 

 tions of that force and of the moving body. 

 But as force is defined as mass X accelera- 

 tion, the physicist, apparently forgetting the 

 difference between pure and applied mathe- 

 matics, methodically divides this force by the 

 mass and calls the quotient acceleration. It 

 simplifies his mathematics. 



Such blind applications of pure mathe- 

 matics, however, sometimes lead to absurd 

 results. In the present case, if this external 

 force is applied in the direction of the move- 

 ment of the body, it adds energy to the 

 moving system, as in the case of a falling 

 body. This is the sense in which engineers 

 use the term acceleration. But if this ex- 

 ternal force is applied perpendicularly to the 

 direction of motion, no energy whatever is 

 added to the moving system, as in the case 

 of bodies rotating around a center. 



The importance of this distinction is shown 

 in the common term foot-pounds, the product 

 of feet and pounds (of force). If both are in 

 the same direction this product represents 

 energy, while if perpendicular to each other 

 it represents torque, which is decidedly not 

 energy. The writer long ago suggested to use 

 the term pound-feet, when it refers to torque, 

 in order to call attention to the difference. 



In the MXT system of dimension of phys- 

 ical quantities, force multiplied by length 



gives energy; hence torque has the dimension 

 of energy, when as a fact they are two 

 entirely different physical quantities. The 

 reason for this inconsistency is that in this 

 system an angle has no dimension, yet we 

 know that torque (which is not energy) when 

 multiplied by an angle gives energy, hence an 

 angle must have some dimensions. This is 

 one of the serious shortcomings of that sys- 

 tem. It is also the cause of the double use 

 of the term' acceleration. 



When force is defined as mass X accelera- 

 tion, it should be imderstood that the angle 

 is eliminated by being zero; acceleraton is 

 then always a change of speed, the sense in 

 which the engineer uses that term. A new 

 term should be used when the force is at 

 right angles to the direction of motion, in 

 which case it adds no energy to the system 

 and produces no change in speed, but merely 

 a change of direction. For any angle be- 

 tween and 90° no further distinction is re- 

 quired as the resultant then is always the 

 vector sum of the two components at 

 and 90°. 



Such a distinction between these two differ- 

 ent meanings of acceleration is very desirable 

 in order that the engineer and the physicist 

 may always understand each other without 

 confusion. 



Carl Hering 



Phtladelphia, 

 October 7, 1919 



AN ORNITHOMIMID DINOSAUR IN THE 

 POTOMAC OF MARYLAND 



A RECENT study of some of the dinosaur 

 specimens in the United States National 

 Museum from the Arimdel formation of 

 Maryland has led to a discovery of more than 

 ordinary interest. It is the recognition of an 

 undoubted Ornithomimid dinosaur, the first 

 representative of this group to be fovmd east 

 of the Eocky Mountain States, or geologically 

 below the Judith River formation of the 

 Upper Cretaceous. 



The materials on which this determination 

 rests consist of various bones of the hind foot, 

 pertaining to more than one individual. 

 Originally some of these elements were in- 



