596 



SCIENCE. 



[N. S. Vol. III. No. 68. 



and also partakes of the motion of its mole- 

 cules, atoms and particles ; a motion of the 

 molar body, as a differentiated motion of that 

 molar body, is a deflection of the motions in all 

 the other bodies of the hierarchy, hut if these 

 other motions be not deflected, as a motion of 

 the molar body differentiated from the other 

 molar bodies, it is at rest. In this case, there- 

 fore, rest is the absence of motion in a molar 

 body which differentiates it from other molar 

 bodies in respect to motion. Best, then, in 

 molar motion is stellar motion and molecular 

 motion. Rest is the motion of a body in its 

 superior and inferior incorporation, undifferen- 

 tiated from the motions of the bodies of its own 

 rank in the hierarchy of incorporations. 



Third. — If by 'motion as speed' is meant 

 ' velocity,' and if by its ' persistence ' is meant 

 invariability of velocity, what possesses this in- 

 variability? bodies, molecules, particles, atoms ? 

 and in reference to what is the velocity constant ? 



So far as can be determined from research, 

 speed is constant in the ultimate particle, but 

 the speed of the atom, if it is a compound 

 body, is not constant. The speed of a mole- 

 cule is not constant, and in general the speed 

 of a body is not constant. The speed of a 

 particle is constant in reference to itself at dif- 

 ferent times. 



Fourth. — As a molecule is considered as a 

 ' body' when reference is had to the atoms which 

 compose it, can it have an ' invariable velocity ' 

 as a molecule and variable velocity as a ' body ? ? 



The molecule has an invariable speed (or sum 

 of speeds) in its ultimate particles, but as a 

 molecule, or one composed of many, this one 

 may have a variable speed. It will be recog- 

 nized that I use the term speed rather than 

 velocity, for the term velocity as it is used in 

 physics does not mean speed. First, velocity 

 is positive and negative ; second, velocity is 

 speed and trajectory. I have been trying to 

 dispel the illusion which inheres in the double 

 use of velocity when we fail to distinguish be- 

 tween speed and path. The abstraction must be 

 perfect when we reason abstractly ; when we 

 reason concretely then abstractions must be 

 combined. Two molar bodies in motion as such 

 may collide with each other, both may be de- 

 flected, both may come to rest, or one may be 



deflected and the other come to rest. All of 

 these cases are concretely explained as velocity 

 in physics. Velocity is a concrete term, not an 

 abstract term. The velocity of a body as speed 

 and path is constant. When a particle or body 

 moves in a straight line its speed and its velocity 

 are the same, but when a particle or body 

 moves in a deflected line the velocity is 

 measured by its speed and the force by which 

 it is deflected. The distinction between speed 

 and deflection is well marked by some English 

 physicists who speak of spirt and shunt. When 

 we consider the rate of motion we consider 

 speed, not velocity, and we may consider speed 

 in every incorporation in which an ultimate 

 particle is found, and its total speed is the sum 

 of all its speeds. 



Let me ask your correspondent to once more 

 consider my definitions and demonstrations, 

 freeing himself from the illusion that velocity is 

 the same as speed, making a perfect abstraction 

 of those things whi6h we are considering ab- 

 stractly and a perfect comprehension of those 

 things which we are considering concretely. 



Finally your correspondent says: 



I cannot refrain from expressing a hope, how- 

 ever, that in addition to these answers. Major 

 Powell will kindly furnish an explanation of 

 what he means when he says that the trans- 

 mission of light at the rate of 299,878,000 

 metres per second, furnishes an example of 

 ' particle motion at a velocity so great that any 

 observed molecular motion sinks into insigni- 

 ficance.' 



This assures me that your correspondent was 

 attentive to my language, and I wonder 

 whether he detected some other misprints in 

 my article. In the same paragraph I say: 

 ' ' The molecular motion of a cannon ball at its 

 mouth is from 518 to 671 metres per second." 

 Of course I should have said the molar motion 

 of a cannon ball. If in these cases he will sub- 

 stitute molar for molecular he will understand 

 what I intended. On reading the published 

 article I discovered this and one or two other 

 errors, which are probably due to my habit of 

 dictation, but thought them hardly worth 

 noticing, as I belived that every intelligent 

 reader would discover the errors and correct 

 them himself. J. W. Powell. 



