480 



SCIENCE 



[N. S. Vol. XLV. No. 1168 



the situation. These discoveries indicate that 

 the pitcher-leafed type may be an older form 

 than I had supposed, and that it may hare a 

 rather wide distribution. As the peculiar 

 form of the leaflets is readily observed, espe- 

 cially on the young trees, the fact that the 

 occurrence of pitchers in this species has 

 never been published except by myself, and 

 in relation to the trees at Cold Spring Harbor, 

 would seem to indicate that this form prob- 

 ably does not occur in any considerable abun- 

 dance over extensive areas. 



In order to work out their probable evolu- 

 tionary history, it is necessary to have more 

 complete information regarding the present 

 distribution of these pitcher-leafed ash trees. 

 The reader is requested to assist in securing 

 this information during the present spring 

 and svimmer, by carefully examining as many 

 young ash trees as may be accessible to him, 

 and reporting the result to the undersigned, 

 giving approximately the extent of area 

 covered by the observations, and the number 

 of normal ash trees observed, as well as the 

 number of pitcher-leafed trees — if any of the 

 latter should be discovered. A report is just 

 as desirable in case only normal trees are 

 found as if pitchered specimens are found. 

 All communications should be addressed to 



Geo. H. Shull 

 60 Jefferson Eoad, 

 Princeton, N. J. 



"KEEP YOUR EYE ON THE BALL" 



Evidently my last letter to Sciencei ^^lb not 

 very clear and convincing, for Mr. Patterson^ 

 in a recent nvimber insists on making the in- 

 ertia-reaction of an accelerated body act upon 

 the body itself and thus oppose the accelerating 

 force. To him the term " unbalanced force " 

 means " a force opposed only hy inertia-reac- 

 tion." 



In avoiding confusion as to the part played 

 by this force of reaction in any case, I have 

 found it useful to adopt a motto from the royal 



i"Can a Body Exert a Force upon Itself?" 

 Science, Vol. XLIV., p. 747, 1916. 



2 "When Is a Force Not a Force?" Andrew H. 

 Patterson, Science, Vol. XLV., p. 259, 1917. 



game of golf — " Keep your eye on the ball." 

 When a ball is swung on the end of a cord, the 

 centripetal force exerted by the stretched cord 

 on the tall is unbalanced and produces the 

 centripetal acceleration of the ball. There is 

 the whole story as far as the ball is concerned. 

 The inertia reaction of the ball acts upon the 

 cord and through the cord acts upon the hand. 

 The law of motion states that the rate of 

 change of momentum of any body is at each 

 instant proportional to the resultant of all the 

 forces acting upon that body. In applying the 

 law to a given body A, keep your eye on A and 

 consider only the forces acting upon A. 

 Among these forces, the inertia-reaction of A 

 is never to be included since it always acts 

 upon some other body or bodies. 



Mr. Patterson would have us believe that in- 

 ertia-reaction and friction are not full-fledged 

 forces in the single definite sense implied in 

 the laws of motion. He says neither can pro- 

 duce positive acceleration. Let us see if this 

 is true. 



Consider the experiment in which two 

 masses, connected together by a string and free 

 to slide along a horizontal rod, are rotated 

 about a vertical axis. If the distances from 

 the axis are in the right ratio, the masses will 

 rotate without sliding, the inertia-reaction of 

 each mass accelerating the other. 



Then take the case of a passenger leaning 

 forward as he stands on a starting train. The 

 forces acting on him are his weight, the up- 

 ward elastic reaction of the floor which bal- 

 ances his weight, and friction. He is being 

 accelerated, and friction is doing it. 



Even though some forces are always pulls, 

 others always pushes, and others neither, we 

 need not difPerentiate between them since each 

 tends to produce acceleration in a certain di- 

 rection. 



Let us agree, then, that a body can not exert 

 a force upon itself; that all forces are similar 

 in their effects; and that in applying the laws 

 of motion to any body, care should be taken to 

 consider only the forces acting upon it. 



Gordon S. Fuloher 

 Wisconsin University, 

 March 24, 1917 



