Apeil 21, 1911] 



SCIENCE 



619 



ence" than has yet been explained or sug- 

 gested by " western scientific research." 



It never can be too strongly emphasized 

 that, granting suitable climatic and physical 

 soil condition, the fundamental of crop pro- 

 duction is crop feeding, and that crop hunger 

 (and thirst) has been the prime condition 

 determining reduced yield oftener than any 

 other. These have been the tenets of practical 

 men through all the past and are likely to 

 remain so to the end. Disease, parasitism, 

 phagocytism, degeneration of seed, toxic sub- 

 stances or vsfhat not may at times reduce 

 yields and the advance of knovfledge which 

 shall make it possible to diagnose these cases 

 and apply the proper remedy, for each will 

 augment the efficiency of plant food but make 

 the demands for it greater nearly in propor- 

 tion to increase of yield, and will accelerate 

 soil exhaustion where nature or man makes 

 inadequate return. 



It is difficult to see on what basis of knowl- 

 edge one may contend that the increase in the 

 productivity of soils of western Europe, re- 

 ferred to as occurring in recent years, has 

 been due to improvements along any of these 

 minor lines rather than to better physical soil 

 condition and to the increasing application of 

 the three most essential plant-food elements 

 which have certainly been coincident with 

 these increases of yield; and even more diffi- 

 cult does the case become when referred to 

 the long and high maintenance of soil pro- 

 ductivity in China where plant feeding has 

 been the heaviest burden of the people. 



F. H. King 



Madison, Wis. 



\, 



A KINETIC THEORY OP GRAVITATION 



To THE Editor of Science: 



Imagine a pound-weight of iron raised from the 

 surface of the earth to a point near the moon, the 

 point so chosen that the opposing attraction of 

 the earth and the moon shall exactly balance each 

 other. In the surface of the earth the pound- 

 weight had some so-called " potential energy of 

 position " because it was capable of falling into 

 a pit: Dut in its new position near the moon this 

 potential energy has disappeared entirely; the 

 pound-weight, left free to move, remains station- 



ary. We can not believe that the whole or any 

 part of it [the energy] has been annihilated: it 

 must, in some form, be resident somewhere. 1 

 believe it was absorbed by, and is now resident in, 

 the ether through which the weight was raised. 

 Conversely if this be true, a falling body must 

 acquire its energy from the ether through which 

 it falls. ' ' ' 



Since the ether is as yet a hypothetical sub- 

 stance, postulated to explain certain physical 

 phenomena, it may be allowable in discussing 

 some phenomena to postulate its non-exist- 

 ence. We do not know that if the ether were 

 non-existent and only an imaginary substance, 

 that gravitation would also be non-existent. 

 Assuming the non-existence of ether, but 

 gravity acting as usual, would not the pound- 

 weight act just as is described by Dr. Brush? 



Consider a simple case. A ball weighing 

 one pound is lifted five feet from the floor, 

 and placed on a shelf. It has a potential 

 energy of five foot-pounds, with reference to 

 the floor, but it can not exert this energy, or 

 convert it into kinetic energy, for it is pre- 

 vented by the shelf. So if the ball is raised 

 to the point near the moon, it has 20,000,000 

 foot-pounds of potential energy, referred to 

 the earth, and this energy could be made kin- 

 etic, if the body were " free to move," which 

 it is not; it is restrained by the attraction of 

 the moon, just as it was restrained by the 

 shelf. Suppose the ball is of iron, and that 

 on being raised five feet it comes within the 

 field of attraction of an electromagnet which 

 attracts it and prevents it falling to the fioor. 

 It has five foot-pounds of potential energy, 

 just as it had on the shelf, but it is for the 

 time being unavailable. Let the current 

 which actuates the electromagnet be inter- 

 rupted for a fraction of a second, the ball 

 begins to fall and the potential energy be- 

 comes kinetic. In neither of these cases has 

 the potential energy " disappeared entirely," 

 it has only been rendered unavailable by the 

 attraction of the moon or the electromagnet, 

 or by the shelf. It has not been annihilated 

 nor is it " resident in the ether." 



' Extracts from an article, ' ' Kinetic Theory of 

 Gravitation," by Charles B. Brush, Science, 

 March 10, 1911. 



