Apeil 23, 1915] 



SCIENCE 



611 



acceleration even though at rest relatively to 

 the earth. 



Professor Huntington objects to the defini- 

 tion " force of gravity = attraction of the 

 earth " on account of " complications con- 

 nected with the spheroidal shape of the earth, 

 the influence of the earth's rotation, etc." 

 From -what has been said above it is quite 

 evident, however, that if the complications^ 

 connected with the earth's rotation are evaded 

 by his definition it is only by a sacrifice of 

 clearness in the force concept. 



Clear thinking about the concept of force 

 would seem to be promoted by the more usual 

 method of distinguishing between true and 

 apparent force of gravity; the former being 

 the actual earth-pull on a body, the latter the 

 pull or push exerted by a body upon its sup- 

 port. Each of these is a true force (a pull or 

 push exerted by a specified body) ; to assume 

 them equal is a first approximation to the 

 truth. The reason they are not exactly equal 

 can be explained rigorously when the student 

 is in a position to understand the dynamics of 

 circular motion; before that stage is reached 

 it is sufficient to stop with the explanation 

 which neglects the effect of the earth's 

 rotation. 



L. M. HosKiNS 



THE NATURE OP THE ULTIMATE MAGNETIC 

 PARTICLE 



For many years scientists have agreed in 

 ascribing the magnetic properties of bodies to 

 the action of exceedingly small elementary 

 magnets, but the nature of these ultimate 

 magnetic particles has been an open question. 

 The influence of temperature, chemical com- 

 position and other factors has received the 

 simplest explanation on the theory that mole- 

 cules, or possibly groups of molecules, are the 

 ultimate magnetic particles. On the other 

 hand the electron theory of magnetism, devel- 

 oped by Langevin, Curie, Weiss and others, 

 seems logically sound and is the only theory 



9 The spheroidal shape of the earth introduces 

 no complieation whatever as regards the definition 

 "force of gravity = attraction of the earth." It 

 is not necessary to be able to compute the attrac- 

 tion in order to understand the definition. 



which has successfully accounted for dia- 

 magnetism. 



The recently developed method of determin- 

 ing the positions of atoms within a crystal by 

 X-ray photography and the ferromagnetic 

 properties of magnetite, hematite and pyrrho- 

 tite crystals suggested a direct experimental 

 method of eliminating one or the other of 

 these two theories. By comparing photo- 

 graphs taken through these crystals while 

 magnetized and unmagnetized it can be deter- 

 mined with certainty whether or "not the atoms 

 have moved from their positions of equilibrium 

 during the process of magnetization. We have 

 obtained experimental results with magnetite 

 and hematite which indicate that the atoms do 

 not leave their positions of equilibrium during 

 magnetization. These results are consistent 

 with the electron theory of magnetism and 

 prove conclusively that magnetism is not a 

 molecular phenomenon, -g-^ rj, Compton, 



E. A. Trousdale 



Eeed College 



the new glacier park 



To the Editor of Science : Eeferring to the 

 pleasing intelligence communicated by Dr. 

 John M. Clarke, in Science for March 12, 

 relative to the new glacier park near Syracuse, 

 a further note on the history of its investiga- 

 tion may well be added. It would seem that 

 the earliest clear interpretation of the glacial 

 stream channels about Jamesville came from 

 a master of physiographic study who has 

 strewn many seed thoughts by the way during 

 the past forty years — Mr. G. K. Gilbert. The 

 record is in " Old Tracks of Erian Drainage in 

 Western New York," an abstract published in 

 the Bulletin of the Geological Society of Amer- 

 ica, Vol. 8, 1897, pp. 285-286. Dr. Quereau's 

 paper, which appeared in the Bulletin of the 

 following year, cites Mr. Gilbert's interpreta- 

 tion by way of acknowledgment, and both 

 papers have been followed by the full exposi- 

 tions of Professor Fairchild in the publica- 

 tions of the Geological Survey of New York. 



Albert Perry Brigham 



Colgate TJniveesitt 



