November 24, 1916] 



SCIENCE 



747 



saved together with part of the apparatus. 

 Chancellor Elliott has announced that a new 

 chemistry building will be erected as soon as 

 possible. 



Final plans have been drawn for a head 

 house for the school of applied science of the 

 Carnegie Institute of Technology, which is to 

 cost $300,000. A portion of the building will 

 be four stories high and the remainder ten. 

 Construction work will start as soon as steel 

 deliveries can be made. The structure will 

 house the executive offices and library of the 

 engineering school, and the departments of 

 modern languages, machine design and com- 

 mercial engineering. 



Dr. H. E. Eggers has been appointed pro- 

 fessor of pathology and bacteriology, Dr. Amos 

 W. Peters, assistant professor of biochemistry, 

 and Dr. John T. Myers, instructor in bac- 

 teriology, in the college of medicine of the 

 University of Nebraska, Omaha. 



Professor J. Versluys, who has held the 

 chair of zoology and comparative anatomy at 

 Giessen since 1907, has been appointed to the 

 corresponding chair in the new Flemish Uni- 

 versity at Genth. 



The Journal of the American Medical As- 

 sociation indicates that negotiations are pend- 

 ing that may bring Professor R. Barany, of 

 Vienna, to the University of Stockholm as 

 professor of otology and rhinolaryngology. He 

 recently delivered at Stockholm the customary 

 address describing his research when presented 

 with the Nobel prize. It will be remembered 

 that he was a war prisoner in Russia when 

 notified that the prize in medicine had been 

 conferred on him. 



DISCUSSION AND CORRESPONDENCE 



CAN A BODY EXERT A FORCE UPON ITSELF? 



In connection with our annual attempt to 

 give our students a few clear ideas about ele- 

 mentary dynamics, the question of the mean- 

 ing to be assigned to the word force peren- 

 nially arises. May I call attention to a well- 

 known phenemenon which seems well suited 

 to serve as a shibboleth in distinguishing be- 

 tween clear and hazy conceptions of force? 



Let a liquid be uniformly rotated in an open 

 vessel. What are the forces acting on each 

 surface particle? Why is the free surface 

 parabolic ? 



In answering these questions one recent 

 author finds it necessary unwittingly to deny 

 all three of the laws of motion. He states that 

 "When a liquid is at rest or in equilibrium 

 the resultant of all the forces acting on a par- 

 ticle in its free surface is perpendicular to the 

 surface at that point " [whereas according to 

 the first law the resultant force must be zero]. 

 In the case of a rotating liquid, we are told, 

 " the resultant force acting on the surface 

 particles is due not only to gravity, but to 

 centrifugal force. ... It will be noted that 

 the resultant force [shown drawn perpendic- 

 ular to the free surface] is greater at points 

 higher up on the surface, so that a surface 

 particle near the top presses against the sur- 

 rounding liquid with far more force than it 

 would if at the bottom of the curve." But 

 according to the second law the resultant force 

 must be in the direction of the resultant ac- 

 celeration, which in this case is obviously 

 centripetal; and according to the third law, 

 if the particle presses against the surrounding 

 liquid, the liquid must press back upon it with 

 an equal and opposite force not mentioned by 

 the author. 



Such an explanation is evidently completely 

 misleading. Yet another recent text-book does 

 equal violence to the laws of motion in ex- 

 plaining the same phenomenon. " The result- 

 ant force," we are told, " is made up of two 

 components; one of these is the weight of the 

 particle, mg, the other is the reaction which 

 the particle offers against acceleration toward 

 the center by the centripetal force mr<o 2 ." 



Of course the trouble is that among mathe- 

 matical physicists it has been customary to 

 reduce such problems to purely statical ones 

 by introducing centrifugal forces in accord- 

 ance with D'Alembert's principle; but authors 

 of elementary texts sometimes forget that the 

 forces so introduced are purely imaginary. 



Does not the third law mean this : A body 

 A can not exert a force upon itself as a 

 whole; any force acting on it must be due to, 

 that is, associated with, the existence of, some 



