950 



SCIENCE 



[N. S. Vol. XXXI. No. 807 



iutellectual and moral philosophy. He has 

 been an officer of the college since 1883. 

 Dr. W. H. Sheldon has been transferred to 

 the professorship made vacant by the retire- 

 ment of Professor Campbell. Dr. Walter 

 Van Dyke Bingham, now instructor in educa- 

 tional psychology in Teachers College, Colum- 

 bia University, vcill join the Dartmouth fac- 

 ulty as an assistant professor of psychology. 



At the University of Missouri, Dr. O. D. 

 Kellogg has been advanced from the rank of 

 assistant professor to that of professor in 

 mathematics. 



Dr. a. S. Pearse has been promoted to the 

 position of assistant professor of zoology at 

 the University of Michigan. 



At Dartmouth College advances in grade 

 from instructorships to assistant professor- 

 ships have been voted to Charles E. Hawes, in 

 anthropology, Leon Burr Richardson, in chem- 

 istry, and Dr. George Sellers Graham, in 

 pathology. 



H. S. Jackson has been appointed professor 

 of botany and plant pathology in the Oregon 

 Agricultural College. Mr. Jackson has been, 

 since August, 1909, research assistant in plant 

 pathology at the Oregon Agricultural Experi- 

 ment Station. 



Nels C. Nelson and Thomas T. "Waterman 

 have been appointed instructors and assistant 

 curators in anthropology at the University of 

 California. 



Jacob Parsons Schaeffee, instructor in 

 medical anatomy in the Ithaca division of the 

 Medical College, has been promoted to an as- 

 sistant professorship of medical anatomy. 



Mr. T. Townsend Smith, at present the 

 holder of the Tyndall fellowship in physics 

 in Harvard University, has been elected in- 

 structor in physics in the University of 

 Kansas. 



DISCUSSION AND CORRESPONDENCE 



■' the definition of force 

 The discussion now going on in Science 

 concerning the language to be used in ex- 

 plaining to students what force " is," must 



be of great interest to students. They will 

 observe that there is good reason for the ob- 

 scurity of their own vision. In the physics 

 department, the student might finally learn 

 to distinguish between the pound and the 

 weight of a pound. In the engineering de- 

 partment he learns that a pound is a pound, 

 and that the weight of a pound is also a 

 pound. In the physics class he will be as- 

 sured that the weight of a pound is differ- 

 ent at different places. He will learn that 

 the weight of the earth is equal to the weight 

 of any other body which it attracts. The 

 weight of the earth is equal to the weight of 

 a pound, of a gram, of a ton or of the moon. 

 In the engineering department he will be 

 taught that the weight of the earth is equal 

 to the weight of 1.35 X lO""" pounds. There 

 was a time when the use of the phrase " con- 

 servation of forces " was excusable. We do 

 not discredit Helmholtz for saying in 1854 

 that " nature as a whole possesses a store of 

 force which can not in any way be either in- 

 creased or diminished," or that " all force will 

 finally pass into the form of heat." The 

 words had not yet been given definite mean- 

 ings, which would enable one to say what he 

 had in mind. 



The electrical engineers of our time have 

 no difficulty in using modern notation. The 

 mechanical engineers continue to use the 

 good old definitions of Weisbach and Eankin. 

 " Thus the British unit of force is the stand- 

 ard pound avoirdupois." 



The notation which makes a proper dis- 

 tinction between the pound and the weight of 

 a pound, or between mass and weight, or 

 force, does not require us to say that force 

 " is " a rate of change of momentum. Some 

 of us prefer not to say this. In a lecture be- 

 fore the British Association at Glasgow in 

 18Y6, Tait made a rather strenuous attempt 

 to enlighten Tyndall on the nature of force. 

 In this lecture we are informed that " force 

 is the rate of change of momentum." Again, 

 it is stated that " unit force is thus that 

 force, which, whatever be its source, produces 

 unit momentum in unit time." In the dis- 

 cussion which followed this lecture a vn-iter 



