Febbuart 5, 1915] 



SCIENCE 



209 



balance, in the form, for example, of a grip- 

 testing macliine, lie could measure the strength 

 of the muscles of his hand, or the attraction 

 between two bodies, just as well under those 

 circumstances as if he were on the surface of 

 the earth. 



Secondly, if we are dealing with only a por- 

 tion of the physical universe (as is always the 

 case in practical problems), we must either 

 introduce " forces " to account for the action 

 of the residual portion, or else resort to very 

 artificial conventions in regard to " imaginary 

 masses." (It should be noted that the " mass- 

 acceleration " of a body can not conveniently 

 be taken as a substitute for an external force 

 acting upon that body; for the mass-accelera- 

 tion of the body, like its momentum or kinetic 

 energy, is a quantity inherent in the body.) 



Thirdly, the approach to statics, in which 

 the concept of mass plays no part whatever, is 

 peculiarly awkward by this route; whereas if 

 force is taken as the fundamental concept, the 

 problems of statics may readily he taken up 

 either before or after the detailed study of 

 dynamics. 



While therefore it is logically possible to 

 choose either mass alone or force alone as the 

 fundamental concept, the latter choice seems 

 practically preferable. 



Either the force method or the mass method, 

 I say, is logically defensible; but the method 

 which starts with the equation F = ma is 

 neither the force method nor the mass method. 

 My chief objection to this hybrid equation 

 F = ma is precisely this uncertain wavering 

 between the force concept and the mass con- 

 cept as the fundamental notion of the science. 

 This wavering is, I believe, the main source of 

 the very real difficulties which the student ex- 

 periences in regard to " units " — difficulties 

 which are not necessarily functions of the 

 laziness or immaturity of the student, but 

 which are felt more keenly by those of a 

 scientific and critical turn of mind than by 

 those of a merely practical bent. I quite agree 

 with Professor Hoskins that any student of 

 dynamics ought to have sufficient intelligence 

 to grasp the idea of a systematic system of 

 units, that is, a system in which certain units 



are taken as fundamental, and all others are 

 derived; but I do think that the student has 

 a right to expect that the quantities which 

 appear in the so-called fundamental equation 

 shall be the same as the quantities which are 

 taken as fundamental in the system of units. 

 This is not the case with the equation F = ma. 

 The trouble with this equation is not that it 

 contains mass, but that it contains both force 

 and mass, while not both of these quantities 

 are regarded as fundamental in the subsequent 

 treatment. 



The use of the equation F/F' = a/a' seems 

 to me, therefore, not merely a matter of prac- 

 tical convenience, but also a distinct advance 

 in scientific precision of thought. 



Edward V. Huntington 



Harvard University 



geologic history of lake lahontan ^ 

 In reference to the summary concerning the 

 probable history of Lake Lahontan by J. C. 

 Jones, contained in Science, December 4, 1914, 

 while I am. much interested in Professor 

 Jones's conclusions concerning the origin of 

 the tufa, I feel that his statements regarding 

 the interpretation of the age of Lake Lahontan 

 need some important qualifications, and that 

 his conclusions as to the probable accumula- 

 tion of salines in Lahontan waters are not at 

 all the necessary deductions from the evidence 

 that he has cited. 



Professor Jones's estimates on the age of 

 Lake Lahontan and the quantity of salines 

 that might have been deposited by the evapora- 

 tion of its waters fail to take into account 

 some very important considerations. The as- 

 sumption that because Pyramid Lake may be 

 and probably is a remnant of Lake Lahontan, 

 which has never been dried up completely, 

 therefore its salines are an index of the age 

 of the whole larger lake seems to me errone- 

 ous. A conception of a closer interpretatioa 

 may perhaps be obtained in the following way. 

 No one doubts that Lake Lahontan formerly 

 rose to a height of approximately 500 feet 

 above present Pyramid Lake and that its 



1 Published by permission of the Director of the 

 United States Geological Survey. 



