DECE3IBEK 23, 1898.] 



SCIENCE. 



913 



formulated by Dr. Mendeuhall in bis vice-presi- 

 dential address delivered before tbe Section of 

 Physics at the Montreal meeting of tbe Ameri- 

 can Association for the Advancement of Science 

 in 1S82, tbe very year when Dr. Gage's book 

 appeared. It is probably safe to say that a ma- 

 jority of American teachers of physics are now 

 agreed in the view that the elementary pupil 

 should not enter the laboratory as an original 

 investigator, because he is utterly unfit to be 

 such until after much training has been re- 

 ceived. What he needs is a well arranged, 

 clear and accurate presentation of principles, 

 with such experimental demonstrations by the 

 teacher as may be needed to ensure the acqui- 

 sition of the truth. After a good introduction 

 has thus been received he should have the op- 

 portunity to make a selected series of tests of 

 these principles in the laboratory, and advan- 

 tage should be taken of such practice to train 

 him into habits of close observation, system, 

 neatness and good order. He needs, there- 

 fore : first, a reliable class text-book, the study 

 of which should accompany the teacher's lec- 

 tures ; and, second, 'a separate laboratory man- 

 ual, or its equivalent in the form of special 

 written or printed instruction cards adapted to 

 the particular apparatus that is put into his 

 hands after the requisite class-room preparation 

 has been secured. Should he in time manifest 

 enough originality to become an investigator, 

 his work will probably be amid surroundings 

 better adapted for research than the school 

 laboratory. 



It is to meet the first of these needs, a reliable 

 class text-book, that the present volume has 

 been written. The author exhibits good judg- 

 ment, not only in the selection of what he in- 

 cludes, but in omitting certain special topics, such 

 as the polarization of light, of which a smatter- 

 ing is often unwisely given. Such subjects as ab- 

 sorption, osmose and crystallization belong now 

 to the newly differentiated science of physical 

 chemistry. Nevertheless, there remain some 

 embarrassments due to the necessity to avoid 

 mathematics, and the attempt to be a trifle too 

 conservative by recognizing certain details of 

 nomenclature that are deservedly passing away. 

 For example, the poundal is recognized as a 

 unit of force. The only'escuse for the inven- 



tion of such a unit has been to bring the clumsy 

 ' British ' system into accordance with the far 

 simpler system, employed by all physicists 

 irrespective of nationally, that for tbe sake of 

 contrast is often called the absolute system. 

 Engineers in England and America express 

 weights in pounds, but they have no use for 

 poundals in either theory or practice. The 

 physicist thinks in the simpler system, but 

 often has to translate his final results into the 

 British system. There is hence no use in be- 

 fogging the minds of young pupils with more 

 than one unit of force, the dyne. Nor is any 

 advantage to be derived from specifying two 

 methods of measuring force, calling one the 

 statical or gravitational system, and the other 

 the dynamical or absolute system. The latter 

 is the only one needed ; the former is always 

 expressible in terms of the absolute system. 

 The suggestion of duality is confusing and often 

 misleading. The confusion culminates where 

 the pupil is confronted with poundal, foot- 

 poundal, pound, foot-pound, dyne, erg, gram, 

 gram-centimeter and kilogram, all grouped in a 

 single diagram (p. 71) for the purpose of con- 

 trasting the units of force and of work. If all 

 calculations are made in terms of centimeters 

 and grams there is but little trouble in trans- 

 lating final results if necessary. 



The author in similar manner speaks of den- 

 sity, specific density and specific gravity. Of 

 these three terms the first is tbe only one that 

 is really needful, though it may sometimes be 

 convenient to employ the term specific gravity, 

 or relative density, to denote that a secondary 

 rather than primary standard is employed. 

 The distinction between specific density and 

 specific gravity is, of course, definable, but in the 

 interest of simplicity it is not desirable. 



In illustrating any subject with numerical 

 examples it is best to employ such as are ap- 

 proximately within the range of practice. After 

 a good description of Joule's apparatus for the 

 determination of the mechanical equivalent of 

 heat, which was operated in Joule's laboratory, 

 an example is given in which the weights are 

 supposed to be raised to a height exceeding that 

 of an ordinary church steeple. There is, of 

 course, no theoretical objection to this. But it 

 is practically of some importance that the final 



