Intelligence and Miscellaneous Articles. 369 



increase of momentum in a given time : in other words, forces will be 



dv 

 proportional to m-rr . From this it is but a step (though it may be 



a false step) to say that the force is the rate of increase of momentum, 

 and nothing else, and respectfully to demand that we shall re- 

 construct the whole of Mechanics in accordance with that definition. 

 Mr. Close takes an illustration from the rate of mortality ; and I 

 will follow him. Disease causes death, and the strength of 

 different diseases, or of the same disease at different times and 

 places, may fairly be measured by the rate of mortality (say the 

 number of deaths per 1000 per diem) which they induce. Mr. 

 Close, if consistent with himself, would therefore define disease as 

 being the rate of mortality — that and nothing else ; and would re- 

 quire all our medical literature to be rewritten, because its language 

 does not square with the new definition. 



I think it is time that the advocates of the new view should 

 themselves bring out a book, showing that the ordinary laws and 

 propositions of Mechanics can be worked out on their principles in 

 a way intelligible to the ordinary student. Prof. Tait, in a recent 

 communication to the Eoyal Society of Edinburgh, made a sort of 

 commencement of such a work : I should be glad to hear of any 

 ordinary student who has derived much comfort and clearness of 

 thought from a study of the result. For an attempt of the kind 

 I refer to, but on the opposite lines, I may perhaps be allowed to 

 point to a recent work of my own, entitled ' The Student's Me- 

 chanics : an Introduction to the Study of Force and Motion.' 



May I be allowed to conclude with one word in reply to Mr. von 

 Tunzelmann's letter on the Conservation of Energy : it is simply 

 to point out that a periodic function is not the same thing as a 

 constant. Walteb E. Bkowi^e. 



ON A MODIFICATION IN THE PYCNOMETEE. 

 BY EILHAKD WIEDEMANN. 



Since determinations of the specific gravity of solids for the 

 purpose of discovering relations between it and their chemical 

 constitution are now more and more frequently made, the improve- 

 ment of the pycnometer communicated in what follows may not 

 be without interest. By means of it results as exact can be ob- 

 tained with powders as when larger bodies are employed. 



As is well known, in the former method the pycnometer is first 

 weighed empty, then filled with water, then with the substance in 

 question, and finally with water and the substance. If the weights 

 in these cases are P, n, p and 7r, the specific gravity of the sub- 

 stance is 



p-V 



s -[n+<j>-P)]-V 



But air is always contained between the particles of the solid, 



