Mat 19, 1911] 



SCIENCE 



lib 



ity and environment. At least we may expect 

 a flood of new light on the subject from histo- 

 metrical investigations, and if the conclusion 

 is different from what the author of the 

 method anticipated it will not at all detract 

 from the credit due him for its development. 

 Geo. H. Johnson 

 Brooklyn, N. T. 



^ metals on metals, wet 



To THE Editor of Science: A year or two 

 ago I repeated to a class in elementary physics 

 the statement, familiar to generations of engi- 

 neers on the authority of General Morin, sup- 

 ported by the approval of Eankine, that the 

 coefficient of friction of metals on metals, wet, 

 is considerably greater than that of metals on 

 metals, dry. 



Thereupon a thoughtful youth in the class 

 asked me why, if this were the case, it was 

 customary to put sand on wet car-rails to 

 prevent the slipping of the driving-wheels. 

 Taken aback by this unexpected scepticism, 

 I begged for time to find the right answer to 

 the disturbing question and set to work ex- 

 perimentally on the problem. The student 

 reported after a time that trackmen had told 

 him the water they had trouble with was 

 usually slimy, which seemed to be a fairly 

 satisfactory explanation of the puzzle; but 

 meanwhile my experiments had shovTn some 

 interesting facts, which I will here set forth. 



Using a disk of brass, about 7.5 cm. in 

 diameter and about 0.6 cm. thick, on a flat 

 brass plate, I found: 



1. That, when there was no load on the 

 disk, a few small drops of water placed be- 

 tween it and the plate multiplied by a factor 

 which might be as great as 3 the friction 

 between the two. 



2. That, when the disk was heavily loaded, 

 the presence of the few small drops of water 

 between it and the plate made little, if any, 

 relative difference in the friction between the 

 two. 



3. That when plenty of water was used, so 

 that it covered the whole space beneath the 

 disk and extended somewhat beyond the edge, 

 the disk without load was drawn along the 



plate quite as easily, apparently, as when both 

 were dry. 



From these facts I came to the conclusion 

 that the increase of friction observed in case 

 1 was not due to an increase in the coefficient 

 of friction caused by the water, but merely to 

 the increase of pressure between the disk and 

 the plate, caused by the suction of the capil- 

 lary perimeters of the water-spots between 

 them. When there is much water, its peri- 

 meter is outside the edge of the disk, is wide, 

 or thick, and has little effect. 



Some little search in books dealing with the 

 subject of friction has failed to show there 

 any recognition of this possible explanation, 

 and refutation, of the Morin-Eankine state- 

 ment of the large value of the coefficient for 

 metals on metals, wet; but I should hardly 

 have written you about the matter if I had 

 not recently found this statement repeated in 

 the " Smithsonian Physical Tables " published 

 in 1903. I hope the new edition of these 

 tables will not quote Eankine on this partic- 

 ular without further evidence. 



Edwin H. Hall 



Cambridge, Mass., 

 April 29, 1911 



SCIENTIFIC BOOKS 



The Stability of Truth. By David Starr 



Jordan. New York, Henry Holt & Co. 



1911. Pp. ISO. 



" This little book," says the author, " repre- 

 sents the substance of a course of lectures 

 delivered on the John Calvin McNair founda- 

 tion in the University of North Carolina, 

 January, 1910." 



The chapter headings are: Eeality and Sci- 

 ence, Eeality and the Conduct of Life, Eeality 

 and Monoism, Eeality and Illusion, Eeality 

 and Education, Eeality and Tradition. 



Evidently something has happened in phi- 

 losophy, in science or in both when a scientist 

 of the first class, not to say the author of this 

 volume in particular, puts out a book with the 

 good old philosophical term, " Eeality " at the 

 head of every chapter. Doubtless in the minds 

 of most scientists there will be little ques- 

 tion about where " something has happened." 



