446 



Prof. J. Perry on Liquid Friction. 



As an error of *0004 inch in the diameter measurement leads 

 to an error of 4 per cent, in the modulus of rigidity, and as 

 the modulus of rigidity usually taken for steel is 8*19 X 10 11 , 

 we believe that our constant 1531, as directly measured, is 

 sufficiently correct for practical purposes. 



In using (6), then, we take a to be 1531. 



The virtual addition X ought, by Maxwell's formula, to be 

 0*52 centim. in our case. But the bottom of the trough H 

 was only 0*5 centim. from the edge B of the suspended 

 cylinder in most of our experiments, and we do not know how 

 to calculate for this. Our experiments have shown that when 

 this distance is 0'5 centim. the twisting moment at a given 

 speed is practically the same as when the distance is much 

 greater ; but we did not know this from any theory, and, 

 besides, it is always rather dangerous to depend upon a 

 theoretical calculation of X such as Maxwell was compelled to 

 use. It is possible, also, that a correction of the same kind 

 ought to be introduced for capillary and other actions at the 

 surface of the liquid. The action of the atmosphere was in 

 any case negligible, because when there was no liquid present 

 in the trough, so that there was an action of the air several 

 times greater than ever occurred during the experiments, the 

 deflexion was quite imperceptible at much higher speeds than 

 those used in the experiments. 



The temperature being kept as nearly as possible constant, 

 but probably varying between 18°* 9 C. and 20 o, l C. (stated 

 as 19°*5 C), the following experiments were made with 

 sperm-oil, beginning with a small quantity in the trough and 

 ending with a large quantity. The bottom of the trough was 

 in every case 0'5 centim. below the edge of the suspended 

 cylinder. 



Table I.— June 9th, 1892. 







Deflexion D when 







1 



1 







! l=0'o cm. 



l=2'5 cm. /=5 cm. l=7'ocm. 



1=99 cm. 



50 



1 24 



67 112 158 





49 







196 



48 









40 



18 



132 



163 



39 





53 91 





23 



10 







17-5 



8 



57 



73-5 



17 





24 39 





12 



! 5 



40 





11-5 



i 



16 27 





11 



: - 



i 



49 



