AT THE BOUNDARY OF A LIQUID IN MOTION. 
575 
greater (by about one-fourth) than that given by Poiseuille’s experiments. This seems 
to suggest that some slight modification in the application of the formulse may be 
necessar}^, which will reduce tlie value deduced for the viscosity of the liquid,, and 
increase that for its adhesion to the vessel to the value requisite for the condition of 
no slip. 
By a preliminary series of experiments Piotrowski claims to have shown that the 
friction on a body oscillating in contact with a liquid depends on the nature of the 
surface. He suspended a glass flask bifilarly, filled it with water, and observed the 
time of swing and the logarithmic decrement. He then silvered-the inner surface and 
repeated his observations. The results are as follows ;—- 
Time of swing. 
Logarithmic decrement. 
Unsilvered. 
23 9333 
0-0622182 
23-9333 
0-0628467 
23-9333 
0-062.5325 
Silvered. 
24-0088 
0-0600305 
24-0076 
0-0599622 
24-0082 
0-0599964 
As the result of these observations, Piotrowski calculates that the ratio of the 
friction on glass to the friction on silver is as 1 ; ’95645. 
Independently of the fact that no account is given of any precautions to keep the 
temperature constant, or even to measure it, it is evident that the above determina¬ 
tion is liable to errors due to changes in the suspension, which are very apt to occur, 
and that the agreement between the pairs of readings is nut very close. 
However, in order to test whether such an effect were appreciable, I undertook a 
series of experiments with an apparatus similar to that used by Piotrowski. 
A glass bulb was blown as nearly as possible spherical, and the neck drawn off 
sideways Into a fine tube. It wa.s filled with water by means of an air pump, and 
always kept completely full ; when left, a piece of india-rubber tubing filled with 
water was attached, so that if the temperature of the room sank, water, and not air 
was drawn In. During working the temperature was always slowly rising, and before 
