254 Dr. Buchanan and Mr. Malcolm : 



electromagnet. The current through one electromagnet was 

 controlled by a metronome approximately beating seconds. 

 The metronome was compared from time to time with a 

 chronometer. The current through the other electromagnet 

 was " made " by the horizontal movement o£ the trigger 

 which released the drum; and was " broken " by the falling 

 load touching a trigger when almost at the bottom o£ its 

 descent. 



The observed times o£ fall never differed by more than 

 one-tenth of a second ; usually the differences were of the 

 order of two or three hundredths of a second. 



In order to measure h, a common pin to serve as a mark 

 was pushed at right angles through the cord near to the 

 load, and the highest point to which it rose was read off 

 against a vertical scale. A telescope was used to avoid 

 parallax. The observations were repeated until the mean 

 height was certainly known to a millimetre. 



Three liquids — water, Price's u cycle-lamp oil," and castor- 

 oil — were chosen so as to give a wide range of viscosity. 



In making a set of observations, the values of H, h, and t 

 were found for a series of loads, first with the drum empty. 

 The loads extended from 50 grms. to a maximum of 300 grms. 

 A larger load than this maximum was not considered safe to 

 use, on account of the severe jerk experienced by the cord 

 when the bottom of the descent w*as reached. 



A measured quantity of the liquid under examination 

 was next run into the drum, and H, h, and t again observed 

 for the same series of loads as before. And so on, until the 

 drum was completely filled with the liquid. Each time the 

 drum was empty, check observations were taken. 



Sometimes the order of the observations was altered, by 

 beginning with a full drum, and gradually emptying it in 

 order to get a series of readings. 



The bearings of the drum were kept w r ell lubricated, and 

 were not tampered with during the taking of each set of 

 readings. 



The value of / evidently depended on the extent of the 

 tightening up of the end cones. But w^ith the drum empty 

 /' was found to be independent, within the errors of obser- 

 vation, of the load used. Moreover, it was found that/ was 

 not greatly different whether the drum was empty or was 

 completely full. 



As is well known, in quantitative observations connected 

 with viscosity, the difficulties arising from varying tempe- 

 rature are by far the most serious. Unless very elaborate 

 precautions be taken, they render nugatory any attempts at 



