44 Messrs. Playfair and Joule on 



The freezing-points of the thermometers were carefully de- 

 termined -within a few hours of the experiments. Each vessel 

 was furnished with a stirrer, consisting of a disc of tinned 

 iron 4 inches in diameter attached to the end of a slender rod 

 of iron, by means of which the water was thoroughly stirred 

 before each determination of temperature. 



In order to measure the motion of the water in the trough 

 connecting the tops of the vessels, a hollow glass ball of 

 about three-eighths of an inch diameter was placed in it. The 

 weight of this glass ball was carefully adjusted so as only 

 just to float : a matter of great importance, as the slightest 

 buoyancy is accompanied by a certain degree of capillary at- 

 traction, and makes the ball liable to adhere to the sides of 

 the trough. 



The toater employed in the experiments was distilled by 

 ourselves in clean vessels of tinned iron ; and the additional 

 precaution was taken to prevent, as far as possible, the solu- 

 tion of air. 



Our method of experimenting was as follows : — Having 

 filled the vessel with distilled water at a temperature of about 

 37°, we increased the temperature of one of the vessels to 

 41°*5 by the addition of a small quantity of hot distilled water. 

 We then placed two delicate tliermometers upon a proper 

 stand, so that their bulbs dipped in the water to the depth of 

 6 inches. Having then closed the stop-cock and adjusted the 

 slide, we stirred the water in each vessel thoroughly, and 

 noted the temperatures indicated by the thermometers. The 

 stop-cock was then opened and the slide carefully removed 

 from the trough. After Malting three minutes the glass ball 

 was put into the trough, and its motion watched for two or 

 three minutes with the help of a graduated rule placed at the 

 top of the trough. In conclusion, the stop-cock was again 

 turned, the slide readjusted, the water stirred, and the tem- 

 peratures again noted. The mean of the temperatures thus 

 observed before and after each trial of the velocity of the cur- 

 rent was taken as the temperature of the observation. 



The following table contains the results of a series of ob- 

 servations taken in the above manner. The temperature of 

 the laboratory being about 38°, the water in the warmer 

 vessel cooled down more rapidly than the water in the other 

 vessel increased in temperature ; and therefore after two or 

 three hours had elapsed, the water in the cooler vessel was 

 found to have acquired greater buoyancy than that in the 

 warmer vessel, although at the commencement of the experi- 

 ments the current indicated a greater degree of buoyancy in 

 the warmer water. 



