122 Pte. J. A. Hughes on the 



In the case of the large cylinder, 20 cm. length, 15'5 cm. 

 diameter, the arrangement of the apparatus is shown in 

 fig. 3. The condensed water was allowed to collect in the 

 cylinder C itself, the whole being suspended from a balance B 

 placed on the top of the channel, the steam connexion being 

 made by rubber- tubing R. When the balance-beam is down, 

 the cylinder rests on a stand S in the channel ; the excess of 

 steam escapes through the exit E. 



The water condensed, which measures the heat lost, is 

 estimated by taking the weight of the cylinder at the 

 beginning and end of the interval of time, viz. 10 minutes. 

 Steam is passed through for about 5 minutes, and after the 

 steady temperature has been attained, the weight is taken 

 and the motor started. After 10 minutes the motor is 

 stopped and the weight again taken. The weight of the 

 water condensed will be the sum of three parts : (1) that 

 condensed by convection and radiation in still air in the 

 interval from the first weighing till the motor was started; 

 (2) the condensation during the wind-current; (3) the con- 

 densation in still air during the interval between the stopping 

 of the motor and the time when the final weight is taken. 

 The " water equivalent " of the excess of time above 

 10 minutes is lound, and this subtracted from the total 

 weight gives the weight of water condensed by the wind 

 alone. Still-air condensation is small, as can be seen from 

 Table II., and therefore no serious error is introduced by 

 this method. 



Corrections, 



Most of the heat is lost from the cooling cylinders by 

 convection, but some also by radiation. A correction can 

 be applied for the heat lost by radiation, and this subtracted 

 from the total loss of heat gives the heat lost by convection 

 alone. The radiation correction was calculated from the 

 equation 



where R is the heat lost by radiation; 



k the coefficient of emissivity in gram-calories per sec. 

 per sq. cm.; 



— the excess of temperature of the hot cooling 

 body above the temperature of the surroundings. 



k* was calculated from Bottomley's results to be 

 •63 X 10~ 12 gm.-cals. per sec. per sq. cm. for tar- 

 nished copper; 



* Phil. Trans, vol. clxxxiv. p. 591 (1893). 



