and Conduction, and on the Propagation of Sound. 313 



accelerate the mean cooling. In order to diminish the dis- 

 turbance due to capillarity, the bore of the manometer-tube, 

 which stood in a large open cistern, was increased to about 

 18 millim.*, and suitable optical arrangements were intro- 

 duced to render small movements easily visible. By degrees 

 the raDge was diminished, with a prolongation of the. time of 

 half recovery to 18, 22, 24, and finally to about 26 seconds. 

 The minimum range attained was represented by 3 or 4 

 millim. of water, and at this stage there did not appear to be 

 much further prolongation of cooling in progress. There 

 seemed to be no appreciable difference whether the air was 

 artificially dried or not, but in no case was the moisture 

 sufficient to develop fog under the very small expansions 

 employed. The result of the experiments maybe taken to be 

 that when the influence of gravity was, as far as practicable, 

 eliminated, the time of half recovery of temperature was 

 about 26 seconds. 



It may perhaps be well to give an example of an actual 

 experiment. Thus in one trial on Nov. 1, the recorded times 

 of passage across the divisions of the scale were 3, 6, 11, 18, 

 26, 35, 47, 67, 114 seconds. The divisions themselves were 

 millimetres, but the actual movements of the meniscus were 

 less in the proportion of about 2J : 1. A plot of these 

 numbers shows that one division must be added to represent 

 the movement between s and 3 s , and about as much 

 for the movement to be expected between 114 s and oo . 

 The whole range is thus 10 divisions (corresponding to 

 4 millim. at the meniscus), and the mid point occurs at 

 26 s . On each occasion 3 or 4 sets of readings were taken 

 under given conditions with fairly accordant results. 



It now remains to compare with the time of heating 

 derived from theory. The calculation is complicated by 

 the consideration that wdien during the process any part 

 becomes heated, it expands and compresses all the other 

 parts, thereby developing heat in them. From the investi- 

 gation which follows f, we see that the time of half recovery t 

 is given by the formula 



' — -^r> < 6 ) 



in which a is the radius of the sphere, y the ratio of specific 

 heats (1*41), and v is the thermometric conductivity, found 

 by dividing the ordinary or calorimetric conductivity by the 



* It must not be forgotten that too large a diameter is objectionable, 

 as leading to an augmentation of volume during an experiment, as the 

 liquid falls. 



t See next paper, 



