Temperature of the Sea and the Air 117 



reading it when it had assumed a constant temperature. The 

 temperature of the air is thus determined in exactly the same 



as that of the water, namely, by agitating a thermometer 

 in it, and the comparison of the two is therefore likely to 

 lead to trustworthy conclusions. 



Temperature of wet-bulb thermometer. The series of observa- 

 tions with this instrument is not so complete as that with 

 the dry thermometer, but they possess some interest. The 

 mot hod of observation was the following: the temperature 

 of the air having been determined by whirling the thermometer 

 in it, a bucket of sea-water was fetched and its temperature 



n ; the thermometer was then exposed, with its bulb 



wot with sea-water, to the breeze in a suitable part of 

 the ship, and its temperature observed when it became constant. 

 The exposure of the instrument requires some care. The 

 bulb must be quite free from grease, which can be readily 

 secured by washing it with soap and water. It is then dipped 

 into the water and allowed to drip for a se'cond. It is then 

 held somewhat inclined to the direction of the wind and to 

 the horizon, and rotated gently on its axis so that the bulb 

 be kept covered with a continuous film of water which is 

 locally thickened by gravity, which tends to form a drop on 

 the lower side of the bulb. The reading of the thermometer 

 is observed whilo it U being rotated. Had I intended from the 

 beginning to make a series of wet and dry bulb observations, 



uld probably have used fresh water from the first. I began 

 xpose the thermometer, merely in order to have an in< 

 tion whether the atmosphere was saturated or not, and I 

 expected, in the damp equatorial regions, to find the atmosphere 

 so heavily saturated as to be incapable of producing ;mv 

 ible lowering of the thermometer with damped bulb. 



purpose it seemed to be quite sufficient to expose the 

 thermometer wet with sea-water. I ':>< -.nun with sea- 



he observations were continued with it. A 

 parative obs ^ were made in order to determine 



the effect of replacing the sea-water by fresh water. On 



.prr.t urc of the air was 

 25C. When \v, t with Ml tin- temperature of the 



