n6 Temperature of the Sea and the Air 



was a calm sea, a clear sky, and a very powerful sun. The 

 result was a comparatively great rise of temperature in the 

 afternoon ; and yet the greatest difference between any neigh- 

 bouring maximum and minimum in this region was only 1-1 C. 



The maximum temperature of the sea surface observed 

 during the voyage was 27-4 C. (81-3 F.) at 2 P.M. on 

 January 31, in lat. 7 35' S., the Brazilian coast being about 

 TOO miles distant. The temperature of the water will be 

 further considered in connection with its density; at present 

 its connection with the temperature of the air will be more 

 particularly considered. 



Temperature of the air. Along with the temperature of 

 the water, that of the air during daylight was determined. 

 It is probably very rare, in any part of the ocean, to find 

 the mean temperature of the air agreeing exactly with that 

 of the surface water, and in many places the differences are 

 considerable. In order to be able to compare the temperature 

 of the air with that of the water, it is necessary that both 

 should be determined with equal accuracy. The temperature 

 of the water is easily and accurately determined by agitating 

 the thermometer in a bucket of it freshly collected. With 

 the air it is somewhat different. Having only one thermometer 

 with me, I was obliged to use it for all purposes, and I could 

 not hang it up in a thermometer box, even if I had had one, 

 and had deemed it advisable to do so. On board ship, however, 

 I am convinced that it is quite impossible to fix a thermometer 

 box in such a position as always to secure such an exposure 

 as to justify the assumption that the indications of the thermo- 

 meter may be taken as the true temperature of the air. Even 

 on shore and under the most advantageous circumstances, 

 the temperature of the thermometer in the atmosphere of the 

 best constructed box is too much dependent on the temperature 

 and capacity for heat of the material of the box for it to be 

 assumed always to be identical with that of the air outside, at 

 the moment of reading. I was obliged, therefore, to adopt the 

 method of whirling the thermometer, at the end of a short 

 string, in the air, in whatever part of the ship happened at 

 the moment to afford the most favourable conditions, and 



