I2O Temperature of the Sea and the Air 



in the relative dryness of the atmosphere over the ocean. 

 If the observations with the wet-bulb thermometer be con- 

 sidered, it will be seen that the least difference of reading 

 between the dry-bulb and the wet-bulb thermometers was i C. 

 on January 28, when the ship was in the middle of the equatorial 

 belt of calms and rains. In this region perfectly saturated 

 air might be expected, and with instruments exposed in the 

 usual form of box I have no doubt that here, and in the very 

 oppressive weather of the northerly monsoon, the two instru- 

 ments would have given identical readings. The readings of 

 the air-temperature on January 28 were perfectly trustworthy, 

 as the sky was thickly overcast with dense rain-clouds all 

 day there was thus no risk of overheating; the readings 

 with the wetted bulb were equally satisfactory, so that the 

 results of the observations on that day may be taken to 

 represent fairly the normal state of things in the "Doldrums." 

 The temperature of the sea varied from 26-3 to 26-6 C., the 

 mean of five observations during the day being 26-42 C. 

 The mean temperature of the air during the day was 0-92 

 lower than that of the sea, or 25-5 C., and the temperature 

 of the wet-bulb thermometer 1-3 lower still, or 24-2 C. It 

 will be seen that, on the two exceptional days, January 31 

 and February i, the difference between the wet-bulb and the 

 dry-bulb thermometers is greater than would be expected from 

 the oppressive damp feeling of the air; it is therefore all 

 the more likely that the dry-bulb readings are too high as 

 indicated above. However, it is important to observe that 

 in all the regions passed through, whether in the westerly 

 winds of the North Atlantic or the equatorial calms, or the 

 monsoon of the South Atlantic, the temperature of the wet- 

 bulb thermometer is always very markedly below that of 

 the dry-bulb thermometer. In fact, such is the mobility of 

 the atmosphere that it rarely has the opportunity of saturating 

 itself; and if the effect which must be produced when this 

 air meets the surface of the water be considered, it will, I think, 

 afford some explanation of why at sea the temperature of the 

 air, even by day alone, is usually markedly below that of the 

 sea surface. 



