xx ii Contents 



PAGE 



temperature of ice due to the nature of the medium 



in which it melts . . . . . . .114 



General description of the voyage. It began in 

 mid-winter in the northern hemisphere and finished in 

 mid-summer in the southern hemisphere . . . 115 



In equatorial latitudes great uniformity of the 

 temperature both of sea and air . . . . .116 



Particulars respecting the method of determining 

 the true temperature of the air. Observations begun 

 before sunrise and concluded after sunset, in both cases 

 while there was day light enough to read the thermometer. 



On the many occasions when I have enjoyed the 

 privilege of making meteorological observations at sea 

 I have always made my most important terms the 

 time between dawn and sunrise and that between sunset 

 and darkness. The temperature observed at the former 

 is influenced principally by the integral effect of the 

 nocturnal cooling, and that observed at the latter is 

 influenced principally by the integral effect of the solar 

 heating. 



Impossibility, on board ship, of giving a thermo- 

 meter-box a fixed position which shall secure an expo- 

 sure such as to justify the assumption that the tempera- 

 ture of the thermometer in it coincides at any moment 

 with the true temperature of the atmosphere outside of it. 



Method of observing the wet-bulb thermometer with 

 a film of sea-water supported by capillarity on the 

 chemically clean bulb . . . . . . .117 



Effect of replacing the sea -water by a film of fresh 

 water on the bulb of the thermometer . . .118 



When the wet-bulb thermometer is used in this way 

 the continuous film of water resembles the surface of 

 the sa which is exposed to the same atmospheric 

 influences and the effect of these on the bulb of the 

 thermometer resembles that produced on the water 

 immediately below the surface. 



Observations were made only during day-light. 

 Description of the table of observations. With two 

 exceptions the temperature of the sea was found to be 

 higher than that of the air. Great heat in the northerly 

 monsoon off the Brazilian coast. The least difference 

 between the readings of the wet and dry thermometers 



