n8 Temperature of the Sea and the Air 



thermometer was 23-5 C., and with rain-water 23-1 3 C. 

 Similarly, at noon on the same day, the following temperatures 

 were observed: dry bulb, 26-1 C. ; wet bulb (sea), 24-5 C., 

 wet bulb (rain), 24-2 C. The air, at this time, appeared to 

 the sensation to be damper than at any other time, and yet, 

 when suitably exposed, there was a difference of nearly 2 C. 

 between the wet and dry thermometers. 



There is an advantage in having the bulb of the thermo- 

 meter wet with a continuous film of water, instead of being 

 surrounded with damp muslin, namely, that it more nearly 

 resembles the surface of the sea, which is exposed to the 

 influence of the atmosphere. Observations with the wet 

 thermometer were not made as regularly as those with the 

 dry instrument, and no observations were made with either 

 of them after dark, owing to the difficulty of securing proper 

 exposure and reading the instrument with a lantern, without 

 heating it. 



The temperature of the air and of the water were taken 

 generally every two hours from 6 A.M. to 6 P.M., but the 

 intervals between the observations were not always the same. 

 These observations showed that only on two days, January 31 

 and February I, between lat. 6 S. and 12 S., did the mean 

 day temperature of the air exceed that of the surface water. 

 On these days the temperatures were taken every two hours 

 from 6 A.M. to 6 P.M., and the means of the groups of seven 

 observations gave : on January 31, air, 27-13 C. ; sea, 26-90 C. ; 

 difference, 0-23 C. ; and on February i, air, 27-26 C. ; sea, 

 26-96 C. ; difference, 0-30 C. These differences would have 

 been reduced in amount if the observations had been carried 

 on through the night, though, from the very high temperature 

 of the air just before sunrise on February I and 2, they would 

 not have been reduced to zero. 



In the table (p. 121) all the simultaneous observations 

 of temperature of air and water made during the voyage, 

 except those of the last day, when approaching the mouth of 

 the River Plate, are collected in small tables for each day. 

 The time of day is given in hours, from o to 24 ; the temperatures 

 are in Centigrade degrees; t denotes the temperature of the 



