8 



THE VOYAGE OF H.M.S. CHALLENGER. 



During the same months, which gave on the mean of one hundred and twenty-six 

 days a daily range of temperature of 3°*2 over the open sea, the Challenger was 

 lying near land on seventy-six days. The observations made on these days showed a 

 greater daily range than out on the open sea. The minimum, — 2° - l, occurred at 

 4 a.m., and the maximum, 2° - 3, at noon, thus giving a daily range of 4°*4. It is 

 interesting to note the frequency with which the mean daily maximum occurred as 

 early as noon when the Challenger was in harbour, a result probably due to the 

 diurnal period of the sea breezes in such situations in tropical and subtropical regions. 

 Generally speaking, at High Level Stations and in situations within the influence of 

 well-marked sea breezes, the time of occurrence of the daily maximum temperature 

 is about two hours earlier than in inland open situations. 



Brewster made the remark many years ago, that, as regards land observations, the 

 mean of any pair of hours of the same name, such as 2 a.m. and 2 p.m., 4 a.m. and 4 p.m., 

 etc., does not differ very materially from the mean temperature of the day. 



The following are the deviations from the mean temperature of the air of the 

 separate pairs of hours for the one hundred and twenty-six days of the North Atlantic 



given above 



2 a.m. and 2 p.m., 



4 4 



6 „ „ 6 „ 



8 ,i ,j 8 „ 

 10 „ „ 10 „ 

 Noon and midnight, 



Deviation from 

 the Mean. 



+ 0°-3 



+ 0°d 



-0°-3 



0°'0 



0°-0 

 + 0°-2 



The result for the six hours, 4, 8 A.m. and p.m., noon and midnight, is + 0° - l, and for 

 the six hours 2, 6, and 10 a.m. and p.m., 0°'0. 



In the Isothermal Maps for the globe given in this work, the isothermals for the 

 North Atlantic have been drawn from the data published in the " International 

 Meteorological Observations " of the United States. But as the observations are made 

 as near as possible at the same physical instant, they were first corrected for Diurnal 

 Range from the results given in this table. 



Variation of the Humidity of the Air. — The observations on the humidity of the 

 atmosphere were made with the ordinary dry and wet bulb thermometers, from which 

 the absolute and relative humidities have been calculated by Glaisher Tables. 



If the aqueous vapour remained permanently and unchanged in the atmosphere, 

 that is, if it were not liable to be condensed into cloud or rain, the mixture would 

 become as complete as that of the oxygen and nitrogen of the air. The equilibrium of 

 the vapour atmosphere, however, is being constantly disturbed by changes of temperature, 

 by every instance of condensation, and by the unceasing process of evaporation. Since 



