126 



The Realm of Nature 



CHAP. 



temperature of the air reaches its maximum about 2 P.M. 

 local time, or about 2 hours after the Sun has passed the 

 meridian. Cooling then sets in, and the temperature reaches 

 a minimum about 5 A.M., or shortly before sunrise. These 

 hours apply to the tropics and vary slightly in different parts 

 of the world, but the air is always coldest in the early 

 morning and always warmest in the early afternoon. Sir 

 David Brewster discovered, by comparing a long series of 



FIG. 23. Daily Range of Atmospheric Temperature in temperate 

 and tropical climates (after A. Buchan). 



hourly observations, that the average temperature at any 

 pair of hours of the same name (e.g. 9 A.M. and 9 P.M.) was 

 almost exactly the average temperature for the whole day. 

 Fig. 23 shows the range of temperature above and below 

 the average for the day, the hours being marked along the 

 top and the temperature in degrees above and below the 

 average on the side (see 444). The solid curve refers to a 

 station in the tropics, the lighter curve to a temperate region. 

 183. Daily Pressure Changes. The pressure of the 

 atmosphere is least about 4 A.M. and 4 P.M. and greatest 

 about 10 A.M. and 10 P.M. In Fig. 24 the diurnal range 

 of the barometer above and below its mean value is given, 

 the range in fractions of an inch being marked on the side, 

 the hours from noon to noon along the top. The solid 

 curve shows the typical range in the tropics, the lighter 

 curve that in a temperate region. This regular increase 

 and decrease of pressure twice daily, was for a long time 

 supposed to be a tidal effect caused by the Moon's differen- 

 tial attraction, but Dr. A. Buchan in discussing the 

 barometric observations made on the Challenger Expedi- 



