SECRETARY'S REPORT. 79 



These statistics show in the most striking light the charac- 

 teristic difference in the climate of the two countries. The 

 dry extreme, which never approaches the dry extreme of our 

 climate, occurs in England but once in five years ; the wet 

 extreme once in ten years. 



But we must not assume that more rain falls in England 

 than in Massachusetts. The mean annual fall of rain near 

 London for the period of 35 years, from 1797 to 1831, was 

 25.42 inches; while the mean annual amount for 12 years at 

 Cambridge, Massachusetts, was 43.06 inches : at Boston, for 

 30 years, it was 42.43 inches; and at Amherst, for 17 years, 

 42 inches. Thus it appears most conclusively that we have far 

 more rain during the year than falls in the eastern parts of 

 England. 



The mean temperature of England appears from observa- 

 tions made near London to be 48°.5 Fahr., the latitude being 

 51° 31'; while the mean temperature at Boston, latitude 42° 

 21', is 48°.9. The mean annual temperature of the summer 

 months in England is 61°.7; at Boston for the same months it 

 is 69°. 1. Thus the mean temperature of the two countries ap- 

 pears to be about the same, though the summer months present a 

 very marked difference. But our climate is subject to sudden 

 and marked changes, while that of England is comparatively 

 equable. We have many days in winter when the thermometer 

 falls far below zero, and in summer we frequently have it 

 above 80°, or even 90°, for several days in succession ; while 

 in England it very seldom rises above 80° in summer with- 

 out being followed very soon by thunder and rain, and very 

 rarely falls as low as zero in winter. 



The difference in the amount of evaporation of the two coun- 

 tries is a fair index of the difference in the climate of these 

 countries. Evaporation will generally be found to be in pro- 

 portion to the height of the temperature and the extent of 

 water or land surface. Ordinarily, in temperate zones, it is 

 about 37 inches a year, but in the tropics it amounts to from 

 90 to 100 inches. The atmosphere when at the freezing point 

 contains about a two-hundredth part of its weight of water, 

 while at 52° it contains about a one-hundrcth part, at 74° a 

 fiftieth part, at 98° a twenty-fifth part ; and so on. The evapo- 



