74 PNEUMATICS. 



taking the average of six years, was only 21 '781 

 inches: viz. 



1816, 24 inches. 1819, 21 inches. 



17, 19 inches. 20, 17 inches. 



18, 23 inches. 21, 25 inches. 



In 1821, the average quantity that fell at Totten- 

 ham was 33-84 inches. Mr. Dalton reckons the 

 annual deposit of water in England at 31 inches of 

 rain and 5 inches of dew. 



Rain in the Torrid Zone. 



The following estimate by Humboldt is given as 

 an approximation to the depth of rain that falls in 

 different parallels : 



Inches, English. Inches, English. 



Lat. - 96 Lat. 45 - 29 

 19 - 80 60 - 17 



Although the annual depth of rain is greatest 

 within the torrid zone, the number of rainy days fol- 

 lows an inverse order, and diminishes as we advance 

 from high latitudes to the torrid zone. But even 

 within the torrid zone there are tracts where rain 

 seldom or never falls, such as the Sahara desert of 

 Africa, the low coasts of the Caraccas, and the desert 

 shore of Peru, between 15 and 30 south latitude. 



Total Weight of the Annual Rain. 



Taking the specific gravity of mercury to be to 

 that of water as 14 to 1, if we suppose the whole 

 quantity of rain that falls in a year (say 28 inches) to 

 be dissolved and suspended in the air at one time, it 

 would only support two additional inches of mercury, 

 and on the supposed fall of the wftble quantity the 

 mercury would only sink two inches. 



Dr. Hutton's Theory is, 



That as the power of the air to imbibe moisture in- 

 creases in a higher ratio than the temperature, two 



