^-w 



Dr. Holtoke's Estimate of the Excess of Heat and Cold. Yi 



short; for the mean evaporation of those seven places docs 

 not amount to quite 45 inches, for the year 1785 ; whereas 

 the evaporation at Cambridge the same year, by Dr. Wil- 

 liams' account, was upwards of 56 inches. 



The great difference in the quantity of rain, which falls 

 in different countries annually, makes this a remarkable ar- 

 ticle in the meteorolo«rical reo;ister. We are informed by 



o^" o 



Dr. Lind,* that at Senegal in Africa there falls, in the four 

 rainy months, 115 inches depth of rain ; and by the EpTiem- 

 rerides so often quoted, that at St. Petersburg, in Russia, in 

 the year 1785, there fell short of 12 inches.f Now I think 

 it almost certain, that the quantity of rain, that falls yearly 

 in any country, provided it be sufficient for the purposes 

 of vegetation, must be very much in proportion to the an- 

 nual quantity of evaporation, in the same region. For a 

 very trifling quantity would any where be enough, if none 

 of it were to pass off by evaporation ; as, on the other hand, 

 scarcely any assignable quantity w^ould be sufficient, if the 

 whole of it were suddenly exhaled. The drier the air is 



large extent of country, the more rain is required 

 to support vegetation in its full vigour. So that the com- 

 parative dryness of the atmosphere, in any two countries, 

 may be pretty fairly inferred from the annual quantity of 

 rain, which 'falls in each of those countries respectively, for 

 a course of years; provided vegetation be equally vigorous 



any 



in both. 



From 



* Diseases of hot climates. 

 + And Tre arc not Informed 



m 



^ 



