INTllODUCTOllY ESSAY. 81 



of Afghanistaiij and the winter rains of the lower hills and of 

 the plains at the foot of the mountains. These last are irre- 

 gular in amount and period, and dependent perhaps on local 

 disturbances of the great current, the causes of which are 

 still obscure and require careful investigation. During the 

 south-west monsoon, a similar return current from Siberia 

 and Tartary probably flows almost uniformly from the north- 

 ward at a very great elevation, and joins the ascending cur- 

 rent from the plains of India. 



When the causes and direction of the periodical winds are 

 clearly indicated, there is no diiSculty in understanding why 

 it is that in some parts of India the climate is always moist, 

 both monsoons being rainy, while iu others one monsoon only 

 is rainy, and in others again there is no rain at any period of 

 the year. The only permanently rainy province is the Ma- 

 layan peninsula, and the only absolutely arid ones are Sind 

 and the neighbouring deserts of the Panjab. Throughout the 

 greater part of India one monsoon is rainy, and that gene- 

 rally the south-west one, blowing from May or June till the 

 end of September. 



The amount of rain varies prodigiously in different parts of 

 India, from almost none to six hundred inches, but the details 

 must be reserved for notice under the several districts. It 

 is very essential to bear in mind that the rain-fall affords no 

 direct criterion of the humidity of any climate, for the atmo- 

 sphere may be saturated with moisture without any preci- 

 pitation taking place. The influence upon vegetation of the 

 vapour suspended iu the air, and thus brought in contact with 

 every surface of the foliage, is most important, and can only 

 be ascertained by means of daily observations with the hy- 

 grometer. This instrument is indeed, generally speaking, of 

 far more importance to the botanist than the thermometer ; 

 the distribution of tropical plants especially, in so far as it is 

 influenced by climate, being so by its moisture*. 



* To make our meaning clearer, we may say that any part of the tropics is 

 hot enough for the growth of a tropical plant, but that whole natural orders, 



m 



