DEW. 177 



saturated, the same quantity ol' moisture. But since the 

 temperature diminishes with the latitude, a given vohime 

 of air in a perfect state of saturation must contain less and 

 less moisture as we approach the poles. From a similar 

 cause the moisture of the atmosphere must diminish as we 

 ascend above the earth. Local circumstances also affect 

 the moisture of the atmosphere : thus, over coasts it is much 

 moister than in the interior of continents, as is well exem- 

 plified in the moist atmosphere of Western Europe, when 

 contrasted with the very dry atmosphere of Asia. The at- 

 mosphere over wooded districts is moister than over those 

 sparingly covered with vegetation ; and the driest reposes 

 upon arid sandy tropical plains. In India, for the ruost 

 part, between December'and June,' while the general motion 

 of the air is southward to the sea, the atmosphere is com- 

 parativelj- dry. It attains its maximum of humidity during 

 the blowing of the south-west monsoon. After the rains 

 eease, fogs in the mornings continue for some time, and re- 

 appear before the commencement of the iainy season. Such 

 fogs are useful, Heyne remarks, to the growth of plants, as 

 they clear them from dust, open their pores, and supply 

 them with nourishment, which they could not obtain from 

 the earth in this season. Without these irrigations very 

 little saltpetre could be made, as the earth which contains 

 it can be recognised only after it has attracted this moisture. 



The only accurate mode of ascertaining the quantit}"^ of 

 moisture in the air is by means of the hygrometer, an in- 

 strument which has hitherto been but sparingly used in 

 India. W^e regret this, as the extensive employment of this 

 instrument would throw much light on the climate of our 

 eastern empire. Hygrometers ought, along with other me- 

 teorological instruments, to be distributed by government 

 throughout India. 



10. Dew. — Dews, in many parts of India, are hea\'iestin 

 December and January, before the fogs set in. They be- 

 come perceptible before eight or nine in the evening, when 

 the atmosphere is perfectly serene. On the Coromandel 

 coast, according to Heyne, the inhabitants are not so much 

 afraid of exposure to them as those of other countries. 

 During the foggy season the vinegar of Sennagalu (the 

 acid dew of sbme authors), so much prized by the Moormen 

 and rich Hindoos, is obtained. It is made by spreading 



