B. TERRESTRIAL PHYSICS AND METEOROLOGY. 127 



the southwest monsoon sets in on both coasts of the penin- 

 sula. 



The relation of the winds to temperature, humidity, and 

 barometric pressure is most carefully presented, in detail. In 

 a general summary he states that the northeast monsoon of 

 tlie Indian seas is produced by the cooling and condensation 

 of a comparatively calm atmosphere over the land surface of 

 India. Its origin is in the plains of the Punjab, and Upper 

 and Central India and Assam, probably also on the southern 

 slopes of the Himalaya. These wind currents are fed by 

 an upj^er current which he terms the anti-monsoo7i^ of which 

 he traces two distinct branches. 



The double system of upper and lower currents is present- 

 ed by him in admirably constructed charts, on which both 

 are shown in their proper relations to each other. The south- 

 west monsoon is produced by the heating of the land surface 

 of the peninsula. As the heat increases the pressure falls 

 steadily, and the sea winds are drawn from a greater dis- 

 tance south. At length, in general, the ridge of high press- 

 ure over the sea, which has been steadily receding southward 

 since February, is obliterated, and the southeast trade, or 

 perhaps only a portion of it, crossing the equator, brings the 

 monsoon rains to Bengal and the western coast of India. 

 Phil. Trans. Royal jSoc. of London, QIjXIY.^ 363. 



ORIGIN OF CYCLONES. 



The origin of the cyclones of the Bay of Bengal has been 

 studied by Blanford, of Calcutta, who concludes that they 

 are not necessarily produced by two parallel currents blow- 

 ing in opposite directions, but rather that a calm state of 

 atmosphere, or one in which the winds are light and variable 

 over an open sea, is a condition favorable to the formation 

 of these storms ; and that a second condition is high or mod- 

 erately high temperature. In consequence of this collocation, 

 a large quantity of vapor is produced, ascends into the at- 

 mosphere and is condensed, with a liberation of its latent 

 heat over the place of its production instead of being carried 

 to some distant region. The atmospheric pressure is thus 

 locally lowered, tending to cause an indraft of air. Tlie for- 

 mation of cyclones is, then, finally determined by an inrush 

 of a saturated storm-current of air from the southwest or 



