1870. J Proceedings of the Asiatic Society. 89 



this approaches with the cloud and is accompanied by much thun- 

 der and lightning. The southerly breeze still, however, continues 

 until the line of cloud-front has reached, or even passed, the zenith, 

 when, for a few seconds, there occurs a complete calm, followed by 

 a sudden and mighty rush of wind from the N. W. — the rain 

 arrives, — there is a heavy downfall — the storm passes by — and for 

 a time there is almost a complete absence of wind with a lowered 

 temperature. 



My explanation is this : — The air coming from oceanwards is 

 largely charged with vapour at a comparatively high temperature, 

 and I suppose that a much colder current of air is suddenly (so to 

 speak) poured out upon it from the N. W. and passes over with con- 

 siderable velocity. Rapid condensation takes place along the sur- 

 face of contact of the upper and lower strata of air — dense cloud is 

 formed ; there is electrical disturbance ; and this state of things 

 advances with the front of the advancing N. W. current. As the 

 result of the condensation, a large mass of water is precipitated to 

 the surface of the earth. This, in falling, displaces air, the water 

 as it nears the ground driving the air out of site with increasing 

 force. By the continuation of the process, as the front of the up- 

 per stream of cold air passes on, an uninterrupted sheet of falling 

 water is produced, which apparently advances with the cloud above 

 and wind below. Actually, however, the path of each drop (except- 

 ing those of the forefront) may, I imagine, possibly be pretty nearly 

 vertical : before condensation, the vapour, if not brought to rest by 

 the friction of the two currents, was moving with the lower current 

 of air towards the N. or N. W. ; it does not acquire any new hori- 

 zontal velocity merely by becoming condensed, and as the drop falls 

 (if the height be great) it will manifest, if anything, relative to the 

 earth a slight increment of velocity towards the west. So far, 

 then, there is certainly no reason why the drops should move in 

 the direction of the storm ; on the contrary, they ought to have a 

 velocity towards the N. and W. But the formation and falling of 

 the water effects a transfer of matter from above to below ; a par- 

 tial vacancy, or rarefaction, thus takes place beginning at the point 

 of condensation, and the relatively dense cold air of the upper cur- 

 rent is at once ready to press into the opening : it pursues the 



