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6ih April, 1886. 



The President, Mr. W. H. Patterson, M.R.I.A., in the Chair. 



Mr. Joseph John Murphy, read a Paper on 

 WET AND DRY WEATHER. 



The treatment of the subject was chiefly based on some pub- 

 lications by Dr. Hann, printed in the journal of the Austrian 

 Meteorological Society. 



The motive power of all winds ultimately consists in the heat 

 of the sun. When one region becomes warmer than another, 

 as, for instance, land heats more rapidly under the sun than 

 water, or bare ground than ground covered with vegetation, 

 the air flows upward over the heated space, and a wind is formed 

 by the inflow of air along the surface of the earth ; just as the 

 fire in a room draws the air towards it in a draft along the 

 floor. The trade winds consist of such a draft towards the 

 warm regions of the equator. 



Storms, as distinguished from mere winds, are due to the 

 condensation of watery vapour in ascending currents of air. 

 When air flows upward the pressure on it from the air above is 

 diminished, because of the less thickness of the aerial strata 

 above it ; the diminution of pressure causes expansion, and the 

 expansion produces cold, whereby the heat that was latent in the 

 vapour is liberated : — and though when vapour is condensed 

 into water the volume of the water is destroyed, yet this is com- 

 pensated for four or five times over by the liberated heat expand- 

 ing the air ; which expansion increases the force of the ascending 

 current, and the consequent indraft of wind at its base. The 

 motive power of storms is thus steam power. But storms would 

 not be produced but for another agency, namely, the earth's 

 rotation ; which, though it has no power whatever to set a wind 

 in motion, has a most important modifying influence on winds, 

 as is to be explained further on. 



