470 Proceediufjs . 



and this slight obstruction affects the black bulb so as on rare occasions to 

 render the readings no higher in Sydney that in Southland. And I may add, 

 as a "rider" to the above, that those who contend that the atmosphere, 

 whether charged with vapour or not, is a vacuum to the sun's heat rays, are 

 left with no explanation of the heat being diminished when that luminary is 

 low down in the heavens. 



In connection with the Meteorology of New Zealand, I wish to offer an 

 explanation of the Hot Winds of the Canterbury Plains. The N.W. dry, 

 hot winds of Australia, in their passage over the ocean, become surcharged 

 with vapour at the expense of a noticeable part of their heat. In the 

 philosophic language of the day, the heat of the air is made to do work by 

 converting water into vapour, and by maintaining it in that state. The 

 sensible heat thus absorbed by vapour is converted into latent heat — that is, 

 the heat is not lost, but is engaged in the veiled work of maintaining water in 

 an invisible state. The air in thus dissolving water becomes a carrier, or 

 distributor of heat. It is not difficult to give an approximate estimate of the 

 quantity of heat made sensible to the thermometer by the reconversion of 

 vapour into water. The problem has been solved by experiments, and it is 

 found that the amount of heat stored up in aqueous vapours from one pound of 

 water is sufficient to heat 1000 pounds of water one degree of Fahrenheit, or, 

 as Tyndall puts it, to fuse five pounds of cast iron. 



Next as to the phenomena arising out of the fall of heavy rains on the 

 flanks of the ranges on tlie West Coast. The quantity of heat liberated would 

 be considerable did not other causes interfere with its escape. The rain itself 

 carries down with it a small part of the sensible heat, but the greater portion 

 of that set free is at once taken up by the atmosphere in rising to the altitude 

 of the lofty ranges in its passage to the plains of the South Island. The 

 sudden rarefaction is due to the loss of pressure of the column of air lying 

 between the level of the sea and the average height of the ranges. This 

 dilatation is accompanied by what used to be termed an increased capacity for 

 heat, so that heat is absorbed by the air itself. This too, in the current 

 phraseology of the day, is called work done at the expense of heat. So that 

 altogether we have a diminution of sensible heat, but not an actual loss of 

 heat, by the passage of the air over the ocean, and next by the rarefaction of 

 air in its ascent to the higher regions of the ranges. When air expands by 

 reason of the loss of pressure, the undulations of the molecules of air, which we 

 terra heat, diminish also — or, in other words, the amplitude of the vibrations 

 is diminished, and the distance between the undulations increased. Motion 

 imposed on the molecules of matter would go on for ever, just as any motion 

 imposed on a mass would never cease, if there were no medium through wliicli 

 it could communicate its motion to other bodies, or in some other way exhaust 



