62 Transactions. — Miscellaneous. 



consider the probable causes of the motions of the atmosphere 

 in cyclones (or " lows ") and anti-cyclones (or " highs "). 



Heat, however, has an additional influence on the motions 

 of our atmosphere, on account of the vapour of water which 

 is present so largely in the atmosphere. Vapour of water 

 contains a considerable amount of heat in an imperceptible or 

 latent condition. This heat has been abstracted from the wet 

 surface where the evaporation takes place (a fact well known 

 to us by the chill experienced as our skin dries when wetted), 

 and it is given up again when the vapour is reconverted into 

 water by reduced temperature and pressure. Thus the appa- 

 rent anomaly results that the cool air saturated with vapour 

 which the westerly winds bring over our Southern Alps loses 

 there its moisture — condensed into rain and snow — and passes 

 on as the warm, or even hot, dry wind experienced in the 

 plains of Canterbury. Qnder suitable conditions this setting- 

 free of the latent heat in water- vapour by condensation gives 

 rise to ascending currents in temperate or cold regions like 

 those produced directly by the sun's heat in the tropics, or in 

 localities where the character and configuration of the surface 

 of the ground causes an abnormal heating of a portion of the 

 surface of the ground, and so of the air in contact with it. 

 This latter condition of things is believed to be the cause of 

 most of the comparatively small, but very violent, tornadoes 

 experienced in tropical regions — in America, and Asia, and 

 other places exposed to great heat from the sun's rays. The 

 former is thought to be one of the main causes of those more 

 widely-extended but generally less violent storms experienced 

 in the temperate zones. 



We see, then, two or three classes of circulation in the 

 earth's atmosphere. First, the constant great exchange be- 

 tween the equatorial and polar districts, which, modified by 

 the rotation of the earth, causes the trade winds north and 

 south of the equatorial belt of calms, the two belts of com- 

 parative calms and high barometer in the vicinity of the 35th 

 parallel of latitude north and south, and the counter-trades 

 on the polar sides of these belts. Secondly, the constantly- 

 recurring phenomenon of circular storms moving eastwards in 

 these regions of the counter-trades. And thirdly, the occa- 

 sional tornadoes or hurricanes which are met with in the 

 tropics. It is with the second class of disturbances that I 

 propose to deal this evening, by the aid of the charts of Aus- 

 tralasian weather prepared day by day at Brisbane under the 

 superintendence of Mr. "Wragge, the Government Meteor- 

 ologist there, and sent regularly to Sir James Hector, who has 

 kindly placed them at my disposal. 



I believe that it is mainly owing to the energy, zeal, and 

 scientific knowledge of Sir James Hector himself that we are 



