Wellington Philosophical Society. 471 



itself. This inertia belongs to molecules as much as to masses. "While the 

 swing of the molecules is diminished, and the distance between the undulations 

 is increased, the actual force, which we call heat, remains undiminished. It 

 is only necessary that the undulations should be again crowded together in 

 order that the energy may be restored. 



So far I hope that I have made my meaning clear, — that air in taking up 

 moisture loses sensible heat. No one who has travelled on the West Coast of 

 the South Island can have failed to notice when he leaves the forest road, and 

 comes on the dreary sands of the coast, with a moderate sea breeze, how 

 miserably cold the air is in the immediate vicinity of the breakers. This is 

 owing to absorption of sensible heat by the solution of the spray in the current 

 of air sweeping over the sea. 



Next, in reference to the heat gained, if any, in the passage of the air over 

 the extensive elevated region between the West Coast and the Canterbury 

 Plains. The air on the mountain ranges gains no addition of temperature 

 from the direct radiant heat of the sun, for the rarefied air is, to use a technical 

 expression, almost transparent to the sun's heat rays — it permits them to pass 

 with very slight loss. The sun warms the ground but not the air. It is not 

 so, however, with the heat radiating from the surface of the ground — this is 

 almost entirely absorbed by the atmosphere, which thus gains sensible heat 

 until at length it reaches the eastern slopes of the mountain ranges. 



Here, where the column of air begins its descent to the plains, we ought 

 to have a complete reversal of every phenomenon that accompanied its passsage 

 from the level of the sea on the West Coast till it reaches the elevation of the 

 mountain ranges. As the column of air sinks down, the increased extent of 

 the vertical column gives increased pressure, and compresses more and more 

 the lower stratum — the latent heat becomes sensible and the thermometer 

 rises. But it is necessary that I should give you here some proof that 

 increased pressure adds to the energy of heat in aeriform bodies, or, in the old 

 expressive phraseology, converts the latent heat into heat recognised by our 

 senses, and registered by the thermometer. Airy has explained that when the 

 changes of volume and pressure are very rapid, the changes of temperature of 

 the air are very great : — " Upon suddenly condensing air it becomes very hot. 

 We have verified the experiment, that if inflammable tinder is placed in the 

 bottom of a cylinder in which a piston fits tightly and slides easily, when the 

 piston is driven rapidly down so as to condense the air very much before it 

 has time to impart the whole of its heat to the surrounding metal, the air will 

 inflame the tinder." And Airy remarked, *'in the powerful air pumps (driven 

 by large steam engines) which were used to exhaust the air tubes upon the 

 atmospheric railway, that when the attenuated air in the tube, having acquired 

 the temperature of the ground, was compressed by the operation of pumping 



