ANNIVERSARY ADDRESS. 27 
fire to rise up slowly, mixing with the air, and making it drier as 
they rise. If it is to have the effect of a wall—that is, making 
the whole of the air passing over rise up 1,800 feet—it must act ar 
an explosion, would do suddenly; or by a constant uprush of such 
violence that it would rise up 1,800 feet. The force necessary to 
do this is easily computed, and we can in this way get a money 
value for the work to be done. At Sydney the average velocity of 
the wind is 11 miles per hour, and all the air passing over is to be 
lifted, and the weight of it on the surface is, say, 14} pounds 
on the square inch, and 13} pounds at 1,800 feet high. At least 
for our present purpose these figures are sufficiently exact. The 
average weight to be lifted, therefore, is 14 pounds on the square 
inch. The fire must have the same length as the proposed wall, 
for the same reason, and a breadth equal to the forward motion 
of the air in a giventime. We have, therefore, to lift a weight of 
14 pounds on the square inch over a surface of 1,000 feet by 10 
miles (52,800 feet), and raise it up 1,800 feet every minute. To 
do this we will assume that coal is employed, and that, as it is 
burnt in the air, the whole of its heat will be effective. The 
mechanical equivalent of good coal is fourteen millions of foot 
pounds for each pound of coal used. We have therefore— 
14x 12 x 12x 1,000 x 1,800 x 5,2800 
14,000,000 x 112 x 20 
or nearly 9,000,000 of tons of coal per day, to increase the rainfall 
60 per cent., at a cost, at 10s. per ton, of £4,500,000. 
=6,110 tons per minute=8,800,000 tons 
in a day, 
Of course this is only a theoretical experiment, and ignores all 
“the heat lost by liation and imperfect combustion ; but it serves 
_ to give some idea of what is necessary to disturb the course of 
‘nature, and I think shows how utterly futile any such attempt 
_ ‘Would be, even near the sea, where the air is moist. Inland it is 
Common thing in summer to find 20° between the dry and wet 
bulb thermometers; and when that is the case, the air would have 
to be lifted 6,000 feet to form a cloud, and in such weather no 
