152 Things not generally Known. 



smaller bore, which contained a coloured fluid much lighter 

 than mercury. When a diminution of atmospheric pressure oc- 

 curred, the mercury in the large tube descended, and by its fall 

 forced up the coloured fluid in the smaller tube ; the fall of the 

 one being indicated in a magnified ratio by the rise in the other. 



THE ATMOSPHERE COMPARED TO A STEAM-ENGINE. 



In this comparison, by Lieut. Maury, the South Seas them- 

 selves, in all their vast intertropical extent, are the boiler for 

 the engine, and the northern hemisphere is its condenser. The 

 mechanical power exerted by the air and the sun in lifting 

 water from the earth, in transporting it from one place to an- 

 other, and in letting it down again, is inconceivably great. 

 The utilitarian who compares the water-power that the Falls 

 of Niagara would afford if applied to machinery is astonished 

 at the number of figures which are required to express its equi- 

 valent in horse-power. Yet what is the Jiorse-power of the Nia- 

 gara, falling a few steps, in comparison with the horse-power 

 that is required to lift up as high as the clouds and let down 

 again all the water that is discharged into the sea, not only by 

 this river, but by all the other rivers in the world ? The calcu- 

 lation has been made by engineers ; and according to it, the 

 force of making and lifting vapour from each area of one acre 

 that is included on the surface of the earth, is equal to the 

 power of thirty horses ; and for the whole of the earth, it is 800 

 times greater than all the water-power in Europe. 



HOW DOES THE RAIN-MAKING VAPOUR GET FROM THE 

 SOUTHERN INTO THE NORTHERN HEMISPHERE ? 



This comes with such regularity, that our rivers never go 

 dry, and our springs fail not, because of the exact compensa- 

 tion of the grand machine of the atmosphere. It is exquisitely 

 and wonderfully counterpoised. Late in the autumn of the 

 north, throughout its winter, and in early spring, the sun is 

 pouring his rays with the greatest intensity down upon the 

 seas of the southern hemisphere; and this powerful engine, 

 which we are contemplating, is pumping up the water there 

 with the greatest activity ; at the same time, the mean temper- 

 ature of the entire southern hemisphere is about 10 higher 

 than the northern. The heat which this heavy evaporation 

 absorbs becomes latent, and with the moisture is carried 

 through the upper regions of the atmosphere until it reaches 

 our climates. Here the vapour is formed into clouds, con- 

 densed and precipitated ; the heat which held their water in the 

 state of vapour is set free, and becomes sensible heat ; and it is 

 that which contributes so much to temper our winter climate. 



