The Weather. 105 



its effect cannot be omitted ; but even in the course of ordinary 

 observations the horizontal arrangement of the clouds, and 

 their cross-currents, one above another, afford daily means of 

 verifying the horizontal direction of the wind. 



Suppose a current of air to descend from above, with the 

 velocity of an ordinary gale of wind, which is sixty miles an 

 hour, from an extreme height of ten miles above the earth. In 

 ten minutes it would arrive at the surface of the earth. The 

 elastic pressure of its mass would at the same time be increased 

 about tenfold. A very ordinary experiment, and one recently 

 improved by Dr. Tyndall, may be taken to illustrate the 

 change that would thereupon occur in the intruding mass. 



When air, confined by means of a piston in a tube, is sud- 

 denly compressed to a tenth or a twentieth part of its original 

 volume, the heat developed by the sudden compression, it is 

 well known, is such that tinder, punk, or amadon attached to 

 the end of the piston is instantly ignited. Such an instrument 

 is called a fire syringe. In its new form the tube is made of 

 glass, and a small portion of combustible gas is included with 

 the air. A bright and vivid flash at the moment of compres- 

 sion then gives ocular demonstration of the violence of the 

 heat. Lest the circumstances of the wind should not be con- 

 sidered as adequately represented by the conditions of this 

 experiment, it may be calculated that air submitted to a ten- 

 fold pressure must have its temperature elevated to the extent 

 of fully a thousand degrees Fahrenheit. This is therefore what 

 would take place in the descending mass, which would be 

 rendered buoyant by the heat developed, and would return 

 rapidly to the summit of the atmosphere by the force of 

 gravity alone. 



In a less extreme case, when wind is only slightly deflected 

 from its horizontal course either upwards or downwards by any 

 interruption, it quickly returns (by the force of gravity) to its 

 accustomed level, on account of the change of temperature that 

 attends its change of elevation. 



Main currents of the air are therefore horizontal, and violent 

 currents of the air, in a vertically ascending or vertically 

 descending direction, are, from the nature of the case, im- 

 possible. 



The mainspring of ventilation in the air is the ascensional 

 force of air and vapour imparted to wide tracts of the atmos- 

 phere by the ardent rays of the sun, scattered again in other 

 spaces by radiation from the earth. The focus of the sun's 

 action describes a diurnal circle round the globe, which is in 

 the equator at the time of the equinoxes, and at either tropic at 

 the solstitial seasons of the year. The latter circles receive 

 their name of tropics from the fact that the sun, although it 



