Ixx INTRODUCTION TO PNEUMATICS. 



degrees on each side of the equator, the regions further distant from it 

 experiencing- only their respective north and south winds. 



The light air about the equator, which expands and rises into the upper 

 regions of the atmosphere, ultimately flows from thence back to the poles, 

 to restore the equilibrium. If it were not for this resource, the polar 

 atmospheric regions would soon be exhausted by the stream of air, which, 

 in the lower strata of the atmosphere, they are constantly sending towards 

 the equator. There is therefore a sort of circulation in the atmosphere: 

 the air in the lower strata flowing from the poles towards the equator, 

 and in the upper strata flowing back from the equator towards the poles. 

 An example of this circulation on a small scale may be seen in the air of 

 a room, which being more rarefied than the external air, a current is 

 pouring in from the crevices of the windows and doors, to restore the equi- 

 librium ; but the light air with which the room is filled must find some 

 vent, in order to make way for the heavy air which enters. If the door 

 be set a-jar, and a candle held near the upper part of it, the flame will be 

 blown outwards, showing that there is a current of air flowing out from 

 the upper part of the room; and if the candle be placed on the floor 

 close by the door, the flame will bend inwards, shewing that there is also 

 a current of air setting into the lower part of the room. The upper 

 current is the warm, light air, which is driven out to make way for the 

 stream of cold, dense air which enters below. 



There are also periodical trade-winds, commonly called monsoons, 

 which change their course every half-year. This variation is produced 

 by the earth's annual course round the sun, when the north pole is 

 inclined towards that luminary one-half of the year, and the south pole 

 the other half. During the summer of the northern hemisphere, the 

 countries of Arabia, Persia, India, and China, are much heated, and 

 reflect great quantities of the sun's rays into the atmosphere, by which it 

 becomes extremely rarefied, and the equilibrium consequently destroyed. 

 In order to restore it, the air from the equatorial southern regions, where 

 it is colder (as well as from the colder northern parts) must necessarily 

 have a motion towards those parts. The current of air from the equa- 

 torial regions produces the trade-winds for the first six months in all the 

 seas between the heated continent of Asia and the equator. During the 

 other six months, when it is summer in the southern hemisphere, the ocean 

 and countries towards the southern tropic are most heated, and the air 

 over those parts most rarefied ; then the air about the equator alters its 

 course, and flows in an opposite direction. 



The breaking-up of the monsoons is the name given by sailors to the 

 shifting of the periodical winds; they do not change their course sud- 

 denly, but by degrees, as the sun moves from one hemisphere to the other. 

 This change is usually attended by storms and hurricanes, so that those 

 seas are seldom navigated at the season of the equinox. 



It is less easy to account for the great variety of winds which prevail in 

 the temperate zones ; but when we consider that so large a portion of the 

 atmosphere is in continual agitation in the torrid zone, these agitations in 

 an elastic fluid, which yields to the slightest impression, must extend 

 every way to a great distance. The air in all climates will suffer more or 

 less perturbation, according to the situation of the country, the position 

 of mountains, valleys, and a variety of other causes : hence it is easy to 

 conceive that almost every climate must be liable to variable winds. On 

 the sea-shore a gentle sea-breeze generally sets in on the land in the 



