CHAMBERS'S INFORMATION FOR THE PEOPLE. 



opposite effect must arise. Not only will the upper- 

 most portion of the long column, which rises to a 

 point where the short column has ended, and has 

 therefore nothing but a vacuum to flow to, have a 

 side-movement towards the other, but portions far 

 beneath the top ^will have a pressure so much 

 greater than the contiguous portions of the shorter 

 column, that they too will flow out laterally, and 

 determine a current tending to equalise the differ- 

 ence of heights and pressures. 



When a difference of temperature exists between 

 two spots that lie near one another, the effect of 

 the aerial currents which take place between 

 them is to equalise the temperature. Its lateral 

 currents are the effect of unequal heating, and in 

 some measure the remedy. These lateral cur- 

 rents are felt and recognised by us under the 

 name of winds. If the differences of warmth that 

 originate them were only limited and temporary, 

 the carrying of heat to and fro would bring about 

 an equality, and then the air would come to a 

 perfect repose ; but these differences of warmth 

 are perpetually kept up ; hence the lateral currents 

 go on for ever without ceasing. The middle 

 band of the earth, or the equatorial zone, is 

 exposed to the burning radiance of a direct sun, 

 while the two polar regions are so faintly heated, 

 that snow never melts upon them ; and this great 

 standing inequality keeps up two grand sets of 

 movements, which encompass the globe. The 

 equatorial air, being warmer and lighter below 

 than the air on each side, is buoyed up by the 

 cold masses moving in upon it from north and 

 south, and thus two great under-currents are 

 maintained between the poles and the equator. 

 The equatorial mass being expanded far upwards, 

 overtops and overpresses the upper portions of the 

 colder columns on each side, and therefore flows 

 in upon them on both sides ; thus causing two 

 .great upper-currents towards the poles, while the 

 under-currents are moving from the poles. The 

 inequality of the equatorial and polar regions in 

 respect of heat is thus in some degree mitigated ; 

 but it is by far too great to be entirely done away. 



We have supposed, for simplicity's sake, that 

 the atmosphere is divided into an equatorial and 

 two polar masses ; but in reality there is a con- 

 stant gradation from the equator to the polar 

 regions, and the movements will take place be- 

 tween every two adjoining portions of atmosphere 

 of which one is farther north or south than the 

 other. 



Although, as a general rule, the temperature of 

 the earth decreases from the equator to the poles, 

 this is not strictly or at all times true. Various 

 causes occur to render a high latitude as warm 

 as, or warmer than a lower, and in such a case a 

 contradiction will arise to the general movement, 

 which will have peculiar local consequences. 



VAPOUR OF THE ATMOSPHERE. 

 Vaporisation Dew Mist Clouds Rain Hail Snow. 



Next in quantity to nitrogen and oxygen, 

 although very small compared with these, is the 

 vapour of the atmosphere ; that is to say, the 

 gaseous water or steam that is constantly present 

 in it. Although comparatively small in amount, 

 this ingredient of the atmosphere is of immense 

 importance in its effects. Unlike the other gases, 

 it is easily reduced from the aerial to the liquid 



34 



form, or to water as we usually find it ; and it is 

 constantly going through the processes of be- 

 coming liquid and descending to the earth, and 

 again rising into the air in the gaseous or in- 

 visible form. The great agency connected with 

 these transformations is heat. When water 

 passes into steam, it takes in a large amount of 

 heat, which is rendered insensible to the feeling or 

 to the thermometer ; and when steam or invisible 

 vapour is condensed into water, all this heat is 

 given out again (see NATURAL PHILOSOPHY). 



It is essential to bear in mind that true gaseous 

 water, steam, or elastic vapour of water, which all 

 mean the same thing, is invisible, like the other 

 portions of the atmosphere ; and that the white 

 cloud that appears at the chimneys of steam- 

 boats and locomotives, and at the spout of a 

 kettle, is not gas or elastic vapour, but vapour 

 partially condensed, whose particles only require 

 to be brought together to become drops of water. 

 It is, in fact, water in the form of something like 

 dust. Mist, fogs, and clouds are of the same 

 character : they are not gaseous steam, but pre- 

 cipitated watery particles destitute both of mutual 

 repulsion and of the latent heat of steam. When 

 the atmosphere contains nothing but true steam, 

 it is transparent and cloudless. 



The formation of steam out of water is most 

 conspicuous in the process of boiling, where the 

 surface is kept in an intense bubbling state, each 

 bubble containing a mass of steam, which forces 

 its way up into the air. This boiling takes place 

 at 212 of the thermometer, called for that reason 

 the boiling-point. The steam thus formed has 

 an elastic force equal to the pressure of the 

 atmosphere, or a force of 14$ pounds to the square 

 inch. The reason for the violent escape of steam 

 at 212 is, that it has attained a force equal to 

 the weight of the atmosphere pressing on the 

 water, and is therefore able to set aside this 

 pressure, or make its way in spite of it. But 

 even at temperatures below boiling, water passes 

 into steam slowly and invisibly. It is well 

 known that a wet surface soon becomes dry ; 

 in other words, that the water upon it disap- 

 pears. Water, however, cannot be annihilated ; 

 in the drying process, the liquid water becomes 

 gaseous invisible water or steam, and mixes with 

 the other steam contained in the air. 



The rapidity of the process of drying up, or of 

 the passing of water into steam, depends, in the 

 first place, on the temperature of the water. We 

 have seen that it is abundant and violent at 212 ; 

 and it is less and less for every degree downwards. 

 The elastic force of the steam produced also 

 depends upon the temperature. The elastic force 

 of steam at the boiling-point is equal to the 

 pressure of the atmosphere ; while the elastic 

 force of steam produced silently at 80 is only -^tli 

 of the elasticity of the atmosphere, or equal to 

 one inch of the barometer. 



Now, the entire quantity of steam that can rise 

 is limited by the temperature in the same manner 

 as the elasticity is limited. Water at 80 will give 

 forth vapour, until as much has been produced in 

 the atmosphere as would counterbalance one inch 

 of mercury ; evaporation then ceases, and the air 

 is said to be saturated with steam. Whether the 

 steam rise freely into the atmosphere, or rise into 

 a vacuum, no more will be produced at 80 than 

 this quantity. If the temperature were raised to 



