18 PHYSICAL PBOPEETIES 



From observations on luminous meteorites and refraction of sun- 

 light it has been estimated that, at a height of 200 miles, air has a very 

 small but appreciable density. It is also impossible to conceive of an 

 actual, defined limit to a gaseous atmosphere. 



The pressure of the atmosphere varies, as is well known, from day 

 to day, and upon the variations of pressure the direction and force of 

 the wind depend. 



From a consideration of the mean pressures of many different parts 

 of the world it has been discovered that there are two broad belts 

 of high pressure, one north, and the other south, running roughly 

 parallel to the equator. Near the equator itself, and also near the 

 poles, are regions of low mean pressure. 



Another fact in connection with atmospheric pressure has been 

 clearly noticed, most distinctly in tropical districts a diurnal variation, 

 there being two maxima, often about 9 a.m. and 9 p.m., and two mini- 

 ma about 3 a.m. and 3 p.m. So regular is this variation in some hot 

 countries that, according to Humboldt, it is almost possible to tell 

 the time of day by reading the barometer. 



The consideration of the relative heights of the barometer obtaining 

 at different places at a given time affords one of the most important 

 criteria in forecasting weather. Though this subject to the farmer is 

 obviously of the greatest importance, and though considerable progress 

 has recently been made in connection with it, this is not the place in 

 which it can suitably be discussed. The reader should consult a 

 modern treatise on meteorology. 



Dry air is almost diathermanous, i.e., transparent to heat rays. 

 Consequently it allows the sun's heat to pass through with but little 

 loss, becoming only very slightly warmed. It also allows a consider- 

 able, though probably much less, percentage of the heat radiated from 

 the earth to pass through it. If, however, any appreciable amount of 

 aqueous vapour or suspended solid matter be present, both forms of 

 radiant heat are to a large extent absorbed. The presence of clouds 

 confers a still greater retentive power for heat. This effect of aqueous 

 vapour or of clouds is often very apparent at night ; it is a matter of 

 common experience that clear starlight or moonlight nights, even in 

 summer, are often cold, because of the free radiation of heat from the 

 earth into space, while cloudy nights are generally much warmer. 

 Water in the air, too, has an important effect in conveying heat from 

 one place to another. Whenever water becomes gaseous, heat is ab- 

 sorbed, and when the vapour condenses again (often in the upper 

 regions of the air) heat is evolved. When air, from any cause, ascends, 

 it cools itself by its own expansion, the rate being about 1 C. for each 

 rise of 340 ft. Consequently, if nearly saturated with aqueous vapour, 

 some is deposited as cloud or mist when the air ascends. 



Air itself is thus little affected by the direct heat of the sun, being 

 heated either by contact with the hot surface of the ground, by the aid 

 of its own aqueous vapour, or by its own contraction. 



The specific heat of air is about 0'24, that of water being unity, i.e., 

 to raise the temperature of a given weight of air through a given inter- 

 val of temperature requires only about one-fourth as much heat as 



