CLIMATOLOGY.] 



UNDULATORY FORCES. HEAT. 



31 



graphy and meteorology, we may state, that there are 

 certain places on the earth's surface which have an equal 

 annual average temperature. Tliis arises from a variety 

 of local and general causes ; and the lines drawn on a map 

 expressing the geographical position of such places, are 

 called isothermal lines a term derived from two Greek 

 words, signifying "equal heat." 



The range of temperature in any country varies for 

 the same latitude, according to the altitude of the place 

 of observation above the level of the sea. In our own 

 climate, the limit of perpetual snow is rarely seen, on 

 account of our highest lulls being below that point. In 

 the north of Scotland, there is, however, one hill, Beu- 

 Wyvis, on which, as we have been informed, snow may 

 generally be found at all periods of the year. This, 

 perhaps, is partly owing to the winter drifts being so 

 deep as not to allow the high temperature of summer heat 

 to melt them entirely. In more southern climates, the 

 Alps, the Pyrenees, the Andes, and Himalayan range, 

 at all times present a place of continual congelation. In 

 some parts of India and South America, every known 

 climate may be met with, from that of a tropical to one 

 of a polar nature, within a few hours', or, at most, a day 

 or two's journey. Hence the inhabitants of those hot 

 regions of the earth's surface, are enabled to seek and 

 enjoy comparatively temperate climes, and thus to 

 escape the effect of that intense heat which so often 

 proves fatal to European constitutions. 



The cause of these temperatures varying with the rise 

 above the sea-level, is easily understood, if we bear in 

 mind *he fact, that as a gas expands, its capacity for heat 

 U increased, and that atmospheric rarefaction is always 

 attended by a loss of heat. The reader will remember, 

 that the loss of latent heat in the vacuum of an air-pump 

 U rapidly effected : when ether is thus evaporated from a 

 watch-glass placed in another containing cold water, the 

 latter liquid is thereby easily frozen. Now, at great 

 altitudes, the atmospheric pressure is greatly diminished, 

 and results analogous to those producible in the vacuum 

 of the air-pump are thus presented. Even in a small 

 tract of country, great difference of temperature may be 

 found. These often are owing to rapid evaporation, as 

 in the case of marsh lands, which always maintain a com- 

 paratively low temperature. In countries abounding 

 with chalk, such as Kent, the temperature does not 

 vary so rapidly, although in summer-time the heat is 

 highly reflected by chalky toils. Many tracts of land 

 powerfully absorb and radiate heat, and hence their tem- 

 perature is undergoing incessant change. 



We cannot here enter into some most interesting 

 instances of the laws of radiation and absorption, which 

 affect the fertility of the soil. We must not, however, 

 omit to remark, that in these inquiries we continually 

 observe evidences of design, and complete adaptation of 

 means to an end. In winter we have the surface of the 

 earth covered with a mantle of snow, which, by its non- 

 conducting power, prevents the escape of heat, and also 

 prevents radiation. So soon as the snow melts, or the 

 shower of rain falls, the colour of the soil is darkened, 

 and it is thus rendered a better absorber of the genial 

 heat of the sun's rays. As the heating process goes on, 

 the earth dries and becomes of a lighter colour, and U 

 mode to radiate less ; it is, indeed, thus changed from 

 being a good absorber of heat in its moist state, to be a 

 comparatively bad radiator in its dry condition. By 

 the*) successive arrangements the soil is enabled to drink 

 in the solar heat, which at the same time so changes the 

 surface as to enable it to store up heat. The seed is thus 

 Blade to germinate to grow and to bear fruit not by a 

 variety of complicated natural causes, but by the appli- 

 cation of one simple and unchangeable law. 



\Ve illicit add a vast variety of illustrations which 

 some under our daily experience, in connection with the 

 influences of heat on the vegetable and animal kingdom. 

 As we must confine ourselves, however, in this section 

 to those considerations which strictly belong to Ther- 

 moties, we shall reserve our further observations on 

 these and allied matters for our section on Agricultural 

 Chemistry. 



HEAT, APPLIED FOR THE PURPOSES OF 

 WARMING AND VENTILATION. 



THE continual variations of the temperature of the 

 atmosphere in climates situated beyond the tropics, 

 necessitate arrangements for the production of artificial 

 heat ; and the different plans adopted for this purpose are 

 but so many accidental or intentional applications of the 

 laws of Thermotics, which we have previously discussed 

 in the earlier pages of this section. 



The contrivances of various nations depend very much 

 on the kind of fuel employed as a source of heat. The 

 interior of an Esquimaux snow-hut may be considered as 

 perhaps the rudest, but, at the same time, a very effectual 

 mode of warming, but certainly not of ventilating, a 

 dwelling. Access is generally obtained to the house by 

 means of a long and narrow tunnel, through which the 

 natives creep on the hands and knees. On emerging 

 from the tunnel, a kind of lamp is observed in some part 

 of the snow-house. The fuel employed is common seal 

 oil, which produces plenty of heat and smoke during its 

 combustion. Such lamps serve the double purpose of 

 affording heat and light, and also are employed for that 

 of cooking food. In the arctic regions, wood of course 

 is very scarce ; yet nature casts at the feet of these boreal 

 natives the snow, which, as a bad conductor, makes an 

 exceedingly warm house ; and the seal, which, like the 

 cocoa-nut of India, serves for food, clothing, fuel, and 

 material for various purposes, to the inhabitants of these 

 inhospitable regions. 



Even in the northern parts of our own country, the 

 cooking and warming arrangements of some classes are of 

 the most primitive kind. We have visited "many huts in 

 the north Highlands of Scotland, where the only fire- 

 place was a large stone, the chimney a hole in the 

 thatched roof, and the fuel dried sticks, gathered from 

 the neighbouring woods. In many instances, the top of 

 the door of the bothy forms the only means of exit for 

 the smoke of the scanty fire. 



The employment of coal as fuel, necessitate*, that 

 arrangements should be made for an adequate supply of 

 air to support combustion, and also that means should 

 be provided by which the dense and offensive smoke 

 produced may be readily got rid of. Hence the invention 

 of fire-grates and chimneys, having the objects just named 

 in view. It is, however, exceedingly curious to remark, 

 that modern arrangements, such as are employed in our 

 houses botli for cooking and heating purposes, are, gene- 

 rally speaking, constructed on principles diametrically 

 opposed to those wliich sound science would suggest. 

 For instance, the whole of the grate intended to contain 

 the burning coal is made of metal, which, being a good 

 conductor, rapidly abstracts a large portion of heat pro- 

 duced by the combustion of the fuel. The chimney, by 

 means of which the smoke should escape, is often so wide 

 at its mouth as to defeat its object, by drawing up cool 

 air, which thus diminishes the draught ; and generally, 

 the grate itself, which is intended to supply warmth to 

 the room, is often " set" so far back, as that a greater 

 proportion of heat escapes up the vent than ever reaches 

 the apartment. It is true that all these objections have 

 been met by the construction of various stoves ; but as 

 the use of tliis kind of heating apparatus rarely finds 

 favour in the majority of cases, our existing arrangement 

 is almost universally the open fire, involving as it does 

 low of heat, and, therefore, of fuel, besides producing 

 strong draughts of air, wliich are frequently prejudicial 

 to health. 



It is, perhaps, no exaggeration to say, that full one-half 

 of the fuel employed in open fire-places is entirely wasted. 

 In the grates used by the working classes, the loss may 

 even be greater, because such are generally constructed 

 of heavy masses of metal, very different to the more 

 complete contrivances used by their opulent neighbours. 

 Owing to the mass of metal containing the fuel, the heat 

 rapidly passes off on all sides. If, however, the back 

 and sides of the grate were made of some non-conducting 

 material, such as fire-clay, etc., this important cause of 



