UNDULATORY FOROES.-HEAT. 



[TRADK WINDS, ro. 



and violently diverge, and thus at ouco show that the 

 rays hare been collected into a focus by the lens. 



"Experiments of this kind require considerable core and 

 attention on account of their extreme delicacy, and also 

 from the fact that the senses cannot judiie of the effects 

 prod in 3d, as can be done when light is the force under 

 examination. Much depends on the careful construction 

 of the thormo-inultiplior to obtain successful results, and 

 on a proper adjustment of every part of the apparatus 

 employed. 



TERRESTRIAL HEAT. 



ALTHOUOH the subject of atmospheric temperatures, and 

 the laws which govern their changes, are properly to bo 

 considered in detail under the section on Meteorology, 

 it will, however, be convenient that we should enter into 

 the examination of some of the causes and effects of the 

 natural heat of the earth in the present section. The 

 idea has long been held, that the centre of the earth is in 

 a state of intense ignition. The gradual increase of tem- 

 perature observed in descending deep mines, certainly 

 gives countenance to such an opinion. It is found, that 

 the temperature of a sliaft sxmk beneath the surface of 

 the ground, increases at the rate of one degree Fahrenheit 

 for a descent of every seventy feet ; and, arguing on the 

 assumption that this increase of temperature continues to 

 progress at the same rate, it would follow that, at a depth 

 of ten miles, all substances would be at a red heat, and 

 those melting at a lower temperature than 1000 Fah- 

 renheit would be in a liquid or gaseous condition. The 

 existence of volcanoes, and the boiling fountains or 

 geysers of Iceland, indicates that, at some depth beneath 

 the average surface of the earth, the temperature must be 

 very high. 



Some very ingenious speculations have been made as 

 to the cause of the internal heat of the earth. Many 

 have ascribed it to chemical action ; and, shortly after 

 the discovery of the metals potassium and sodium, it was 

 suggested that their combustion might, to some extent, 

 account for many of the phenomena observed in volcanic 

 countries. Intense combustion has also been suggested 

 as another cause. Against this idea, however, a difficulty 

 arises, inasmuch as, according to onr daily experience, 

 combustion cannot generally occur except in the pre- 

 sence of oxygen. The most feasible mode of explaining 

 the matter seems to be, that at some remote period the 

 earth's surface lias been at a much higher temperature 

 than at the present time, but that it has cooled down 

 gradually, by the radiation of heat, into space. It is higl ily 

 improbable that any definite notion of the cause of this 

 internal heat of the earth will be ever arrived at, because 

 the depth to which we can penetrate in mining and other 

 operations, is very trifling compared with that which 

 would be required to verify speculation, or even to obtain 

 reliable data for calculation. 



The application of the laws of the conduction, absorp- 

 tion, and radiation of heat, as applied to the earth's 

 surface, is of the most extended kind. We have already 

 noticed some of the phenomena which arise from the 

 action of those laws. There are, however, two special 

 instances of such in the gulf streams and the trade winds, 

 which deserve further notice, inasmuch as both pheno- 

 mena depend on the expansive powers of caloric acting 

 on liquids. 



There are three parts of the earth's surface where 

 extremes of temperature naturally exist ; namely, at the 

 north and south poles, and under the equator. Bearing 

 in mind the laws which we have mentioned in our 

 previous pages, in reference to the expansion, and conse- 

 quent diminution of density, which all gases undergo 

 when heated, it follows that an ascending current of air 

 should always be found in the equator, which is replaced 

 by the flow of air from the cooler regions of temperate 

 climes, .and the still colder polar blast. As the heated 

 air, proceeding at a great altitude, approaches the poles, 

 its temportiiro is gradually diminished; and, eventually, 

 the hot current is so cooled down, as to become succes- 

 sively the return cold current towards the tropics. It 



will thus bo observed, that by the heat of the sun's ray.) 

 absorbed, and afterwards radiated by the earth at the 

 tropical regions, an enormous current of air is constantly 

 kept in motion, having a definite direction of its own, 

 nml comparatively independent of minor disturbing 

 causes which it may meet with during its progress. 

 There is, however, one cause which modifies the direction 

 of this great calorific current which is the rotation of 

 the earth on its axis. Now, owing to the levity of the 

 atmosphere, it does not partake so completely of the 

 motion of the earth as it would do if solid. The result 

 is, that the current just spoken of does not take a due 

 north or south direction from either polo, but divides 

 into one passing towards the south-west from the north 

 pole, and another blowing towards the north-west from 

 the south pole. Hence the origin of the trade winds so 

 well known to, and so highly prized liy, navigators. 



Besides the currents hero referred to as the result of 

 the agency of heat, there are an infinity of minor and 

 modifying effects produced by various disturbing can 

 The land and sea breeze, before mentioned, owe their 

 origin to partial or local radiation of heat ; and the same 

 may be remarked in reference to the mountain storm of 

 wind and rain, where similar causes are always operating 

 on the small scale. The monsoons of India, and the hot 

 winds of the arid Arabian and African deserts, simply 

 result from the effects of the heat absorbed by the sandy 

 plains on the air resting over their surface. A little 

 reflection will at once point out, that the law of the 

 convection of heat by gases, produces results of the 

 utmost importance, and of the most beneficial character 

 throughout nature. Without heat was thus conveyed 

 by ! means of these expanded air-currents from the 

 tropics, those regions would become a scene of desola- 

 tion, and of the absence of vegetable and animal life ; 

 and were it not that such currents convey the excess of 

 heat from the tropics to colder regions, the latter would 

 lie in one ocean of ice, or in a winter of perpetual frost. 

 The grand arrangement or chain of these laws of heat 

 prevents either catastrophe, and insensibly but surely 

 spreads an airy mantle of fertility and fruitfulness over 

 every part of the surface of our world. 



It remains for us to speak of another current which 

 prevails in the waters of the ocean. This is called the 

 gulf stream; and it has a great effect in modifying the 

 annual temperature of the polar regions. An immense 

 body of water, of a temperature of about 84, proceeds 

 continually from the Gulf of Mexico, in a northerly 

 direction, eventually reaching the shores of Great Bri- 

 tain, where it divides into two streams one passing 

 northwards, and the other bearing easterly on to the 

 coast of Franco. The heat of the tropics is thus con- 

 veyed towards our own coasts, and hence the prevalence 

 of rain when the wind is in the south-west, because the 

 air is loaded with moisture, which condenses on reach- 

 ing land. The force of this great current of heated 

 water is such, that plants and fruits, the products of 

 tropical countries, are often cast on the shores of the 

 Western Isles, along the coast of Scotland. 



The thermal effect of the gulf stream may be judged 

 of from the fact, that whilst the whole of the American 

 coast, in the same latitude as our own country, is locked 

 in frost during winter, and unbenefited by the genial 

 warmth of tliis stream, our sea-shores never present the 

 appearance of a mass of ice, through the whole length of 

 the western sea-coast. There is even a less extreme 

 annual range of temperature in the north-west of Scot- 

 land, than on the south-east coast of England, although 

 the latter is about 400 miles nearer the equator. The 

 same comparison may bo instituted between the climate 

 of Scotland and that of some portions of Denmark, 

 situated in the same latitude. 



It has been discovered that there are return currents 

 in the ocean, corresponding to those wo referred to as 

 occurring in the atmosphere, and as producing the trade 

 winds. The gulf stream is bounded, and, in fact, rests 

 over such currents ; and hence the analogy between it 

 and the trade winds is complete in all respects. 



Without trenching on the subjects of physical geo- 



