TEMPERATURE. 



555 



abandoned." If we except these cases, and that ol 

 volcanoes and hot springs, the temperature of the 

 interior of the earth seems to be the mean of that 

 at the surface ; and hence it is inferred that it is 

 derived from the same source. The diurnal varia- 

 tion of heat, so considerable at the surface, is not 

 to be perceived at the depth of a few feet, although 

 here there is a gradual change that becomes sensible 

 at intervals of a month. At the depth of thirty 

 or forty feet, the fluctuation is still less, and takes 

 place more slowly. Yet at this distance from the 

 surface there is a small annual variation ; and the 

 time of midsummer, or greatest heat, is ordinarily 

 about the last of October, and that of midwinter, 

 or greatest cold, is about the last of April. These 

 times, however, are liable to vary a month or more, 

 according as the power of the earth to conduct heat 

 is increased by unusual moisture or diminished by 

 dryness. But at the depth of eighty or a hundred 

 feet, the most sensible thermometer will hardly 

 exhibit any change throughout the year. So, on 

 the other hand, if we ascend above the earth's sur- 

 face, we approach more and more to a region of 

 uniform temperature, but of a temperature much 

 below the former. The tops of very high moun- 

 tains are well known to be covered with perpetual 

 snow, even in the tropical climates. The same, or 

 rather a still greater degree of cold, is found to 

 prevail at the same height, when we make the 

 ascent by means of a balloon. The tops of high 

 mountains are cold, therefore, because they are in 

 a cold region, and constantly swept by currents of 

 cold air. But what makes the air cold at this 

 height? It is comparatively cold, partly because 

 it is removed far from the surface of the earth, 

 where the heat is developed, but principally because 

 it is rarefied, and the heat it contains is diffused 

 over a larger space. Take a portion of air near 

 the surface of the earth, and at the temperature of 

 79 of Fahrenheit, for instance, and remove it to 

 the height of about two and a half miles, and it 

 will expand, on account of the diminished pressure, 

 to double the bulk, and the temperature will be 

 reduced about 50. It will accordingly be below 

 the freezing point of water. This height varies 

 in different latitudes and at different seasons. It 

 increases as we approach the equator, and dimin- 

 ishes as we go towards the poles. It is higher 

 also, at any given place, in summer than in winter. 

 It is, moreover, higher when the surface of the 

 ground below is elevated, like the table land of 

 Mexico. At a mean the cold increases at the rate 

 of about 1 for every 300 feet of elevation. In 

 addition to the above, it ought to be mentioned 

 that the tops of mountains part with the heat they 

 receive from the sun more readily on account of the 

 radiation taking place more freely in a rarer medium, 

 and where there are few objects to send the rays 

 back again. 



The question has been much discussed, whether 

 the winters in the temperate latitudes have become 

 milder or not. There is abundant evidence, it 

 seems to us, in favour of the alleged change. 

 Rivers which used to be frozen over so as to sup- 

 port armies, and which were expected to be covered 

 in the winter season with a natural bridge of ice, 

 as a common occurrence, now very rarely afford 

 such facilities to travellers. The directions for 

 making hay and stabling cattle, left us by the Roman 

 writers on husbandry, are of little use in modern 



Seo Edinburgh Review, No. riii, p. .';0, &c. 



Italy, where, for the most part, there is no suspen- 

 sion of vegetation, and where the cattle graze in 

 the fields all winter. The associations with the 

 fireside, annually referred to as familiar to every 

 one, can be little understood now in a country 

 where there is ordinarily no provision for warming 

 the houses, and no occasion for artificial heat as a 

 means of comfort. The ancient custom of suspend- 

 ing warlike operations during the season of winter, 

 even in the more southern parts of Europe, has 

 been little known in campaigns of recent date ; 

 not because the soldier of our times is inured to 

 greater hardships, but because there is little or no 

 suffering from this cause. In the northern parts of 

 America, also, the lapse of two centuries has pro- 

 duced a sensible melioration. When New England 

 was first settled, the winter set in regularly at a 

 particular time, continued about three months 

 without interruption, and broke up regularly, in 

 the manner it now does in some parts of Canada 

 and Russia. The quantity of snow is evidently 

 diminished, the cold season is more fluctuating, 

 and the transition from autumn to winter, and from 

 winter to spring, less sudden and complete. The 

 period of sleighing is so much reduced and so pre- 

 carious as to be of little importance compared with 

 what it was. The Hudson is now open about a 

 month later than it used to be. We are not, how- 

 ever, to conclude that so great a melioration has 

 taken place as might at first be inferred from this 

 fact. The change, whatever it be, seems to belong 

 to the autumn and early part of winter. The 

 spring, we are inclined to believe, is even more cold 

 and backward than it used to be. The supposed 

 mitigation of winter has usually been ascribed to 

 the extirpation of forests, and the consequent ex- 

 posure of the ground to the more direct and full 

 influence of the solar rays; and there can be little 

 doubt that a country does actually become warmer 

 by being cleared and cultivated. The favourable 

 change experienced in the New England and the 

 Middle States may, it is thought, be referred to 

 this circumstance. But the alteration that is ob- 

 served in the similar latitudes of Europe can hardly 

 be accounted for in this way. It is doubtful 

 whether Italy is more clear of woods, or better cul- 

 tivated, now than it was in the Augustan age. No 

 part of the world, it is believed, has been cultivated 

 longer or better than some parts of China ; and yet 

 that country is exposed to a degree of cold much 

 greater than is experienced in the coi responding 

 latitudes of Europe. 



The science of astronomy makes us acquainted 

 with phenomena that have a bearing upon this sub- 

 ject. The figure of the earth's orbit round the 

 sun is such that we are sometimes nearer to this 

 great source of heat by 3,000,000 of miles, or one 

 thirtieth of the whole distance, than at others. 

 Now it so happens that we have been drawing 

 nearer and nearer to the sun, every winter, for 

 several thousand years. We now actually reach 

 the point of nearest approach about the first of 

 January, and depart farthest from the sun about the 

 first of July. Whataver benefit, therefore, is de- 

 rived from a diminution of the sun's distance, goes 

 lo diminish the severity of winter; and this cause 

 has been operating for a long period, and with a 

 power gradually but slowly increasing. It has, at 

 length arrived at its maximum, and is beginning to 

 decline. In a little more than ten thousand years, 

 this state of things will be reversed, and the earth 

 will be at the greatest distance from the sun in the 



