TEMPERATURE

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TEMPERATURE ; a definite degree of sensible heat, as measured by the thermometer. Thus we say a high temperature, and a low temperature, to denote a manifest intensity of heat or cold. According to Biot, temperatures are the different energies of caloric in different circumstances. Different parts of the earth's surface are exposed, as is well known, to different degrees of heat, depending upon the latitude and local circumstances. In Egypt it never freezes, and in some parts of Siberia it never thaws. In the former country, the average state of the thermometer is about 72°. The following table exhibits a general view of the variation of heat resulting from dif*brence of latitude: Latitude. Places. M. Temp. 86° 30' . . . Wadso, Lapland . . 36°59 56 ... St. Petersburg .... 4048 51 ... Paris.........5441 54 ... Rome.........6130 03 ... Cairo.........7320 00 ... Ocean........7900 00 . . . Ocean........81 The annual variation of heat is inconsiderable between the tropics, and becomes greater and greater as we approach the poles. This arises from the combination of two causes, namely, the greater or less directness of the sun's rays, and the duration of their action, or the length of time from sunrise to sunset. These two causes act together in the same place; that is, the rays of the sun are most direct always when the days are longest, or at the solstice. But while (the season being the same) the rays become more and more oblique, and consequently more feeble as we increase our latitude, the days become longer, and the latter very nearly makes up for the deficiency of the former, so that the greatest heat in all latitudes is nearly the same. On the other hand, the two causes of cold conspire. At the same time that the rays of the sun fall more obliquely, as we increase our latitude, the days become shorter and shorter at the cold season; and accordingly the different parallels are exposed to very unequal degrees of cold: while tropical regions exhibit a variation of only a few degrees, the highest habitable latitudes undergo a change amounting to 140°. Both heat and cold continue to in crease long after the causes producing them have passed their maximum state. Thus the greatest cold is ordinarily about the last of January, and the greatest heat about the last of July. The sun is generally considered the only original source of heat. Its rays are sent to the earth just as the rays of a common fire are thrown upon a body placed before it; and, after being heated to a certain point, the quantity lost by radiation equals the quantity received, and the mean temperature remains the same, subject only to certain fluctuations depending upon the season and other temporary and local causes. According to this view of the subject, the heat that belongs to the interior of the earth has found its way there from the surface, and is derived from the same general source, the sun; and in support of this position is urged the wellknown fact, that, below eighty or onehundred feet, the constant temperature, with only a few exceptions, is found to be the mean of that at the surface in all parts of the earth. But how are we to explain the remarkable cases in which the heat has been found to increase, instead of decreasing, as we descend ? We are told that in the instance of mines, so often quoted to prove an independent central fire, the extraordinary heat, apparently increasing as we descend, may be satisfactorily accounted for in a simpler way :~1. It may be partly received from the persons employed in working the mines. 2. The lights that are required in these dark regions afford another source of heat. 3. But the chief cause is supposed to be the condensation of the air. which is well known to produce a high degree of heat. The condensation, moreover, becoming greater and greater according to the depth, the heat ought, on this account, to increase as we descend; and as a constant supply of fresh air from above is required to maintain the lights, as well as for the purposes of respiration, at the rate of about a gallon a minute for each commonsized light and for each workman, it is not surprising that the temperature of deep mines should be found to exceed that of the surface in the same latitude. This explanation of the phenomenon seems to derive confirmation from the circumstance that the high temperature observed is said to belong only to those mines that are actually worked, and that it ceases when they are abandoned.* If we except these cases, and * See Edinburgh Review, No. ciii, p. 50, &c. that of 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 variation 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 surface, we approach more and more to a region of uniform temperature, but of a temperature much below the former. The tops of very high mountains 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 eignt, 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 diminishes 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 F 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 favor of the alleged change. Rivers which used to be frozen over so as to support armies, and which were expected to be covered hi 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 Italy, where, for the most part, there is no suspension 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 ot comfort. The ancient custom of suspending 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 our own country, also, the lapse of two centuries has produced a sensible melioration. When New Eng land 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 precarious 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, however, 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 posed mitigation of winter has usually been ascribed to the extirpation of forests, and the consequent exposure 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 favorable change experienced in the New England and the Middle States may, it is thought, be referred to this circumstance. But the alteration that is observed 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 cultivated, 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 corresponding latitudes of Europe. The science of astronomy makes us acquainted with phenomena that have a bearing upon this subject. 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. Whatever benefit, therefore, is derived from a diminution of the sun's distance, goes to 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 middle of winter, and at the least distance in the middle of summer. We are speaking, it will be observed, with reference to the northern hemisphere of the earth. The condition alluded to, to take place rfter the lapse of ten thousand years, is already fulfilled with regard to the southern portions of our globe, since their winter n.ApK'iis at the time of our summer. How f.'.r ih? excessive cold which is known to jiwail about cape Horn and other high Hv;!hcrii latitudes may be imputed to this, w ¦ are not able to say. There is no doubt about the south pole than about the north. Commodore Byron who was on the coast of Patagonia Dec. 15, answering to the middle of June with us, compares the climate to that of the middle of winter in England. Sir Joseph Banks landed at Terra del Fuego, in lat, 50°, Jan. 17, about the middle of summer in that hemisphere; and he relates that two of his attendants died in one night from the cold, and the whole party was in great danger of perishing. This was in a lower latitude by nearly 2° than that of London. Captain Cook, in his voyage towards the south pole, expressed his surprise that an island of no greater extent than seventy leagues in circumference, between the latitudes of 54° and 55°, and situated like the northern parts of Ireland, should, in the very height of summer, be covered many fathoms deep with frozen snow. The study of the stars has made us acquainted with another fact connected with the variable temperature of winter. The oblique position of the earth's axis with respect to the path round the sun, or what is technically called the obliquity of the ecliptic, is the well known cause of the seasons. Now this very obliquity, which makes the difference as to temperature between summer and winter, has been growing less and less for the last 2000 years, and has actually diminished about one eightieth part, and must have been attended with a corresponding reduction of the extremes of heat and cold. It still remains for us to inquire how it happens that the extremes of heat and cold in the U. States are so much more intense than they are in Europe under the same parallels. The thermometer, in New England, falls to zero about as often as it falls to the freezing point in the same latitude on the other side of the Atlantic. The extreme heat of summer also is greater by 8° or 10°. This remarkable difference in the two countries, as to climate, evidently arises from their being situated on different sides of the ocean, taken in connexion with the prevalence of westerly winds. With us, a west wind is a land wind, and consequently a cold wind in whiter and a warm wind in summer. The reverse happens on the opposite shore of the Atlantic. There, the same westerly current of air, coming from the water, is a mild wind in winter, and a cool, refreshing breeze in summer. The ocean is not subject to so great extremes of heat and cold as the same extent of continent. When the smvs rays faL upon the solid land, they penetrate to only a small depth, and the heat is much more accumulated at the surface. So, also, during our long, cold nights, this thin stratum of heated earth is more rapidly cooled down than the immensemass of the ocean through which the heat is diffused to a far greater depth. At a sufficient distance from land, the temperature of the sea, in the temperate latitudes, is seldom below 45° or above 70°; that is, the ocean is exposed to an annual change of only 25° or 30°, while the continent, in the same latitude, is subject to a variation of 100° or more. We are confirmed in the cause here assigned for the excessive severity of our climate, by finding that the parts of China, situated like the Atlantic states, have a similar climate; and that the western coast of this continent, without the benefit of much cultivation, enjoys the same mild temperature that belongs to places similarly situated in the western parts of Europe. The principal causes of the unfavorable character of our climate seem, therefore, to be of a permanent nature; and, although it is somewhat meliorated, and may, in time to come, be still more so, yet we are probably never destined to enjoy, in New England, the fine seasons and delicious fruits of the corresponding latitudes of Europe.For more information on the natural history of the weather, see the American Almanac for 1832, from which this article is taken.