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ing as the difference between the July and the January temperatures is increased. Thus the mortality is 8 per cent greater in England than in Scotland, the climate of the latter country being more equable or insular in its character; and it is found on advancing into the continent of Europe, that the more extreme the climate becomes, so much the more is the death-rate increased.
275. The combined effect of the disturbing causes is seen at once if we compare the observed temperature with the normal temperature—that is, the temperature due to a locality in respect of its latitude alone. Dové has published an elaborate set of maps, constructed on this principle, in which he shows by a system of Thermic Isabnormals the deviations from the mean of each month, and of the year, in the different parts of the globe. Those maps show that in January, in the northern hemisphere, the sea and the western parts of the continents are in excess of their normal temperature, but that elsewhere there is a deficiency. There are two centres of excess—one to the north-east of Iceland, amounting to 41° of excess; the other in Russian America, amounting only to 18o. On the other hand, there are two centres where the temperature is deficient—one at Irkutsk, amounting to 41°; and the other west of Hudson Bay, amounting to 27o. In July, the United States of America, Europe, Asia, the Indian Ocean, the north of Africa, and the extreme north of South America, have their temperature in excess, while elsewhere it is deficient. The centres of greatest excess are the north of Siberia, 130.5 of excess; the Red Sea, 11°; and the northwest of the United States, 40.5. The centres of greatest defect of temperature are the entrance to Hudson Bay and the Aleutian Islands, the defect in each case being 11°.
276. While the temperature of each place on the earth's surface is undergoing more or less change from day to day, rising in summer and falling in winter, it might have been supposed that the temperature of the earth itself, considered as a whole, would remain constant, from day to day, throughout the year, since the quantity of solar heat poured into it is the same from day to day. Such, however, is not the case; for from July to December, when the sun is south of the equator, a large amount of solar heat no doubt passes into the latent state owing to the extensive evaporation from southern oceans, and is conveyed by the winds into the northern hemisphere. But from January to June, when the sun is north of the equator, the evaporation being much less, owing to the larger proportion of land in the northern hemisphere, less heat passes into the insensible state in this way. From this it follows that the earth itself has an annual march of temperature, reaching the maximum at the time of midsummer, and the minimum in the middle of winter of the northern hemisphere. Sufficient materials have not yet been collected for determining with accuracy the annual range of the earth's temperature. Professor Dové has, however, attempted an approximate solution of the problem, of which the following is the result :
N. Hemisphere. S. Hemisphere. Whole Earth. Temperature for July, 70°.9
530.6 Do. for January, 480.9
From this investigation the temperature of the whole earth, as shown by the thermometer, would appear to be 8°.1 warmer in July than it is in January.
277. Decrease of Mean Temperature with the Height.The decrease of temperature with the height is perceptibly felt on ascending mountains, and is still more evident in the snow-clad summits which may be seen even in the tropics. For this decrease several reasons may be assigned. In rising from the surface of the earth, we recede from a warm body and approach nearer the cold regions of space. Since comparatively little of the sun's heat is absorbed in passing through the atmosphere, but the greater part reaches the surface of the earth, it is evident that the lower strata of the air, in contact with the earth, will be most heated by the sun's rays, and the upper strata least. But suppose that the same amount of absolute heat (latent and sensible combined) was in the atmosphere at all heights; then since the air in the higher regions of the atmosphere is subjected to less pressure, it occupies a greater space, or the particles are further apart. Now to keep the aërial particles further asunder, more heat will be required to pass into the latent state, and hence the temperature of the higher regions of the atmosphere will be colder. Again, since at elevated situations the atmosphere of invisible vapour intervening between them and space is much less than at lower levels, such situations are less protected from the chilling effects of terrestrial radiation. This supposition is confirmed by the circumstance that the low mean temperatures which universally prevail in high places are chiefly caused by the low temperature during the nights, and during the winter season when terrestrial radiation is the main influence at work modifying the temperature. On the other hand, during calm summer weather, when solar radiation is greatest, the temperature generally rises as high in many situations 1000 feet in elevation as it does at places adjacent, but near the level of the sea. As a practical illustration of this, it may be stated that peaches and apricots ripen in Strathdon, Aberdeenshire, at about 1000 feet above the sea, whereas all along the west coast to the extreme south of Scotland, the heat received from the sun is not sufficient to ripen these fruits.
278. From what has been here advanced, it is plain that the rate at which the mean temperature falls with the height, is a very variable quantity-varying with the latitude, the situation, the dampness or dryness of the air, calm or windy weather, and conspicuously with the season of the year. Accordingly, much diversity of opinion exists regarding the rate of decrease to be allowed in reducing temperature observations to sea-level. For the five years ending with December 1861, the mean annual temperature of Montrose, Forfarshire, 14 feet above the sea, was 47o.2, and of Braemar, Aberdeenshire, 1110 feet high, 430.5. This gives the rate of decrease, 1° to every 296 feet, or nearly lo for every 300 feet, the rate of decrease most generally adopted. But the law, through its different variations, requires yet to be stated.
TEMPERATURE-ITS RELATION TO ATMOSPHERIC PRESSURE.
279. The relation which exists between the temperature of a portion of the earth's surface and the atmospheric pressure at that place compared with the atmospheric pressure of neighbouring regions at the same time is all-important, especially in its bearings on many questions affecting the practical business of life. The relation is a simple one, and admits of clear illustration. The examples will be taken from the recent numbers of the Journal of the Scottish Meteorological Society, and almost all of them will be selected from the weather of 1867—a year so remarkable for illustrations of violent alternations of heat and cold, drought and rain, brilliant sunshine and the dullest weather, as well as for the many great storms which have swept all parts of the globe, that it may well be regarded as the annus mirabilis of meteorology.
280. Examples 1 and 2.—During the severe frost which prevailed in Great Britain from the 1st to the 21st of January 1867, the following are the mean pressures, reduced to 32° and sea-level, at different places from the English Channel to Iceland for these three weeks :
· · ·
29.758 Edinburgh, . . . . .
29.692 Jersey, . .
29.604 Now, since air flows from the region of high to that of low
pressure, these figures show at a glance the immediate cause of the singularly low temperature which prevailed; for it is plain that Great Britain was then in the stream of a powerful polar current descending from Iceland over Western Europe. The geographical distribution of the cold is instructive. Thus in Orkney the temperature of the month was 69.9 below the average, whilst on the Solway Firth it was only 4°.2; on the Moray Firth it was 89.4, but on the Firth of Forth it was only 4°.6 below the average, and at Jersey 3o. Hence places nearer the source of the cold suffered a greater depression of temperature than places further south. The wind during the time blew from the N., N.E., and E. ten days more than the average of the month. The same principle is illustrated by the equally severe weather of March 1867. From the 15th to the 18th, during which the severity of the cold was greatest, the pressure in Iceland was 30.191 inches; in the south of Norway, 29.941 inches; in Orkney, 29.943 inches ; and at Brest, in France, 29.619 inches; and as a consequence, the mean temperature of Scotland fell to 29o.7, being about 11° below the average temperature of the season—a degree of cold which, in our equable climate, is happily of rare occurrence in this or any other season.
281. Example 3.—During the mild weather of February 1867 the following were the mean pressures :
Thus during the month there prevailed over this part of Europe a remarkably strong equatorial current, bringing over Great Britain the warmth of southern latitudes. The mean temperature of Scotland for the month rose to 4° above the average, being absolutely the highest mean temperature for