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it otherwise would, the mean of the maximum and minimum temperatures gives a mean temperature a little higher than the true mean temperature of the day. In dry climates, where little or no dew falls, these two means may be expected to approach each other. This is the probable explanation of the small difference at Nertchinsk during summer, and at St Petersburg during winter. Again, in cloudy rainy weather, when the daily temperature falls less frequently to the dew-point, the difference between the two means may also be expected to be less. During the rainy season at Calcutta, which lasts from about the middle of May to October, the difference is less than during the dry season; the observations at Pekin, the climate of which resembles that of Calcutta, point in the same direction. At the Scottish stations, the smallest differences between the 9 A.M. and 9 P.M. and the maximum and minimum observations occur in the western and northern islands, where the climate is eminently cloudy and rainy. An examination of this whole question by observations of the maximum daily relative humidity would go far to settle the point.
155. The more this subject is considered, the more caution will one be inclined to use in applying “ Corrections ” (usually so called) of temperature, derived from the long averages of one place, to the long averages of another place, with the view of determining the true mean temperature. When this is attempted for averages of a few years, and a fortiori for averages of single months, it results in frequent error and confusion, and renders the examination of such “corrected ” observations at best a very perplexing, and, in many cases, a hopeless inquiry. A few examples will illustrate this. At Calcutta the excess of the mean of the maximum and minimum temperatures was 1°.4 in January 1866, but in the same month in 1863 it was only 0°.7; in May 1864 it was 1o.6, in May of the following year it was only 0°.7. Hence, if the correction was applied for May 1864, the supposed true temperature would be 0°.6 above the true temperature, and in the following year it would be 0°.3 below it. 156. At Dundee, in June 1867, the mean of the maximum and minimum temperatures for the month was 54°.8, and the mean of the 9 A.M. and 9 P.m. observations was 54o.4. Suppose now, we apply to these the Greenwich corrections, in order to arrive at the true mean temperature of the month : From the self-registering thermometers the mean temperature would be 53.0; and from the 9 A.M. and 9 P.M. observations the mean temperature would be 54°.1; and, further, if we combine these two results, the mean temperature will be 530.6. Is any one of these correct? The chances against it are at least as many as those for it. Judging from the greater prevalence than usual of cloud and east winds, and being guided by the figures in the table given above, the mean temperature was probably 54o.4. Owing to the uncertainty in which this question is at present involved, the Council of the Scottish Meteorological Society, in their abstracts of observations issued quarterly, publish the means without applying any corrections whatever to them —a practice which we think commends itself to meteorologists.
157. It is from the vague and uncertain meaning attached to the phrase mean temperature, that not a few writers of Local Climatologies are led to claim for particular watering-places a general amenity or mildness of climate over other sanatoria, which has no existence in fact. It is not by their excess of mean temperature, but by their situation and local surroundings, that our principal British sanatoria have their repute. Thus Bridge of Allan owes its celebrity not to any difference in its mean temperature, which doubtless is the same as that of the district around it, but to the well-wooded heights behind it, sheltering it from the east winds, and to the sloping nature of the ground on which it is built. From its sheltered position, the east wind passes over it at a reduced rate and force, and there being consequently less evaporation, our bodies are deprived of less heat, and thus the sensation of greater warmth which we feel is a reality. Again, being built on rising ground, the cold air flows down to the valley below it, so
that excessive cold rarely occurs. Ventnor, in the Isle of Wight, possesses similar advantages in the protection afforded by the Undercliff, and in its proximity to the sea, towards which its cold heavy air flows, and being there counteracted by the higher temperature of the sea, low temperatures rarely occur. These advantages not only give greater facility for open-air exercise, but by offering, to a great extent, protection from the hurtful effect of the weather which is most trying to invalids—viz., east winds, and the severe cold of winterare far more to be prized than would be an annual temperature several degrees higher. If meteorologists would investigate the drying qualities of the air in different parts of the same town when the east wind prevails, and the march of the temperature, also in different parts of the same town, when the weather is considered to be hurtful or beneficial to particular diseases; and if at the same time the physician would note the occurrence of particular diseases in his district, their commencement, culmination, and cessation,-solid contributions would be made to meteorology, which could not fail to result in alleviating suffering and prolonging life. It is in this direction that the question of local climate should be prosecuted.
158. Value of Extreme Temperatures.-Self-registering thermometers are of great practical value in recording the extreme temperatures, which in reference to their effects on health and vegetation must be regarded as most important elements of climate. The subject of extreme temperatures is one of paramount importance, especially where the transitions of temperature are sudden and violent. In the north-western parts of the United States of America the temperature in spring often rises to 830 during the day, and falls to freezing during the night. Under such a climate the vital functions of plants the tissues of which abound in sap are called into activity during the day ; but the sap being frozen during the night, the vessels containing it are ruptured by expansion, and the plant, if not totally destroyed, is so seriously injured that its successful cultivation becomes precarious and uncertain. The same risk is not incurred in such places as Great Britain, where the seasons shade into each other by nice and almost insensible gradations. It is, however, everywhere desirable to arrive at some definite information as to the probability of occurrence of certain extreme temperatures that may take place in the different seasons, and this can only be done by observations of extremes carefully made and recorded from day to day.
159. Importance of resolving Mean Temperature into the Extremes which compose it.—Every mean temperature may be considered as a composite element, made up of the mean temperature of the day and the mean temperature of the night. Hence the same mean temperature often stands for two things essentially different. Thus, Madrid in the centre of Spain, and Menton on the Gulf of Genoa, had the same mean temperature of 72°.8 during September 1865. But the climates of the two places were widely different; for the temperature at Madrid during the warmest period of the day was on an average 86o.2, whereas at Menton it was only 77o.6 ; and during the coldest period of the night the temperature was 59o.5 at Madrid, whilst at Menton it was as high as 68.0.
160. Again, in the same country and in the same month, but in different years, the same mean temperature when resolved into the extremes that compose it represents very different things. Thus, the mean temperature of Scotland in August 1860 was 54o.4, and in the same month of 1864 it was also 54°.4 ; but in 1860 the mean of the highest day temperatures was 60°.8, whilst in 1864 it was 62°.5. This higher day temperature in 1864 was the chief cause of the productive harvest of that year, and the lower day temperature of 1860 was the chief cause of that year's deficient harvest, and yet the mean temperature of both years was the same. Thus, in considering the relations of mean temperature to health or to agriculture, it is most essential to know the separate elements which compose it.
161. Range of Temperature.—This points out the importance of considering the daily range of temperature, or the difference between the extreme day and night temperatures. Additional interest is added to the subject by the consideration that the rate of mortality is to a very large extent determined by the range of temperature of the climate. Everywhere the range of temperature is least in winter, augments rapidly in March and April, reaches the maximum in May or June, continues high during summer, and diminishes rapidly in October and November to the minimum in the winter months. As regards climates, it is least in wet climates, and in the tropics and polar regions; and greatest in dry climates and in temperate countries. Hence it is less in Ireland than in Scotland, greater in England than in both these countries, and still greater on the continent of Europe. In Shetland, Orkney, and the Hebrides, whose climates are perhaps the most strictly insular in Europe, the summer range is only about 10°; on the west of Great Britain it rises to 12o and 14° ; in the central districts to 15°; and in the south to 20°. At Paris, Utrecht, Vienna, and other places on the Continent, the range is still higher. In the dry climate of Jerusalem, in Syria, it amounts to a mean of 22°.5 from May to October inclusive, and during October 1865 the mean was 24°.6; in the still drier climate of Madrid a range of 27°, and in one month 31° was observed in 1865. A similar high range is frequently recorded in the Russian and Siberian steppes. At Trivandrum, in Southern India, during the dry season in January, the daily range is about 17°, but during the rainy season in July it is only half that amount. The greatest differences among the months as compared with each other, are observed in the polar regions. Thus, in Spitzbergen and Boothia Felix, the range in winter varies from 1° to 0°; in May, when the sun has reappeared and continues to rise and set, it is about 14° ; but in July, when the sun does not set, the range is only 10°.