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 it was 59o.5 at Madrid, whilst at Menton it was as high
as 68o.o.

112. 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
549.4, and in the same month of 1864 it was also 54o.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
theślower 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 agriculture, it is most essential to know
the separate elements which make up the mean temperature.

113. 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 it 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°; and in the central districts to 15°; and in the south to 20o. At Paris, Utrecht, Vienna, and other places on the Continent, the range is still higher. In the dry climate of Jerusalem, in Syria, it amounted to 23° in four months of 1864; 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

At Trevandrum, 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

Siberian steppes.

1

of

monthly differences are observed in the polar regions. Thus, in Spitzbergen and Boothia Felis, 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%.

114. But in comparing climates there is not a more misleading element than range, unless care be taken to be informed of the circumstances under which the range was obtained. Thus, in the above instances the term has been employed to signify the difference between the mean of the maximum and the mean of the minimum temperature, at whatever hour these may chance to happen from day to day. This is the true daily range of temperature. In some books, howerer, the daily range of temperature means the difference between the mean of the coldest and the mean of the warmest of the twenty-four hours, and is therefore always less than the former. Owing to the restricted use of selfregistering thermometers orer the earth, the latter of these two is more widely known than the former.

115. Again, the amount of the range depends to a great extent on the degree to which the thermometers are protected from, or exposed to, direct and indireet radiation, and also to their height above the ground. For if the louvre-boarded box is behind a wall to which the sun has little or no access, the range will be sereral degrees less than if it was in an open situation on which the sun shines during most of the day; the range will be greater urer black earth than orer grass; greater over long grass

than over short grass ; greater with than without a proper ventilation to the box ; greater the more the louvre-boards are apart; greater if the box is always kept open to the north than if louvre-boarded all round ; and greater the nearer the thermometers are to the ground. Hence the extreme desirableness of uniformity of observing in all parts of the world. It may be noticed here as remarkable, that in most of the above cases, though the range differs greatly, yet the mean annual temperature is found to be nearly the same.

116. Of the daily registered highest and lowest temperatures, the following, for July 1866, is a specimen of the monthly abstract published by the Scottish Meteorological Society, as regards Edinburgh :

Degrees.
Highest temperature in month,

82.7
Lowest do. in month,

45.0 ,

37.7 Greatest daily range,

29.7 Mean of all the highest

65.5

117. The above analysis continued from month to month through a number of years, is sufficient as a basis on which the main element of climate may be established. But there are other points of the greatest practical importance in climate which require a more searching analysis for their elucidation, such as the frequency of occurrence at different seasons of certain temperatures exercising a powerful influence on vegetation and on health.

118. The most important question in connection with low temperature is the occurrence of frost. Both to the farmer and to the physician it is of the utmost moment to know how often and with what severity frost may be expected to occur, and when for all practical purposes it ceases to occur, or happens so seldom as to cause no alarm, and call for no precaution on the part of those whose interests

may be affected by it. On the other hand, the question to be inquired into in an investigation of high temperatures as affecting agriculture, is not so much injury received as advantage gained by their occurrence. The growth and maturing of crops depend chiefly on the heat they receive from the sun. And in countries such as Scotland, whose temperature but barely exceeds the minimum heat required for the proper ripening of the staple objects of agriculture, the inquiry becomes invested with a peculiar interest, especially in examining places and localities differing in latitude, proximity to the sea, exposure, and elevation. As regards Great Britain, if the day temperature rises occasionally to 65°, the degree of heat thus received by the grain crops may be looked upon as sufficient for their growth up to the period of flowering ; but after this a higher temperature is required, and a frequent day temperature of 70° is necessary to produce the finer qualities of wheat and barley. For other countries different temperatures would require to be investigated—namely, those essential to the successful cultivation of their staple products.

119. In order to render tables of this description practically as well as popularly useful, the occurrence of the critical temperatures should be given separately for every week of the year. Little has yet been done in preparing such tables, owing to the heavy tedious labour in compiling them. Perhaps Scotland is the only country whose climate has been thus examined; it is to be hoped that observers in other countries may be induced to publish the results of their own observations. In this way a storehouse of the most valuable information would be collected, by which agri

culturists might arrive at a knowledge of the character of the climate they have to deal with; and by which physicians might reason with more certainty than at present regarding the spread of diseases, the rates of mortality peculiar to different countries, and the places to which invalids may be sent, so as to enjoy the greatest safety or receive the most advantage that can be procured from a change of climate.

120. The interchange of temperature among bodies takes place by conduction, convection, and radiation.

121. Conduction.-The communication of heat by conduction proceeds from particle to particle, and implies contact with, or very near approach to, a hotter body. As a class, metals are the best conductors ; solids are better conductors than liquids, and liquids better than gases, which are the worst conductors.

122. The most important illustrations of conduction in meteorology, are the propagation of the changes of temperature downwards through the earth's strata from the surface as it is heated during the day or cooled during the night; and the communication of the same changes of temperature to the lowest stratum of the atinosphere resting on the surface. As regards the relative conducting powers of different substances, dense soils, or soils having their particles closely packed together, are much better conductors of heat than loose porous soils, because the latter imprison large quantities of air in the interstices between the particles, thus diminishing the conducting power of the soil. From this it follows that light loose soils are subject to higher temperatures, and to a greater degree of frost, near the surface, than dense heavy soils; but, on the other hand, frosts and extreme temperatures do not penetrate so far down into light as into heavy soils. In Scotland, during the past nine years the temperature at three inches below the surface has fallen to 26°.5 in loose sandy soils, and at a depth of twelve inches the freezing point has only once been observed. But in clay soils, at three inches the lowest is 28°, whilst at twelve inches the temperature often falls to freezing, and even at twenty-two inches 32° has once been recorded.

123. Damp air is a much better conductor of heat than dry air. Damp air consequently feels colder than dry air of the same temperature, because it takes away the heat from our bodies more