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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 temperatures, at whatever hour these may chance to happen from day to day. This is the true daily range of temperature. In some books, however, 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 self-registering thermometers over the earth, the latter of these two is more widely known than the former.

163. 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 indirect radiation, and also to their height above the ground. For if the louvre-boarded box containing the thermometers be placed behind a wall to which the sun has little or no access, the range will be several degrees less than in an open situation on which the sun shines during most of the day ; the range will be greater in a hollow than on a knoll ; greater over a soil of sand than of loam ; greater over black earth than over 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 be 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. On considering these different conditions under which temperature may be observed, it is evident that if any of the above conditions were adopted as the standard the results could not be comparable. The arrangement least open to objection is that already recommended in par. 136 to 138, which has the great merit of being easily carried out in all places. An objection, sometimes urged against it, is this : The wood of the louvreboards being a bad conductor of heat retains during night part of the heat acquired during the day, and as a consequence the minimum thermometer does not fall so low during night as it otherwise would do. Grant there is something in this objection, the error must be small in the case of boxes in which the ventilation is so very complete, and which are largely protected from the sun's rays by a thick coating of white paint, and by a double system of louvre-boards. But since, as already stated, par. 136, a perfectly unobjectionable arrangement is impossible, the above is the least objectionable, and to be preferred to the practice of exposing the thermometers in a box open in one direction and placed against a wall. For, from experiments made with a number of boxes so constructed, the indications are found to vary with the amount of sky exposed to the thermometers, and their proximity to walls, trees, or other objects which radiate their heat to the thermometers; whereas with boxes louvre-boarded all round, such variation does not occur, or if it does, only to a very slight degree.

164. 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 March Hall, Edinburgh :

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165. The above analysis, continued from month to month through a number of years, is sufficient as a basis on which this 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 that exercise a powerful influence on vegetation and on health.

166. 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 mean summer 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 the staple products of the respective countries.

167. 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 analyse their observations and

publish the results. In this way a storehouse of the most valuable information would be collected, by which agriculturists 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 greatest advantage that can be procured from a change of climate.

CHAPTER V.

TEMPERATURE-SOLAR AND TERRESTRIAL RADIATION.

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

169. 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.

170. 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 atmosphere which rests 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, that frosts and extreme temperatures do not penetrate so far down into light soils as into heavy soils. In Scotland, during the past nine years, the

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