strument given at 160° and upwards, which is about the temperature of the engine room of a steamer off the coast of Aden. Can it be fair to quote any such reading as the true indication of solar heat in this country? It very rarely happens that bodies exposed to the direct rays of the sun attain, in this latitude, a higher temperature than 120°; but De Saussure, by means of a little box constructed of wood, and lined with charred cork, obtained a temperature of 221°, the temperature of the external air not being higher than 75°. Professor Robinson, of Edinburgh, by means of a somewhat similar contrivance, frequently obtained an indication of 230°, and once, under favourable circumstances, 237°! The sun's rays, therefore, not concentrated in any way, but merely accumulated, can be made to indicate a temperature above that of boiling water. In a course of experiments made some years ago by Colonel Sabine the remarkable fact was elicited that the intensity of the sun's rays (the excess of temperature above that in the shade) is least at the equator, and increases with the distance from it. In July, 1822, the maximum effect of a nearly vertical sun was 57°. On Melville island in March, a thermometer placed in the sun, at a distance from the ship's side, and the weather calm and clear, indicated an effect of 55°. At the time this experiment was made the temperature in the shade was 25°, and in the sun, + 30°. The greatest effect in the vicinity of London in the same month upon a thermometer covered with black wool was only 49°. In Captain Scoresby's account of the Arctic regions are several remarks upon the intensity of the sun's rays. He says, "The force of the sun's rays is sometimes remarkable where they fall upon the snow-clad surface of the ice or land, for they are, in a great measure, reflected without producing any material elevation of temperature; but when they impinge on the black exterior of a ship; the pitch on one side occasionally becomes fluid, while ice is rapidly generated at the other; or, while a thermometer placed against the black paint work on which the sun shines indicates a temperature of 80° or 90°, or even more, on the opposite side of the ship a cold of 20° is sometimes found to prevail." In his last voyage to Greenland the radiating power of the sun was found to be nearly 80° in the month of April. In Humbolt often endeavoured to measure the power of the sun between the tropics by two mercurial thermometers, one of which remained exposed to the sun, while the other was placed in the shade. The difference of their readings never exceeded 6.6°, while sometimes it was not more than one or two degrees. But to return to table 16. In column E are given the mean results of the second series of observations upon solar radiation, which extends over a period of ten years. The removal of the thermometer from four feet to four inches above the surface of grass caused a very large increase in the readings in column E over those in column D. It varied from rather more than 3° in January to 13° in April, 11° in May, and nearly 10° in June and July; the greatest variation occurred in April. column F are given the results of twenty-five years' observations upon the mean monthly temperature of radiation upon the surface of short grass. In columns G and H are given the differences of temperature respectively between columns F D and F E. The latter shows the enormous range of temperature to which vegetation is exposed in this locality, which will be still more apparent by consulting table 15, where may be seen the extraordinary ranges of temperature which occurred in several months. Among the numerous meteorological registers which have been kept in this country during the last twenty years we rarely find that regular observations have been made of solar and terrestrial radiation. So great is its value in the general economy of nature, that without a certain proportion of its genial influence the fruits of the earth do not come to perfection. Moreover, the public health is affected to a greater or less extent by a peculiar type of disease which may be referable to a deficiency of intensity in the solar rays, the full influence of which is an advantage, for which no elevation of temperature, under a cloudy sky, can compensate. But if the power of solar radiation, when tested by the work it does, is inconceivably enormous, its exact adjustment to the measure of that work is no less wonderful. There is just enough of radiant force poured down upon the earth to keep its intricate machinery in proper motion-no less, no more. The latent and specific heat of water, with all its other properties, are each exactly fitted to the amount of heat received. The geological changes of long bygone ages have issued in results symmetrical and harmonious with cosmic changes and arrangements made in still earlier times, and both combine to give this little atom in creation the bounteous store of blessings which it enjoys. The matter of the earth and of the sun are common workers together to one end—the lighting, warming, moistening, and feeding of the living things which dwell upon our planet. It might be expected that the sun's greatest power would follow the course of its declination. Such, however, is not the case. Both the period of difference and the amount of greatest intensity appear to vary with the height of the point of observation above the soil. Thus in the first series of observations the greatest intensity of solar heat, above the temperature in the shade, occurs in September, while in the second series it happens in July. The monthly mean highest temperature occurs also in July, but the reading for August exceeds that for June. At the distance of four feet from the ground both the increase and decrease of solar power during the respective seasons is very gradual and regular, with the exception of the month of September, when it arrives at its maximum for the year. At the lower elevation of four inches the greatest intensity occurs in July. The foregoing tables will, I trust, prove a valuable standard of comparison for determining the causes of fruitful and unfruitful seasons, as the difference of range of temperature which has existed in the corresponding months of consecutive years has been very remarkable. It is a subject for regret that the number of observers engaged in the investigation of this most important branch of meteorological science is comparatively few. Careful enquiries made respecting the power of different varieties of soil, to absorb and radiate solar heat, might lead to the discovery of important peculiarities of local climate in reference to the origin of certain endemic diseases. CHAPTER III. IIYGROMETER-MOISTURE IN THE ATMOSPHERE-RAINFALL OF THE SEVERAL MONTHS, SEASONS, AND YEARS-DIREC- TION If a mass of air be gradually cooled, it will descend to a degree of temperature at which it will be saturated by the quantity of vapour contained in it. This temperature is called the "Dew Point." Dew is the visible form of aqueous vapour, which it assumes under circumstances favourable to its production. This occurs when the sun is absent, the sky clear, and when the atmosphere, replete with moisture, is chilled by contact with any surface colder than itself. The interposition of clouds almost invariably prevents the continued formation of dew. For a long time it was supposed that dew dropped "like gentle rain from heaven," and it was not until a comparatively recent period that any other theory was believed. The dew drop is familiar to every one. Resting in luminous beads upon the surface of leaves, or pendant from the finest blades of grass, or threaded upon the floating lines of the gossamer, it varies in size from the diameter of a small pea to a very minute atom. Each of these, like the rain-drop, have the properties both of refracting and reflecting light; and hence, as from so many prisms, the unfolded rays of the sun are sent up to the eye in brilliant colours similar to those of the rainbow. When the sunbeams traverse horizontally a very thickly bedewed grass plot, these colours are so arranged as to form an iris or dewbow; and if we select any one particular drop for observation, and steadily regard it while we change our position, we shall find the prismatic colours follow each other in their regular order. The deposition of dew was first satisfactorily explained by Dr. Wells. When the sun is below the horizon, or for a short period before sunset, bodies upon the surface of the earth, exposed to the aspect of a clear sky, cool by the radiation of the particles of heat absorbed, and at a more rapid rate than the atmosphere. The air in immediate contact with these bodies, replete with humidity in the form of transparent aqueous vapour, is chilled by their cold embraces; and, owing to the increase of its density, it becomes incapable of holding in suspension the moisture with which it is charged in the same quantity as before. The surplus is, therefore, disengaged, and appears upon the surface of the refrigerating object in globules of dew. It is essential to this process that the night should not be a cloudy one; because when the sky is overcast, the radiant heat proceeding from the surface of the earth, and which would otherwise go off into free space, is interrupted by the clouds, and returned by them in sufficient quantity to prevent the decrease of temperature necessary to compel the atmosphere to surrender a portion of its hoard of aqueous particles. On nights which are perfectly cloudless, therefore, the deposition of dew is greater than when the sky is partially overcast; on those which are both cloudy and windy there is none whatever formed; but a gentle motion in the air on a clear night is favourable to its production in the greatest quantity, by bringing fresh portions of the atmosphere, laden with moisture, into contact with the colder bodies at the surface. In opposition to the moisture of dew, that of mists is deposited upon all substances exposed to it alike; while another distinction is, that the moisture of mists exists previous to any deposition, in a visible state, and is produced quite independent of the bodies which receive it. With the object of ascertaining the amount of moisture existing in the atmosphere of this district, I made very careful observations of the temperature of the dew point during eight consecutive years, the monthly results of which, from observations taken daily at 9 a.m., are given in Table 17. G |