drying up of the river Scamander in the Troad, and the contracting of the Euphrates in its channel, may be referred to as illustrations of the same effect of the cutting down of forests, and of diminished vegetation.

183. (2.) On Water.-When the solar heat falls on water, it is not, as in the case of land, arrested at the surface, but penetrates to a considerable depth below the surface. Also, since water is a bad conductor, the heat cannot penetrate lower down by conduction as it does through land, but can only be communicated by the agitation caused by currents or winds. Thus the heat received daily by the ocean is diffused through a considerable depth of the water floating near the surface.

184. Specific Heat of Water in relation to Climate.-The specific heat of a substance is the number of units of heat required to raise the temperature of one pound of it by one degree. Thus the same amount of heat that will raise one pound of water one degree will raise one pound of mercury 33°. If therefore we call the specific heat of water 1000, the specific heat of mercury is only 33. Of all known substances water has the greatest specific heat. As compared with the specific heat of the soil and rocks which compose the earth's crust, it is in the proportion of about 4 to 1. It follows that the surface of the sea cannot be raised to nearly the same degree of heat by the sun's rays as that of the land; and that when the temperature is falling the sea cools much more slowly than the land. Thus, while the surface temperature of the soil has been frequently observed to be as high as 140°, the surface of the sea has in only a few cases been observed to exceed 85°. It is to the great specific heat of water that the enormous effects of the ocean in modifying climate are chiefly due.

185. The temperature of the sea is also diminished by the process of evaporation constantly going on from its surface.

186. Mode of Observing the Temperature of the Surface. -It is necessary to distinguish between the amount of solar heat which arrives at the surface of the ground, and the tem

perature to which the surface is raised by the heat which thus falls upon it. It is with the latter-viz., the temperature of the surface—that as practical meteorologists we have principally to do. In attempting to ascertain the effect of this element on the climate of such a country as Great Britain, which has so large a portion of its surface covered with grass or analogous plants, it is evident that it is to the temperature of grass that we should direct our attention. In vine-growing countries, where a great part of the surface of the soil is exposed to the sun, and in countries whose surfaces consist of rocks or tracts of sand destitute of vegetation, the temperature of the surface exposed to the solar heat will be very different from that of Great Britain. Care should be taken to place the thermometer in positions which will enable the observer to ascertain the temperature of the general surface. The temperature to which the leaves of plants, the ears of wheat, and other objects of agricultural and horticultural produce, are raised by the direct rays of the sun shining on them at different heights and in different climates, though a subject of great practical importance, has not yet been inquired into.

187. The instrument used in observing the temperature of the surface is a Maximum Black-bulb Thermometer, which is an ordinary maximum thermometer, having its bulb covered with a thin coating of lamp-black, and quite exposed to the sun and air. It should be placed horizontally over short grass, so near the ground as just to be above the grass, and in such a position that the sun shall shine directly on the bulb for as large a portion of the day as possible. This is best accomplished by making the bulb project a little beyond the framework of the thermometer, and point directly to the sun at twelve o'clock. It is only by attending to such directions as these that any tolerable uniformity can be obtained among black-bulb observations. Observations do not admit of comparison where the exposure and duration of exposure of the bulbs of the thermometers to the sun, their height above the surface, and the character of the surface, are different.

188. It frequently happens in cloudy weather, when there

is mist or drizzle, that these exposed black-bulbs register a lower temperature than thermometers in shade, being for the time almost under the conditions of the wet-bulb thermometer. But in such cases the difference is always small. Again, during calm, foggy, or dull weather, suddenly following cold, such as frost in winter, it often happens that the exposed maximum thermometer is several degrees lower than the protected one-a difference due to the proximity of the exposed thermometer to the cold ground, as compared with the protected thermometer four feet above it.

189. Black-bulb thermometers enclosed in thin glass tubes, hermetically sealed, are used for the purpose of ascertaining approximately the amount of the solar heat falling on the surface of the earth.

190. It would be interesting to inquire whether the heat received by the earth from the sun varies from year to year, and if so, to what extent. On account of the obstruction offered to the sun's rays by the vapour of water, and our necessarily imperfect knowledge of the varying quantity of vapour in the atmosphere interposed between the thermometer and the sun, it would be useless to conduct this inquiry at any place in Great Britain, or indeed anywhere except at some place within the tropics, and situated at a great height above the sea, where the amount of aqueous vapour is small and nearly constant. Perhaps the best situation would be Bogota, South America, lat. 4° 43′ N., and 8727 feet above the sea; or Quito. A black-bulb thermometer hermetically sealed would suit the purpose. From the importance of the inquiry, and from the simplicity of the required observations, which need not occupy more than one minute a-day, it is to be hoped some member of the medical profession, or some other person resident in Bogota, will undertake the observations.

191. TERRESTRIAL RADIATION.-Since the mean temperature of the earth remains practically the same, it is evident that the earth must part with the enormous quantity of heat which is poured on it from the sun from day to day. It is by its own radiation into space that the earth parts with this

heat,- -a process which is continued by day as well as by night. During the day the heat radiated from the earth is less than that received from the sun, and as long as this continues the temperature rises. But as the sun sinks near the horizon, its beams falling more aslant, less heat begins to fall on the earth than is given off by radiation. For a time the loss thus sustained by the surface temperature is compensated for by heat rising to the surface from the heated strata immediately below it. This mode of making up the loss lasts only a short time, because the escape of heat by radiation is much more active than the propagation of heat by conduction. As soon, then, as the radiation from the surface exceeds all sources of supply, the temperature begins to fall, and continues to do so till it is arrested, or till the sun again returns. When the chilling of the surface goes on, it receives additions of heat from the air in immediate contact with it, and from the air above by radiation downwards from it. Since, then, during night, and during the winter, the cooling of the air is brought about by contact with the chilled surface of the earth, it follows that up to a certain elevation the temperature of the air will increase with the height.

192. There is another source from which the chilled surface receives accessions of heat-viz., when the air at the surface is cooled down to the temperature of the dew-point; for when this point is reached dew is deposited, and latent heat is given out, as happens in every case when gaseous bodies are converted into fluids.

193. When the sky is wholly or partly covered with clouds, a large part of the heat radiated from the earth to the clouds is radiated back again by the clouds to the earth; and thus on cloudy nights the loss of heat by radiation is not so great as on clear nights, and not so great when the clouds are low as when they are high.

194. But the vapour of water obstructs radiation, not only when in a visible form as clouds, but also when it is diffused in an invisible state through the air; and hence the drier the air is, the colder is the temperature during night.

195. The surface of the earth is cooled by radiation to a greater degree during calm than during windy nights, because, owing to the agitation caused by the wind, it is warmed by contact with the air of the upper as well as of the lower strata of the atmosphere. The amount of heat radiated is the same, but the effects are not, as on calm nights, chiefly confined to the surface, but are more generally diffused through the air itself.

196. The degree to which the temperature of an object is reduced by radiation, is proportioned to the amount of sky exposed to the free view of the object. Thus if houses or trees be near-that is, if they cut off a part of the sky from the field of view-the temperature will not fall to the same extent. The degree to which this remark holds is very striking when first brought under the notice of an observer. A sensitive mercurial thermometer will be seen to rise at once by the heat radiated from his person, as he approaches to examine it. Nay, even the holding up of the fragment of a wooden rail at a distance of some yards from the thermometer, has been observed perceptibly to raise its temperature. A cloud passing over raw wool much cooled by radiation, has been observed by Mr Glaisher to raise the temperature of the wool 1° per minute, and 15° in the quarter of an hour.

197. Means of Measuring the Cold of Radiation.-A thermometer is exposed in the focus of a concave mirror, polished within and without, with the concavity pointing upwards, in order to cut off all view of the earth. The only heat a thermometer placed in these circumstances can receive, is from the radiation of the air, and from the condensation of moisture. A thermometer so exposed always indicates a lower temperature than thermometers exposed on the surface of the earth.

198. But since it is the temperature of the surface of the earth which it is most important to know, owing to its intimate connection with the temperature of the air, a much simpler instrument is adopted. This is the ordinary Minimum Black-bulb Thermometer, which is a common minimum ther