fore expands 0.3665 of its bulk from 32° to 212°, and its expansion is uniform between these points. A cubic foot of vapour at any other temperature may be calculated from the following formula : 1.3665 × 258.4 gr. × elastic force of vapour at temperature 30 + 0.0020361 × t − 320) 2. Table 9 in the Appendix contains the weight of a cubic foot of air, at the Royal Observatory, Greenwich. 3. Degree of Humidity. With a knowledge of the amount of vapour which saturates the air at different temperatures, and the amount existing in the air at the time of observation, we are enabled to determine another important element, viz., the degree of humidity of the air. In calculating the numbers, saturation may be assumed as 100, or as unity, and air, without moisture, as zero. The degree of humidity is found by dividing the quantity of vapour at the temperature of the dewpoint, by the quantity which would have been present had the air been saturated. The numbers thus found at the Royal Observatory, Greenwich, are shown in Table 10 in the Appendix. 4. Air is most humid at night: as the sun ascends the temperature increases more rapidly than water evaporates to keep the same degree of humidity; the atmosphere, therefore, becomes less and less humid. This is particularly the case in summer, when the temperature of the dew-point is for some hours nearly stationary, whilst the temperature of the air is increasing. When evaporation commences in the morning with the increase of temperature, vapour accumulates near the surface of the soil, till the air becomes heated and the daily ascending current of air sets in. It then ascends and spreads as long as the ascending current continues. Towards evening, when the temperature of the air is decreasing rapidly, the ascending current is checked, then ceases, and gives place to the descending current of night. Therefore there is a rapid increase of evaporation and decrease of humidity during the day, and a rapid increase of humidity during the evening and night hours. 5. Weight of a Cubic Foot of Air.—A cubic foot of dry air at 32°, and under the pressure of 30 inches of mercury, weighs 566.86 grains. As the weight of air varies inversely as the volume, a cubic foot of air at any other temperature, under the same pressure, will be found by dividing 566.86 by the number expressing the volume of dry air after expansion from heat. Reckoning the volume of dry air at 32° as unity, for instance, required the weight of a cubic foot at 60°? We have seen that the expansion of volume is 0.0020361 for every degree of heat; this multiplied by (60°-32) 28=0.05701, so that a cubic foot of air increases from 1 at 32° to 1.05701 for an increase of temperature from 32° to 60° 566.86 and 1.05701 =536 3 grains, the weight of a cubic foot of air at 60°. 6. A cubic foot of vapour at the same temperature and under the same pressure weighs 5.8 grains; the sum of these two weights is 542.1 grains, and a cubic foot of saturated air is 532.8 grains, and this would be the weight of a cubic foot of air under a pressure of 30 inches, and saturated with moisture: in all other cases it is to be calculated from the degree of humidity and pressure of the atmosphere. Fig. 14. NEGRETTI AND ZAMERA'S MAXIMUM THERMOMETER FOR SOLAR RADIATION 7. Table 11 in the Appendix contains the mean weight of a cubic foot of air under its mean pressure, temperature, and humidity at the Royal Observatory, Greenwich. 8. Solar Radiation. The sun's rays pass through the atmosphere, exercising but little influence on its temperature till they reach the earth, accumulate there, and cause the earth to become much more heated than the air. Its amount is an important element in meteorology, and is determined by the excess of reading of a thermometer, placed near the surface of the earth, fully exposed to the direct rays of the sun, above that of the thermometer, placed to determine the temperature of the air in the shade. The solarradiation thermometer (fig. 14), consists of a maximum thermometer, with blackened bulb, graduated on its own stem. In use it may rest on the forks of two Y's, precaution being taken to prevent lateral wind striking the bulb. In this situation its reading may be 150°, whilst that in the shade is 60°. MINIMUM THERMOMETER FOR TERRESTRIAL RADIATION WITH TRANSPARENT BULB. The average daily amount of solar radiation from November to February is 5° or 6°; in March 13°; in April 17°; from May to August 20°; September 18°; and October 10°. In fine summer months the mean reading of the solar-radiation thermometer may exceed 100°, whilst in others it will not exceed 85°. 9. Terrestrial Radiation.—The amount of terrestrial radiation is of equal importance with that of solar radiation. From the surface of the earth heat is constantly escaping, and on cloudless nights the earth throws off heat by radiation more rapidly than the air, and its surface is reduced to a lower temperature than the surrounding air. Its amount is determined by the defect of the readings of a thermometer, with its bulb fully exposed to the sky, and placed on grass, or on a non-conductor of heat, as wool or flax, below those of the thermometer to Fig. 15. determine the temperature of the air in the shade. The terrestrial-radiation thermometer consists of a minimum thermometer with transparent bulb, and graduated on its own stem: in use it should rest on the fork of two Y's, so that the bulb is on the top of the grass, and not covered by a single blade. A thermometer thus placed, when the sky is covered with low dense clouds, will read the same as that placed some feet above it; but on the clouds rising, or the sky becoming less cloudy, will read from 3° to 5° lower; and when the sky is cloudless and bright, and the air calm, the_reading may be from 3° to 20° lower than the air. I once observed a difference of 28°.5 between the readings of two thermometers, the one placed on raw wool, and the other in air at the height of 8 feet. On calm and clear nights a terrestrial-radiation thermometer may read less than 32° in every month of the year. 10. The daily amount of terrestrial radiation is dependent on the amount of cloud. During any period when the nights are generally cloudy, there will be but little difference between the readings of the two thermometers: the mean difference monthly varies from 5° to 10°. The greater coldness of grass than that of air in clear and calm weather, in places sheltered from the sun, but open to a considerable portion of the sky, may continue all day as well as night. The formation of dew depends solely on the temperature of the bodies on which it is deposited, and never appears till their temperature decreases below that of the dew-point in their locality. The amount of water deposited in the shape of dew is the largest on those substances which radiate heat freely, and on which the reading of a thermometer is lowest. 11. The great difference in temperature of the surface of the earth between day and night affords an explanation of the current of air denominated land and sea breezes. During the day the air in contact with the heated earth becomes heated, expands in bulk, becomes specifically lighter, and rises in consequence, when the cooler air from the sea rushes in to supply its place, and thus causes the current called the sea breeze. During the night, on the contrary, the earth is cooled by radiation; the air in contact with it is cooled, becomes smaller in bulk, and specifically heavier than the air over the water, which parts with its heat much more slowly than the land, and a current from the land takes place. 12. Condensation of Vapour.-Whenever the temperature of the air is below that of the dew-point, a portion of vapour is deposited; and the forms of water so condensed are various, depending on circumstances. When this depression of temperature is small, as when saturated air is mixed with a stratum of a little lower temperature, the separation takes place in the air in very minute globules, which diffused over a large space, assume the form of clouds, mist, or, if near the earth, of fog. When the depression is large, the quantity of water separated from the air is great; and it falls in the form of rain, hail, or snow. 13. CLOUDS are visible collections of minute globules of water suspended in the atmosphere: they differ very greatly in respect of form and magnitude, depending on the quantity of vapour of which they are composed, the direction and force of the wind, &c. They have been classified by Luke Howard, F.R.S., into three primary formations, the cirrus, the cumulus, and the stratus, which are represented in the accompanying plate, fig. 16; and into four secondary, the cirro-cumulus, cirro-stratus, cumulo-stratus, and the nimbus. Cirrus is composed of fibres or wisps or curling streaks, in appearance like a lock of hair, or a feather, sometimes resembling a brush. It occupies the greatest elevation, and is vulgarly known as "mares' tails." Cumulus denotes a cloud in dense, convex heaps, with rounded, and frequently white, rocky surfaces, upon a |