If these directions, trivial though some of them may appear, be not attended to, the observations will lose their value. 328. Since the mean of the temperatures observed at 9 A.M. and 9 P.M. is nearly the mean temperature of the day, these are the hours to be preferred for observing the hygrometer. An additional observation at 3 P.M., when the temperature is near the maximum, is recommended, as showing the greatest dryness of the air during the day. 329. By means of these two observations, the temperature of the air, as shown by the dry-bulb, and the temperature of evaporation, as shown by the wet-bulb, the following may be either determined or approximated to by means of tables constructed for the purpose: (1) The dew-point; (2) the elastic force of vapour, or the amount of the barometric pressure due to the vapour present in the atmosphere; (3) the quantity of vapour in a cubic foot of air; (4) the additional vapour required to saturate a cubic foot of air; (5) the relative humidity and (6) the weight of a cubic foot of air at the pressure prevailing when the observation is made. 330. The formula of reduction, as deduced from Dr Apjohn's investigations, is as follows: Let F be the elastic force of saturated vapour at the dew-point, ƒ the elastic force at the temperature of evaporation (the wet-bulb), d the difference between the dry and wet bulb, and h the barometric pressure, then d h when the reading of the wet-bulb is above 32°; and d h when the wet-bulb is below 32°. M. Regnault has determined by carefully-conducted experiments the value of the elastic force of vapour; the results are given in Table VII. From this table, ƒ is found; and d and h being obtained by observation, F is calculated. From F the dew-point is found by using Table VII. reversely, and finding the temperature opposite the elastic force calculated. To take an example,— Suppose the dry-bulb to read 50° and the wet 45°, and the barometer 29 inches, then f=.299 inch (from Table VI.); d = 50° - 45° = 5°; and h = 29 inches. Hence And from Table VI. we find the temperature opposite .244 to be 39°.7, which is therefore the temperature of the dew-point when the dry-bulb is 50° and the wet-bulb 45°. 331. To obviate such laborious calculations Mr Glaisher has elaborated a series of factors from the combination of simultaneous observations of the dry and wet bulb thermometers with Daniell's hygrometers. These factors are given in Table VII. We shall find the dew-point of the above example by them. The factor opposite the dry-bulb 50° is 2.06, and the difference between the two thermometers is 5°, therefore 2.06 x 5 = 10.3, and hence the dew-point = 50° – 10°.3, or 39°.7, as before. 332. Relative Humidity. In calculating the relative humidity, saturation is assumed as 100, and perfectly dry air as 0. The relative humidity is found by dividing the elastic force of vapour corresponding to the temperature of the dewpoint by the elastic force corresponding to the temperature of the air, and multiplying the quotient by 100. Thus elastic force at 39.7 is .244, and at 50° .361; dividing and multiplying by 100, we find the relative humidity to be 68 when the dry-bulb is 50° and the wet-bulb 45°. 333. Valuable and copious tables for facilitating the processes of finding the dew-point, humidity, and other elements specified above have been published by Mr Glaisher. These tables are indispensable to every practical meteorologist. We shall conclude this chapter with a few remarks on the dewpoint, elastic force, and relative humidity. 334. Dew-point.-The ascertaining of the dew-point is of great practical importance, particularly to horticulturists, since it shows the point near which the descent of the temperature of the air during the night will be arrested. For when the air has been cooled down by radiation to this point, dew is deposited and latent heat given out. The amount of heat thus set free being great, the temperature of the air is immediately raised. But as the cooling by radiation proceeds, the air again falls to, or slightly under, the dew-point; dew is now again deposited, heat liberated, and the temperature raised. The same process continues to be repeated, and thus the temperature of the air in contact with plants and other radiating surfaces may be considered as gently oscillating about the dew-point. For if it rises higher, the loss of heat by radiation speedily lowers it, and if it falls lower by ever so little, the liberation of heat as the vapour is condensed into dew as speedily raises it. Thus, then, the dew-point determines the minimum temperature of the night. 335. This suggests an important practical use of the hygrometer. If the dew-point be ascertained by it, the approach of low temperatures or of frost may be foreseen and provided against. Thus, suppose on a fine clear spring day, towards evening, that the dry-bulb was 50° and the wet 40°; the dewpoint at the time is therefore 29°.4. Frost on the ground may then be predicted with certainty, and no time ought to be lost in protecting such tender plants as may be exposed in the open air at that season. If, on the other hand, with a sky quite as clear, the dry-bulb was 50° and the wet 47°; the dew-point being thus 43°.8, no frost need be apprehended. The raising or depressing of the dew-point during the night by a change of wind, is the only circumstance that can happen to interfere with the predictions founded on the hygrometer. 336. Elastic Force of Vapour.-In an atmosphere of pure steam, its force at the earth's surface is the pressure it exerts; and in an atmosphere of vapour and air perfectly mixed, the elastic force of each at the surface of the earth is the pressure of each. Hence the elastic force of aqueous vapour would be the pressure of the whole vapour in the atmosphere over the place of observation; this is expressed in inches of mercury L of the barometric column. Thus, suppose the total barometric pressure to be 30.000 inches, and the elastic force of vapour .450 inch, the weight of the dry air or air proper would be represented by 29.550 inches of mercury, and the weight of the aqueous vapour .450 inch of mercury. Thus, then, the elastic force may be regarded as representing the absolute quantity of vapour suspended in the atmosphere subject to the modification stated in par. 303. It may also be termed the absolute humidity of the atmosphere. It is greatest within the tropics, and diminishes towards the poles. It is greater in the atmosphere over the oceans, and decreases as we advance inland. It is greater in summer than in winter, and greater at mid-day than in the morning. It also diminishes. with the height, but the average rate at which it diminishes is not known. The balloon ascents of Mr Glaisher and other aëronauts have thrown some light on the question. But the number of ascents are by far too few to warrant the drawing of general conclusions as to the mean rate of the decrease. The chief point established is, that on particular instances the decrease is generally very far from uniform; different strata are superimposed on each other, differing widely as regards dryness and dampness, and the transition from the one to the other is frequently sharp and sudden. 337. Relative Humidity.-This must not be confounded with absolute humidity. Suppose the temperature of the air to be 40° and quite saturated with vapour, and then to be suddenly raised to 50° without any addition being made to its vapour, its absolute humidity would in each case be the same; but in the former case it would in popular language be said to be very damp, and in the latter case very dry. This essential and palpable difference is expressed by the term relative humidity, or more briefly the humidity of the air. Thus, in the language of meteorologists, humidity of the air means the degree of its approach to complete saturation. When the humidity is 100, the air is completely saturated. If the humidity at 9 A.M. and 9 P.M., when the temperature is about the average of the day, is 73, the air to an inhabitant of Great Britain would feel very dry, 73 being about the lowest mean humidity that occurs in Scotland during May, the driest month. This low humidity is, however, greatly exceeded when the east winds of spring happen to acquire their greatest virulence and dryness. Thus, during May 1866, at Corrimony, in Inverness-shire, the dry-bulb at 9 A.M. on the 21st was 65°, and the wet 47°, thus giving a humidity of 29, perhaps as low a humidity as has hitherto been observed in the British Islands; and of course later in the day this extraordinary dryness must have been still further increased. 338. In the ocean, at a distance from the land, the humidity is always great, and during the night generally approaches 100. In the interior of continents it is less, especially in sandy deserts, which allow the rain-water speedily to sink, thus drying the surface, and in rocky countries, which are never wetted more than on the surface. Thus at Djeddah, in Arabia, on 12th March 1866, the humidity was as low as 11. The humidity is greatest during the night, when the temperature is at the minimum; it is also great in the morning, when the sun's rays have evaporated the dew and the vapour has not yet had time to find its way up into the air. And it is least during the greatest heat of the day and for some time thereafter, or before the temperature has yet begun perceptibly to fall. 339. Now, between the vapour present in the air and the temperature of the air there is a vital and all-important connection, which recent experiments and researches have done much to elucidate. 340. Diathermancy of the Air-Bodies possess perfect diathermancy when they allow rays of heat to pass through them freely and unimpeded, or when they absorb none of the rays of heat which fall on them as they pass through them. Thus perfectly dry air allows heat to pass through it without being sensibly warmed thereby. But it is otherwise with the vapour of water or with a mixture of vapour and air, which presents an obstruction to the free passage of the heat of solar and terrestrial radiation much in the same way as stones in |