=108. Causes of Dew.=--"The aqueous vapour of our atmosphere is a powerful radiant; but it is diffused through air which usually exceeds its own mass more than one hundred times. Not only, then, its own heat, but the heat of the large quantity of air which surrounds it, must be discharged by the vapour, before it can sink to its point of condensation. The retardation of chilling due to this cause enables good solid radiators, at the earth's surface, to outstrip the vapour in their speed of refrigeration; and hence, upon these bodies, aqueous vapour may be condensed to liquid, or even congealed to hoar-frost, while at a few feet above the surface it still maintains its gaseous state."[9] The amount of moisture so deposited will vary with different atmospheric conditions. If the sky be decidedly cloudy or misty, the heat radiated from the earth will be partly restored by counter-radiation from the visible vapour; the cooling of the earth's surface will, therefore, take place slowly, and little dew will be deposited. On the other hand, if the air contain transparent vapour, and the sky appear clear, the counter-radiation will be less, the earth will cool rapidly, and the deposit of dew will be copious; provided the night be comparatively calm, for, when the wind blows, the circulating air supplies heat to the radiating substances, and prevents any considerable chilling. The dew which falls in tropical countries greatly exceeds in abundance what we experience in our climate; because the air is there, from the great heat, capable of sustaining a large amount of vapour in the transparent state, and the conditions most favourable for a maximum reduction of temperature by radiation are present. At those places, or upon those substances which cool the lowest and most readily, the dew falls most copiously. [Illustration: Fig. 83.] =109. Plan of Exposing Thermometers=, &c.--Figure 83 is an illustration of a convenient slab for supporting thermometers in an exposed position attached to a stand (such as Glaisher's, described in Chapter XVI.) for ordinary scientific observations. It has a projecting ledge, _B_, to carry off rain from the instruments, the slab, _A_, being erected vertically. The hygrometer is placed at _E_, with the vase of water at _F_. An alcohol minimum thermometer is represented at _C_, in the position most favourable to its certain action; and at _D_ is shown one of Negretti & Zambra's maximum thermometers, the position of which may be more nearly horizontal than there exhibited, although a slight depression of the bulb-end of the frame is desirable, but not necessary, as this thermometer can be used in any position. CHAPTER XII. INSTRUMENTS USED FOR MEASURING THE RAINFALL. The instruments in use for measuring the quantity of rain which falls on a given spot are of very simple construction. Perhaps the simplest is:--