27
during which vapour has been able to accumulate; a low barometer testifies to rarefaction of the air, which is consequently incapable of sustaining much vapour in suspension.]
Fogs.
Fogs do not occur in windy weather. They are driven away when a breeze springs up, unless dissipated by other causes.
If in winter a cold and a warm current meet, and the latter (a Southerly one) is overcome by the former (a Northerly one) the barometer will rise to a high point at places near the line of contact, and a dense fog will appear. This fog often disappears suddenly and then reappears, and perhaps such alternations may occur several times: the alternate predominance of the two antagonistic currents is indicated by this. If great cold ensue it will be a proof that the Northerly (or Polar) current has eventually gained the mastery.
Dew and Hoar Frost.
Dew is the aqueous vapour of the air deposited on surfaces cooled by radiation. The quantity depends on the degree of the cold and on the radiating and conducting power of the surfaces. Furs, wool, silk, cotton, vegetable substances, &c., being good conductors (relatively) will be much bedewed. Glass, mould, sand, gravel, &c., being bad conductors, will be little bedewed. By a benificent arrangement therefore of the Creator dew falls most copiously on the objects which most require its refreshing influence.[1] It is
- ↑ Buchan.
