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5. In wet weather, when the mercury rises much and high, and so continues for two or three days before the bad weather is entirely over, then a continuance of fair weather may be expected.
6. In fair weather, when the mercury falls much and low, and thus continues for two or three days before the rain comes, then a deal of wet may be expected, and probably high winds.
7. The unsettled motion of the mercury denotes unsettled weather.
8. The words engraved on the scale are not so much to be attended to, as the rising and falling of the mercury for if it stand at much rain, and then rises to changeable, it denotes fair weather, though not to continue so long as if the mercury had risen higher. If the mercury
stands at fair, and falls to changeable, bad weather may be expected.
9. In winter, spring, and autumn, the sudden falling of the mercury, and that for a large space, denotes high winds and storms: but in summer it presages heavy showers, and often thunder. It always sinks lowest of all for great winds, though not accompanied with rain; but it falls more for wind and rain together, than for either of them alone.
10. If, after rain, the wind change into any part of the north, with a clear and dry sky, and the mercury rise, it is a certain sign of fair weather.
11. After very great storms of wind, when the mercury has been low, it commonly rises again very fast. In settled fair weather, except the barometer sink much, expect but little
rain. In a wet season, the smallest depressions must be attended to; for when the air is much inclined to showers, a little sinking in the barometer denotes more rain. And in such a season, if it rise suddenly fast and high, fair weather cannot be expected to last more than a day or two.
12. The greatest heights of the mercury are found upon easterly and north-easterly winds; and it may often rain or snow, the wind being in these points, while the barometer is in a rising state, the effects of the wind counteracting. But the mersinks for wind as well as rain in
THE causes which determine the distribution of heat over the earth's surface are, as has been shewn in Vol. II, Conversation X, either the direct influence of the solar rays, or the communication of heat by the air, from one part of the earth's surface to another. The first of these depends on the latitude of the place, by which the intensity of the heat and light from the sun, and also the length of the day are determined. But the intensity of the sun's rays, when they
strike upon any plane, is as the quantity that falls on a given space; and, of course, the nearer the sun is to the zenith of any place, at a given instant, the greater the intensity of heat produced by his rays.
Moreover, the heat of an entire day depends on the length of the day, as well as on the sun's elevation; and, as the day is longer where the distance from the zenith is greater, the inequality in the distribution of heat, arising from one of these causes, compensates that which proceeds from the other, and brings their combined effects much nearer to an equality than could be imagined.
It has been shown, by M. Fontana, that the heat of the day of the summer solstice at Pavia, in Italy, is scarcely greater than the heat of the same day at Petersburgh; that is, only in the proportion of 65° to 62°, though the latitude of the one is 45° 11', and that of the other 59° 26'.
The same author finds, that when the sun's declination exceeds 18°, that is, from