Cause of the variations of the barometer. —Various theories have been proposed to account for those frequent atmosphe rical changes which cause the rise and fall of the barometer, but none of them can be regarded as very satisfactory. Whatever tends to increase or diminish the vertical pressure will obviously cause the barometer to rise or fall ; but the ver tical pressure may be increased either by an influx of winds and the accumulation of air at any place, or by a diminution of the elasticity of the atmosphere. The presence of heat or of moisture augments the elasticity, and consequently reduces the weight of the vertical column. During the prevalence of northerly and easterly winds the barometer stands high, the elasticity being diminished by the cold. But the real difficulty, Professor Leslie remarks, " consists in explaining why the variations of the barometer should be greater in the high latitudes than be tween the tropics, and why they should exceed in all cases the quantities which calculation might assign. The only mode, perhaps, of removing the difficulty is to take into consideration the comparative slowness with which any force is propa gated through the vast body of the at mosphere. Au inequality may continue to accumulate in one spot before the counterbalancing influence of the distant portions of the aerial influence can arrive to modify the result. In the higher lati tudes, the narrow circle of air may be considered as in some measure insulated from the expanded ocean of atmosphere; and hence, perhaps, the variations of the barometer are concentrated there, and swelled beyond the due proportion." Uses of the barometer.—The barometer is an instrument of great importance in astronomy, its indications forming an essential element in determining the amount of atmospheric refraction. It is also, on account of its application to the measurement of altitudes, indispensable in all researches connected with the cli mate. The purpose for which it is most commonly sought after, is to prognosti cate the state of the weather. On land this is perhaps the least important of its applications, but the case is widely dif ferent at sea.
No certain rules can be laid down for prognosticating the state of the weather from the barometer. The following are probably of as general application as any that can be given. It is always to be re
membered that what the barometer ac tually shows is the present pressure of the atmosphere; and that its variations correspond to atmospherical changes which have already taken place, the effects of which may follow their cause at a greater or less interval.
1. After a continuance of dry weather, if the barometer begins to fall slowly and steadily, rain will certainly ensue ; but if the fine weather has been of long dura tion, the mercury may fall for two or three days before any perceptible change takes place, and the longer time elapses before the rain comes, the longer the wet weather is likely to last.
2. Conversely, if after a great deal of wet weather, with the barometer below its mean height, the mercury begins to rise steadily and slowly, fine weather will come, though two or three wet days may first elapse ; and the fine weather will be more permanent in proportion to the length of time that passes before the per ceptible change takes place.
8. On either of the two foregoing sup positions, if the change immediately en sues on the motion of the mercury, the change will not be permanent.
4. If the barometer rise slowly and steadily for two days together or more, fine weather will come, though for those two days it may rain incessantly, and the reverse ; but if the barometer rise for two days or more during rain, and then on the appearance of fine weather begins to fall again, that fine weather will be very transient, and vice versa.
5. A sudden fall of the barometer in the spring or autumn indicates wind ; in the summer, during very hot weather, a thunderstorm may be expected ; in win ter, a sudden fall after frost of some con tinuance indicates a change of wind, with thaw and rain ;, but in a continued frost, a rise of the mercury indicates approach ing snow.
6. No rapid fluctuations of the baro meter are to be interpreted as indicating either dry or wet weather of any con tinuance ; it is only the slow, steady, and continued rise or fall that is to be at tended to in this respect.
7. A rise of the mercury late in the autumn, after a long continuance of wet and windy weather, generally indicates a change of wind to the northern quar ters, and the approach of frost.