Page:Transactions of the Provincial Medical and Surgical Association, volume 4.djvu/210

 108 occur between them. In general the chance of rain, or other precipitation of moisture from the atmosphere, may be regarded as in an inverse proportion to the difference between the two thermometers, one marking the dew-point, the other the temperature of the air.

An increasing difference, accompanied with a fall in the dew-point, is an almost sure prognostic of fine weather; whilst a diminution in the temperature of the air, and a rising dew-point, portend rain. In winter, when the differences between the dew-point and the temperature of the air are always small, the indications must be taken more from the actual rise and fall of the dew-point, than from the difference between it and the temperature of the air. And at this season I have generally found that if the dew-point at nine a. m. is above the minimum temperature of the preceding night, with a falling barometer, rain or snow may be expected; on the other hand, if the dew-point at nine in the morning is at or below the minimum of the preceding night, with a stationary or rising barometer, fair weather may be confidently expected. A sudden change in the dew-point is generally accompanied by a change of wind, and the former sometimes precedes the latter by a short interval: and the course of the aerial current may then be anticipated before it affects the direction of the weathercock, or even the passage of smoke. A rise in the dew-point, accompanied with a fall of the barometer, is an infallible indication that the whole mass of the atmosphere is becoming imbued with vapour, and a copious precipitation may be expected. If a fall