404> Dr. Cumming's New Hygrometer. 



of the thermometer at the appearance and at the evanescence 

 of the dew, opposite to the bulb of the thermometer, will give, 

 with relative accuracy, the measure of the force of vapour in 

 the atmosphere. 



But, in general terms, it is not desirable to depress the in- 

 strument many degrees below that at which the dew becomes 

 visible, on account of the increased length of time requisite to 

 complete an observation. When the hygrometer is consulted 

 with a view of predicting the greater or less probabiHty of rain, 

 or other atmospheric changes, two things (Mr. Daniell ob- 

 serves) are to be principally attended to — " the difference be- 

 tween the constituent temperature of the vapour, and the tem- 

 perature of the air, and the variation of the dew-point. In 

 general, the chance of rain or other precipitation of moisture 

 from the atmosphere, may be regarded as in inverse proportion 

 to the difference between the " thermometrical indications : " but 

 in making this estimate, regard must be had to the time of day 

 at which the observation is made. In settled weather, the dry- 

 ness of the air increases with the diurnal heat, and diminishes 

 with its dechne ; for the constituent temperature of the vapour 

 remains nearly stationary. But to render the observation most 

 completely prospective, regard must be had at the same time 

 to the movement of the dew-point. As the elasticity of the 

 vapour increases or declines, so does the probability of the 

 formation and continuation of rain. An increasing difference, 

 therefore, between the temperature of the air, and the tempe- 

 rature of the point of condensation, accompanied by a fall of 

 the latter, is a sure prognostication of fine weather ; while di- 

 minished heat, and a rising dew-point, infaUibly portend a rainy 

 season." 



The following extracts from Mr. Daniell's Table, showing 

 the force, weight, and expansion of aqueous vapour, at dif- 

 ferent temperatures, will tend to place the importance of the 

 study of hygrometry to the physician, horticulturist, and man 

 of science, in a favourable point of view. 



