Daniell on a new Hi/grometer. 305 



barometer and hygrometer were at variance, and the result 

 was in favour of the latter. On the 30th and 31st of October, 

 during an uninterrupted rise of the barometer of 0.32 inches, 

 the hygrometer denoted the utmost point of humidity, and 

 much rain fell during the period. 



By combining the observations of both instruments, we learn to 

 modify their results, and by so doing can hardly be deceived in the 

 weather for many hours in advance. The indications are to be 

 corrected according to circumstances in the following manner : — In 

 summer-time when the diurnal variations of temperature are 

 great, regard is to be had to the time of day at which the 

 experiment is made. In the morning, supposing the difference 

 between the temperature of the air and the constituent tem- 

 perature of the vapour to be small, it is to be recollected, that 

 the accession of heat during the day is great, and that the dif- 

 ference will therefore probably increase. If the point of con- 

 densation should at the same time be lowered, it is an indi- 

 cation of very fine weather. If, on the contrary, the heat of 

 both should increase with the day in nearly equal progression, 

 rain will almost infallibly follow, as the heat of the air falls 

 with the setting sun. Thus, on the 14th of September, at nine 

 o'clock in the morning, the temperature of the air was 62°, the 

 point of condensation 570, making a difference of 5**. At three 

 o'clock the heat of the air had risen 10", and the point of con- 

 densation had fallen 2°, making a difference of 17". The day 

 was nearly cloudless. At seven o'clock the heat of the air 

 had fallen to 66o, and at ten o'clock to 61°. The temperature 

 of the vapour at the same time rose 2o. The night was very 

 clear, but there was a great deposition of moisture upon dry 

 wood, slates, Sfc. The 15th of the same month afforded an 

 instance of the contrary case. The morning observation shewed 

 a difference of 7°, the point of condensation having at the 

 same time risen during the night : the heat of the day increased, 

 but accompanied by a rapid rise in the constituent tempera- 

 ture of the vapour, from 59" to 65". The consequence was 

 rain, which increased in quantity as the sun declined. In 

 winter, however, when the range of the thermometer during the 

 X'Z 



