Daniell on Meteorological Observations. 109 



time, the degree of dryness is more than double, and the force 

 of evaporation more than treble. It is also perfectly evident 

 that the height of the barometer has a very material influence, 

 as well as the temperature. To illustrate the former, without 

 the latter, let us take the quarter of a year from Macch 

 to May 1820, and compare it with the same quarter of 1821. 

 The force of vapour was nearly alike, and the temperature of 

 the latter rather higher than that of the former ; nevertheless 

 the rain of this quarter was more than double ; the height of 

 the barometer, in the first, being rather above the mean, while 

 in the latter it was 0.14 inch below it. The power of tem- 

 perature in modifying the precipitation is strikingly illustrated 

 in the two half-quarters of October and November. In 1821 the 

 barometer was lower, and the force of the vapour higher, than 

 in 1820. Nevertheless, the quantity of rain in the latter was 

 double that in the former, for the average mean temperature 

 was 5i° lower. The joint influence of the two will not fail to 

 strike any one who takes the trouble to inspect the tables. 



We now come to the consideration of temperature ; and I 

 have endeavoured to give a complete view of the subject in 

 the third table. I have therein included three series of obser- 

 vations not usually taken into account. The first is the 

 greatest eflect of radiation at night, the second the highest 

 temperature of the sun in the course of the day, and the third 

 the constituent mean temperature of the vapour. 



I can add nothing to the ingenious views of Mr. Howard with 

 regard to mean and extreme temperature in general. My motive 

 for including observations upon radiant heat both from and to 

 the earth were, first, their probable importance to vegetation 

 and agriculture, and, secondly, their obvious connexion both 

 with the production and precipitation of vapour. The medium 

 eflfect of radiation from the earth at night, it will be seen, is 

 about 4° : the maximum which 1 have observed is 13°. It will 

 probably strike many people with surprise, to find that there is 

 but one period of six weekg in the year in which vegetation, 

 in particular situations, is not exposed to a freezing tempe- 

 rature, and only one quarter of a year in which the thermo- 



