100 Daniell ow Meteorological Observations. 



marks the changes in the absence of the observer, is still a sub- 

 ject of some dispute. The most we can hope for is a near ap- 

 proximation. Now the results of the barometer contained in 

 the first column of Table 1 , and those of the hygrometer in the 

 second, third, and fourth columns, must be regarded first as the 

 means of the particular hours at which the respective obser- 

 vations were made. It is farther probable, that the former may 

 be taken, without much fear of serious error, as the medium of 

 the whole 24 hours. No particular known cause tends to pre- 

 vent this assumption. But with regard to the latter, it is very 

 different. The fall of the temperature during the night must 

 exercisea very important influence upon the atmospheric vapour; 

 and therefore the results of the second, third, and fourth 

 columns, cannot be received as those of the 24 hours, without 

 further inquiry. 



The register thermometer has been the means of our attain- 

 ing to a much greater degree of certainty with regard to tem- 

 perature than we could ever have hoped to have arrived at by any 

 other method. The same instrument will assist us much in our 

 present inquiry. By referring to Tab. 3, (which is a table of 

 temperature only, and which will be more fully explained here- 

 after,) it may be observed that the mean lowest temperature of 

 the night is constantly several degrees below the mean consti- 

 tuent temperature of the vapour derived from the three before- 

 named periods of the day. Now it is obvious, that once during 

 the night the constituent temperature of the vapour cannot 

 exceed that amount, nor is it at all probable that it should 

 fall below it Speaking from experiment, observation, and 

 calculation, I have no doubt that a precipitation of vapour 

 takes place during some period of every night in the year. I 

 do not now allude to the deposition of dew from the cold pro- 

 duced by radiation, (the amount of which for the whole year 

 is likewise shewn by the table of temperature,) but to a preci- 

 pitation in the body of the atmosphere itself. In the most 

 cloudless nights of the whole year, when the stars are bright, 

 and the disc of the moon perfectly sharp and well defined, by 

 bringing the hygrometer out of a warm room, it will be found 



