Daniell on a new Hj/grometer, 136 



Barometer at first station .... 29-582 Hygrometer 46-34 



at second station. ., , 28.964 — — — — ^ 44-31 



DiflTerence of barometric pressure 0.618 



Ditto of hygrometric ditto 0.021 



These results, it will be seen, are too discordant, and the dif- 

 ferences are too small, to throw any light upon the laws of the 

 decrease of the aqueous atmosphere at different elevations, a 

 point of the highest interest and importance. We learn, how- 

 ever from them that, in settled weather, such as that in which 

 these last experiments were made, the vapour does decrease 

 in density as we ascend. In showery weather, however, this is 

 not always the case, for I have seen several instances, when 

 there has been denser vapour upon the hill-top than in the plain 

 below, a state of circumstances which, as far as I have beeji 

 able to observe, has always been quickly followed by falling 

 weather. 



I shall now conclude these observations with a series of ex- 

 periments, which I took the opportunity of making during the 

 great eclipse on the 7th September, for the purpose of ascer- 

 taining what effect this rare phsenomenon might have upon the 

 temperature and pressure of the gaseous atmosphere in general, 

 and of the aqueous atmosphere in particular. It had been re- 

 marked at Edinburgh, that, during the great eclipse of 18th 

 February, 1736-7, it was very cold, and that a little thin snow 

 fell, and it was not unreasonable to suppose that a sudden ob- 

 scuration of so large a portion of the sun's rays might produce 

 a very sensible change in the state of the atmospheric vapour. 



The day was altogether very favourable to the purpose ; the 

 morning was hazy, and there were a few cirri; the wind was 

 S.E., and brisk. The following observations were made pre- 

 vious to the commencement of the eclipse : 



Clock. Thermometer. Dew Pdint. Barometer. 



lOJ 65 51 30.12 



IH 65 51 



Ul 68 51 



12i 674 51 



