2i 8 Dr. Herschei/s Observations of the Transit of Mercury 
goodness, so that every cause which impeded their proper action 
must be looked upon as extrinsic. I shall arrange these obser- 
vations under different heads, that, when they have been related, 
there may remain no difficulty to draw a few general conclusions 
from them, which will be found to throw a considerable light 
upon our subject. 
Moisture in the Air. 
(l.J October 5, 1781. I see double stars, with 460, completely 
well. The air is very damp. 
(2.) Nov. 23, 1781. i5 h 30'. The morning is uncommonly 
favourable, and I see the treble star f Cancri, with 460, in high 
perfection. The air is very moist, and intermixed with passing 
clouds. 
(3.) Sept. 7, 1782. I viewed the double star preceding 12 
Camelopardalis,* with 932. In this, and several other fine nights 
which I have lately had, the condensing moisture on the tube 
of my telescope has been running down in streams; which 
proves that damp air is no enemy to good vision. 
(4.) Dec. 28, 1782. 17 11 30'. The water condensing on my 
tube keeps running down ; yet I have seen very well all night. 
1 was obliged to wipe the object-glass of my finder almost 
continually. The specula, however, are not in the least affected 
with the damp. The ground was so wet that, in the morning, 
several people believed there had been much rain in the night, 
and were surprised when I assured them there had not been a 
drop. 
(5.) Feb. 19, 1783. I have seen perfectly well till now-f that 
* See Phil. Trans. Vol. LXXV. Part I. page 68 ; II. 53. 
•j- The time is not marked in the journal ; but, from the number of the observations 
that had been made during the night, it must have been towards morning. 
