1822.] Memoirs of the Astronomical Society, Vol. I. 227 
deterniination depends on it; I mean the minute difference in the 
declinations of stars, the obliquity of the ecliptic, and the altitude 
of the pole, which appear. in their determination by different, 
though very excellent instruments. There is no doubt these 
differences arise from the action of gravity on the different parts 
of each instrument, though hitherto the mode of this action has 
not been clearly pointed out, nor is it possible to pronounce 
decidedly which instrument has afforded the right and which 
the wrong result. We know, in fact, very little of the extent to 
which the yielding of the metals may go; and it seems too 
hazardous to deny the possibility of this cause exercising a nota- 
ble influence on the divisions, and in consequence on the obser- 
vations in any instrument, whatever be its construction, without 
grounding such denial on sufficient proof. In our meridian circle 
the great artist has done every thing to obviate the flexure of the 
telescope by a well adapted system of counterpoises: still a 
doubt may remain, whether all the flexure be done away with by 
that means, or rendered quite insensible; and the only direct 
means of ascertaining the point seems to be, the combination of 
immediate observations of a heavenly body with those of its 
image reflected in an artificial horizon. Such observations must 
of course be frequently repeated to clear up a point of so much 
delicacy. M. Gauss has already entered on this inquiry by 
observing the pole-star in a reflecting surface of water. It is, 
perhaps, the most striking proof of the astonishing optical power 
of the telescope, that the superior culmination may be very well 
observed in this manner even in the day time. The result of the 
first complete observation of this kind was as follows: 
May 13, 1820.—Zenith distance of the north star, free from 
refraction, but including the error of culmination. 
; Fp agp Direct 319° 50’ 20:73” 
Inferior culmination Rafiected 2201/60 4304 
DEERE ETA Direct 323 8 41°51 
uperior ditto ...+++ Reflected 216 46 44:31 
Hence we deduce the true zenith distance. 
Inferior culmination ...... 40° 7” 21°60” 
Superior ditto. ...... perme Oat: NS mba 40) 
And hence (the change of declination in 12 hours being 
— 01) the latitude of the place of the water vessel is 
51° 31’ 48”-45, and that of the centre of the circle 51° 31’ 48-40. 
This being nearly a mean between the two above given, it is 
rendered very probable that the effect of its weight on the 
observations with this instrument are either quite insensible or 
at least extremely small.” 
Such observations as Mr. Baily was able to make on the Solar 
Eclipse which took place on the 7th of Sept. 1820, with those 
Q 2 
