transit instrument at the Cambridge^Observatory. 427 
persons who level are opposite, and contiguous, to the south- 
west and northreast braces. Being in the constant habit of 
examining the meridian mark, in order to know what degree 
of stability the instrument possesses, I found, after levelling, 
that the south meridian mark was to the east of the middle 
wire. In about 10 minutes the middle wire returned to the 
meridian mark, and bisected it. I noted this circumstance a 
second, third, and fourth time, and then began to inquire 
whether I had conjectured rightly in attributing it to the 
expansion of the tubes or braces. For this end, I placed a 
heated blanket across the south-west and north-east braces, 
and found the meridian mark deviating to the east of the 
middle wire : a contrary effect was produced by placing the 
blanket across the south-east and north-west braces. In these 
trials the object glass was towards the south : contrary 
effects took place when it was turned to the north.* 
As yet I am unable to say whether or not the sun’s rays 
falling on the braces, during an observation of his transit, 
affect the accuracy of the observation. I am enquiring into 
that point, and have ordered a screen to be made to protect 
the braces from the rays of the sun. 
After repeated trials, I have been obliged to abandon the 
counterpoises instead of relieving the instrument, they 
render it unsteady. It has happened with them (as it has 
happened in cases of a different nature), they have overpow- 
ered what they were meant only to assist. 
* The effect I have noted is somewhat of the same kind as that which was com- 
plained of in Halley’s transit. See Bradley’s Observations, vol. i. p. 2. 
+ They are now with Mr. Dollond, who is endeavouring to remedy their 
defects. 
SK 
MDCCCXXV. 
