4^3 
of an Arc of the Meridian, 
to indicate any serious warp in the stand. Its greatest variation 
was 4/; but, for several days together, it did not amount to 
30". 
The weight of the plummet, I adjusted to the strength of the 
plumb-line, in the usual way. I suspended it in air, and gra- 
dually increased its weight, till the wire broke. This plummet 
was then immersed in the vessel appropriated for its reception. 
It will, perhaps, not be improper to observe, that I was careful 
to give the plummet its maximum of weight, that its wire might 
not be subject to motion from streams of air. 
As it was to be apprehended that errors would result, from 
the effects of an inequality of temperature in the air within the 
observatory, I placed two thermometers, both adjusted to a 
third, near the telescope. One I elevated as high as the axis, the 
other I laid on the hollow brass cylinders which connect the 
divided arch with that behind it, usually called the back arch. 
In the day, I found (as may be seen in the register of observa- 
tions) the heat a little greater at the top of the tent than towards 
the bottom ; and the reverse was generally the case at night. 
To equalize the temperature at those times when the sun 
shone out, or the weather was hot, I opened the shutters in the 
roof, as well as the door of the observatory, a considerable time 
before the moment of observation. By these means, the air 
within the tent was rendered tolerably uniform in its degrees of 
heat. For the space of a week following the commencement 
of my observations, I suspended a third thermometer from the 
milled-headed key which turns the diaphragm placed inside 
the telescope. As the situation of this thermometer was midway 
between the two others just mentioned, I always found the 
temperature there, a mean between those degrees shown by the 
mdccciii. 3 H 
