58 Mr. Pond on the parallax of a . Lyra. 
of observations made with a given instrument two equations 
h^ve been disengaged, previously considered as unknown in 
amount, but known only as to the law of their variation. Of 
these one is much smaller than the other. Hence it is inferred, 
that as the instrument has faithfully disengaged the smaller 
equation (respecting which there is no dispute), it must be 
admitted with equal fidelity to have disengaged the larger, 
which might be supposed the easier operation of the two. 
This reasoning is strictly logical, as proving the disengage- 
ment of two equations, but it by no means proves the larger 
equation to be caused by parallax. The larger equation here 
to be disengaged is after all so small, that it is impossible, in 
different points of its period, to show that the law assumed 
coincides with observation ; it is only a rude agreement at 
the points of the greatest and least variation that can be de- 
monstrated. The disengagement of the larger equation only 
proves therefore the existence of some regularly recurring 
cause, acting with greatest effect at the extreme seasons. 
The reason, I conceive, why Dr. Brinkley does not find 
parallax in 7 Draconis is, that with respect to the zenith 
point, his instrument, like every one of a similar construc- 
tion, is a perfect instrument. No portion of the arc is em- 
ployed, nor can temperature here occasion any errors by 
its changes. As the star to be examined recedes from the 
zenith, the instrument becomes less and less perfect ; and 
he finds a small parallax in « Cygni, a larger in a Lyras, 
and oftentimes a still larger in stars more remote from 
the zenith. An additional reason for suspecting that the 
discordances observed arise from temperature is this : the 
greatest supposed parallax is found in those stars whose 
maximum and minimum of parallax would fall in the ex- 
