Mr. Pond on the Parallax of a, Lijicv. 293 



Iween y Draconis and « Lyia; : 2dly, tlie absolute parallax 

 of the latter star ; the Dublin observations indicating, it may 

 be remembered, that the parallax of y Draconis is insensible, 

 but that of a Lyra; a very perceptible quantity. The pro- 

 cesses employed in these two investigations being very dif- 

 ferent, I shall consider each of them separately. 



O71 the T)ifference of Parallax bet'doccn y Draconis and a. Lyra:. 



It is impossible to conceive a more sunple process than that 

 of determining with the mural circle the difference of polar 

 distiince between these stars. From their proximity in right 

 ascension, the operation is the same as that of measuring the 

 angular distance of two terrestrial objects, about 12° asuiider,- 

 with a theodolite surrounded by six microscopes : for the 

 mural circle, in principle, exactly resembles a vertical theodo- 

 lite ; with this difference, that its microscopes, instead of be- 

 ing placed on a frame-work of bi'ass, are securely fixed on a 

 stone-pier. Now I find that the angular distance thus mea- 

 sured in winter does not differ one-tenth of a second from the 

 same angular distance measured in summer; and therefore, 

 that the difference of parallax between the two stars is abso- 

 lutely a quantit}^ too small to be measured. In this investi- 

 gation, it is to be considered that any constant error in the 

 determination of the absolute polar distances has nothing to 

 do with the question, it being the difference only of those di- 

 stance.s at opposite seasons that is required. To render all 

 errors throughout the whole course of observation as constant 

 as possible, the telescope remained fixed to the same pai't of 

 the limb of the instrument, and the utmost pains were talen 

 to reduce the temperature in the Observatory to that of the 

 outer air ; the difference throughout the year not exceeding 

 one degree. The winter of 1821-1822 was extremely favour- 

 able for astronomical observation; there was an unusual num- 

 ber of fine nights, and the weather was so mild and uniform, 

 that we were enabled to equalize the temperature, so as to 

 make it of no imjiortance whether the observations were com- 

 puted by the outer or inner thermometer ; and it is to this cir- 

 cumstance, in a great measure, that I attribute the perfect 

 coincidence between the observations at different seasons. 



It has been objected, however, tliat )ierhaps some unex- 

 pected effect of temperature deranges the instrument by the 

 exact quantity of the difference of parallax attributed to these 

 stars by Dr. Brinkley ; if we suppose a derangement from 

 temperature so considerable as to give a sensible error, 

 even after being dimiuished by the effect of six microscopes, 

 wc should expect the error to be much greater when the ex- 

 periment 



