Fea 
XLVI. 
SOME REMARKS ON DIFFICULTIES OF MERIDIAN CIRCLE 
WORK. By ARTHUR EH. LYSTER, M.A. 
[Read January 24; Received for Publication January 26 ; 
Published Marcu 16, 1896]. 
THe author gave a general description of the meridian circle and 
its use, recommending the adoption of the late Dr. Romney 
Robinson’s device for preventing the growth of fungus on the 
spider lines by the substitution, when the telescope was not in 
actual use, of a cylinder containing drying materials for the draw- 
tube of the eye-piece; and noticing that, in the use of the ordinary 
mercury pan reflector without amalgamated copper plate, for the 
determination of collimation correction and nadir point, he had 
obtained improved steadiness of the reflected image by the careful 
protection of the reflecting surface from currents of air. 
In illustration of Chauvenet’s remarks on the importance of 
having the whole divided circle at a uniform temperature, he 
submitted a discussion of a conceivable simple distribution of 
temperature as follows :—Consider a vertical uniform circular disc, 
and suppose that when it is at a uniform temperature, a microscope 
is placed so as to read an angle 45° above the horizontal diameter. 
Now, let each element of the disc experience a fall of temperature 
proportional to its distance from the horizontal line. The fall of 
temperature at the top of the vertical diameter being 4, and 
suppose the temperature of the lower half of the disc to rise 
according to the same law: then if # and y are the co-ordinates 
of any point of the disc, and the fall of temperature at this 
point be ¢; and if #’ and y/’ are the new co-ordinates, 
dy =(l—at)de; dy =(l—at)dy;  t=yb; 
~.@ =x-atay; yo =y-taty’?; alsov’+y/=1; 
the elimination of w and y between these three equations would 
give the equation of the new boundary of the dise. 
SCIEN. PROC. R.D.S., VOL. VIII., PART V. 2E 
