[ 209 ] 



XIII. Some Suggestions relative to the best Method of ernphying the New Zenith Tele- 

 scope lately effected at the Royal Observatory. By John Pond, Esq. A.R. F. R.S. 



Received March 11, — Read March 13, 1834. 



X HE erection of a zenith telescope of twenty-five feet focal length at the Greenwich 

 Observatory was determined upon by the Visitors in the year 1815, for the purpose of 

 measuring the zenith distance of y Draconis with greater accuracy than could be 

 effected by any instrument then existing at the Observatory. 



This instrument was fixed in its place by Messrs. Troughton and Simms, in July 

 1833; and although when first erected it was not complete in some of its minute 

 parts, yet, by various improvements which have been made, it is now approaching to 

 a perfect state. 



During the course of the last summer I made a great many observations with it, 

 with the view of determining the most advantageous method of using it. To describe 

 this method, and not the instrument itself, is the object of the present brief commu- 

 nication. 



Among various experiments that occurred to me, I was led to a mode of observing 

 which has not, that I am aware of, been suggested or put in practice by any other 

 observer; and which appears to me to possess advantages sufficient to justify my 

 making it the subject of a separate communication. 



These relate, not only to the determination of the zenith distance of y Draconis, 

 (for which purpose the instrument was especially constructed,) but to the measure- 

 ment of the minute variations from which the equations of aberration, parallax, nu- 

 tation, and others employed in the reduction of the star, are to be elicited. 



I shall first treat the subject with reference to the zenith distance of y Draconis, 

 presuming that the usual mode of using a zenith sector, altitude and azimuth instru- 

 ment, or other instrument constructed on the same principle, is well known. 



If the star be observed on one night with the instrument facing the east, and on 

 the next or any subsequent night with the instrument turned half round, and facing 

 the west, the double zenith distance will be obtained, on the supposition that the in- 

 strument has continued identical during the interval. 



If, however, either by accident or design, the instrument should have suffered any 

 change between the two observations, it is evident tiiat the result will not give the 

 measure of the required distance. I am now to show how my mode of observation is 

 adapted to overcome this difficulty. 



MDCCCXXXIV. 2 E 



