594 PHILOSOPHICAL TRANSACTIONS. [aNNO I /QS. 



introduced me to the patronage of our most gracious King, the great work I had 

 in view was for a while postponed. 



In the year 1783, I finished a very good 20-feet reflector with a large aperture, 

 and mounted it on the plan of my present telescope. After 2 years observation 

 with it, the great advantage of such apertures appeared so clearly to me, 

 that I recurred to my former intention of increasing them still further ; and 

 being jiow sufficiently provided with experience in the work I wished to under- 

 take, the President of our r. s. had the goodness to lay my design before the 

 King. His Majesty was graciously pleased to approve of it, and with his usual 

 liberality to support it with his royal bounty. In consequence of this arrange- 

 ment I began to construct the 40-feet telescope, which is the subject of this 

 paper, about the latter end of the year 1785. The wood-work of the stand, 

 and machines for giving the required motions to the instrument, were immediate- 

 ly put in hand, and forwarded with all convenient expedition. In the whole of 

 the apparatus none but common workmen were employed, for I made drawings 

 of every part of it, by which it was easy to execute the work, as I constantly in- 

 spected and directed every person's labour ; though sometimes there were not 

 less than 40 different workmen employed at the same time. 



While the stand of the telescope was preparing I also began the construction 

 of the great mirror, of which I inspected the casting, grinding, and polishing ; 

 and the work was in this manner carried on with no other interruption than what 

 was occasioned by the removal of all the apparatus and materials from Clay-hall, 

 where I then lived, to my present situation at Slough. Here, soon after my 

 arrival, I began to lay the foundation, on which by degrees the whole structure 

 was raised as it now stands; and the speculum being highly polished and put into 

 the tube, I had the first view through it on Feb. 19, 1 787- I do not however 

 date the completing of the instrument till much later; for the first speculum, by 

 a mismanagement of the person who cast it, came out thinner on the centre of 

 the back than was intended, and on account of its weakness would not permit a 

 good figure to be given to it. A 2d mirror was cast Jan. 26, 1 788 ; but it cracked 

 in cooling. Feb. 16, we re-cast it with particular attention to the shape of the 

 back, and it proved to be of a proper degree of strength. Oct. 24, it was 

 brought to a pretty good figure and polish, and I observed the planet Saturn with 

 it. But not being satisfied, I continued to work upon it till Aug. 27, 1789, 

 when it was tried on the fixed stars, and I found it to give a pretty sharp image. 

 Large stars were a little affected with scattered light, owing to many remaining 

 scratches in the mirror. Aug. 28, 1789, having brought the telescope to the 

 parallel of Saturn, I discovered a 6th satellite of that planet; and also saw the 

 spots on Saturn, better than I had ever seen them before, so that I may date the 

 finishing of the 40-feet telescope from that time. 



