VOL. LXXXV.] PHILOSOPHICAL TRANSACTIONS. SQS 



Dr. H. then enters on the particular description of this magnificent telescope. 

 But as this paper includes a very full and minute account of all the separate parts, 

 both of the instrument and the stand, &c. with references to no less than IQ large 

 plates, we must restrict our abridgment to a description of the general view and 

 idea of this very curious machine. 



PI. 8, represents a view of the telescope in a meridional situation, as it appears 

 when seen from a convenient distance by a person placed towards the south-west 

 of it. The foundation in the ground consists of 1 concentric circular brick walls, 

 the outermost of which is 42 feet in diameter, and the inside one 21 feet; these 

 measures are reckoned from the centre of one wall to the centre of the other. 

 They are 2 feet 6 inches deep under ground ; 2 feet 3 inches broad at the bot- 

 tom, and 1 foot 2 inches at the top ; and are capped with paving stones about 3 

 inches thick, and 124 broad. After describing the contrivances and structure of 

 the foundations and basements, the construction then proceeds to the uprights 

 and superstructure. These walls were brought to an horizontal plane by means 

 of a beam turning on a pivot fixed in the centre of the circle, which had a roller 

 under it at the end. On this beam and over the roller was fixed a spirit level, to 

 point out any defect in the walls ; and by correcting every inequality that could 

 be perceived, they were by degrees brought to be so uniformly horizontal that 

 the beam would roll about every where on them without occasioning any altera- 

 tion in the bubble of the spirit level. 



The length of the ladders is 49 feet 2 inches, and their construction is as 

 follows. The top of each step is 9 inches from that of the one below it ; and, 

 beginning 12 inches from the bottom, there are two rounds and one flat placed 

 alternately, as far as 40 rounds, and 19 flats. In the place of the 20th flat is 

 the centre of the meeting of the front and back sets of the ladders ; above this is 

 another flat, with a termination of 16 inches at tlie top. The timber of the 

 sides being tapering, a similar diminution of the flats and rounds has been at- 

 tended to, especially as their size, in proportion to the sides, is (ar above what is 

 generally used in building ladders. The flats and rounds are all made of solid 

 English split oak. The lowest rounds are 2 inches thick in the middle, and I4 

 where they enter the sides. At the 21st round the thickness is 14 in the middle, 

 and 1-l-at the shoulder. About the 31st round the thickness is \\ in the middle, 

 and \\ at the shoulder, and this size is nearly preserved up to the end. Those 

 parts of the rounds which enter the sides of the ladders have all been turned in a 

 lathe, and are about \ of an inch tapering, in order to fill the holes properly, 

 which were also made a little tapering so as perfectly to answer the size of the 

 rounds. The lowest flat is 44- inches by 2. The next, as far as the 10th, are 

 3 J- by H ; from the 11 th to the l6th they are 2f by 14 ; and from the 17 th to 

 the last 2-1- by \\ inches. 



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