VOL. LXXXV.] PHILOSOPHICAL TRANSACTIONS. 601 



This machine is called the bar-index. It is of eminent use in giving imme- 

 diately, by means of a table made for that purpose, the place of the point of 

 support for any given altitude or zenith distance of the quadrant. In order to 

 come at every part of the heavens, the vertical motion of the telescope requires 

 the addition of the horizontal one. This has been obtained by another very 

 simple mechanism, here described also. And then it is added, with the 

 assistance of the motions that have now been described, I have in the year 1789, 

 many times taken up Saturn, 2 or 3 hours before its meridian passage, and kept 

 it in view with the greatest facility, till 2 or 3 hours after the passage; a single 

 person being able, very conveniently, to continue both the horizontal and ver- 

 tical motions, at the command of the observer. In this however ought to be 

 included an assisting 3d motion, which I am in the next place to explain. 



In fixing the ladders they were set at 8 feet 2 inches distance in front, to 

 permit the telescope to have a side motion, without displacing the whole appa- 

 ratus, which is designed for a meridional situation. Every celestial object, when 

 it passes the meridian, is then in its most favourable situation for being viewed, 

 on account of the greater purity of the atmosphere in high altitudes. The ad- 

 vantage also of being able to direct the instrument, by means of the quadrant, 

 to the spot in which we are to view the object, is considerable, in so large 

 an instrument as the 40-feet telescope. With unknown obiects, it is also of 

 the greatest consequence to be enabled, by a meridional situation, to ascertain 

 their place. But as a single passage through the field of view, especially with 

 examinations of the heavens in zones, would not have been sufficient to satisfy 

 the curiosity of an observer, when a new object presented itself, it became 

 necessary to contrive a method to lengthen this interval. The tube therefore 

 is made to rest with the point of support in a pivot, which permits it to be 

 turned side-ways. Its diameter being 4 feet 10 inches, and that part which 

 is generally opposite the ladders that confine it in front being about 35 feet 

 from the pivot, it appears that a motion of 3 feet 4 inches may be had, 

 which to the radius of 35 feet gives upwards of 5° of a great circle. Several 

 abatements must be made on account of the disposition of the apparatus 

 that gives this side motion, and the shortness of the ropes in high alti- 

 tudes; but there remains, notwithstanding, a sufficient quantity of this lateral 

 motion to answer the purpose of viewing, pretty minutely, every object 

 that passes the meridian. 



Before I can give the particulars of this side motion, some other things must 

 be explained. The point of support rests in a pivot; but this alone could not 

 have given steadiness to a tube of 4 feet 10 inches in diameter, loaded with the 

 weight of the strengthening bars, and speculum, which rest on it. Two move- 

 able supporters have therefore been provided. They consist of 2 solid brass 



VOL. XVII. 4 H 



