[ M ] 



Academy : and in order to form a more juft eftimate of their 

 value, I have given fome {ketches of this inftrument, as com- 

 monly executed, with a brief defcription of the ufual modes of 

 illuminating the wires, and fubjoined the method adopted in that 

 belonging to our obfcrvatory, with fome other more important 

 particulars. 



Fig. I. Plate III. reprefents the inftrument as commonly con- 

 ftruded, mounted ready for obfervation. The detail of the parts 

 is unneceffary, the inftrument being in fuch general ufe as to 

 be known to every one. The candle or lamp for illuminating 

 the wires by night is here made to defcribe a circle equal and 

 parallel to that defcribed by the refleding furface placed before 

 the objed glafs, the centres of both lying in the fame right line 

 parallel to the horizon. This movement is effeded by a fimple 

 apparatus reprefcnted in Fig. II. : it confifts of a ftrong ring of 

 wood, which furrounds the axis of the inftrument at a fmall 

 diftance, and is made faft to the pillar j this ring fupports and 

 confines another, which has a liberty of circular movement, and 

 carries the two arms A. B. one fuftaining a lantern, and the other 

 a counterpoife ; the lantern is loaded at the bottom, and plays 

 freely upon a pin placed above its centre of gravity, and there- 

 fore always preferves a vertical fituation ; it is fo contrived as to 

 let out the fmoak at the back part, by which means the obfer- 

 vations are lefs affedled by the tremors arifing from the vapour of 

 the candle. 



There is another method, very convenient for fmall inftru- 

 ments, which I firft faw at St. John's College, Cambridge ; it is 



reprefented 



