Description of a Properfij of Caoutchouc. 39 



r.Rtil it makes an angle of about 30° with the top part of the 

 frame in wiiich it turns. Then place a lighted candle before 

 the cometarium, at the distance of 18 feet, and the liffht 

 which passes through the great lens forms a beautiful repre- 

 sentation of the tail of a conjet upon the wall ; the licht 

 which passts through the small lens represents its head, and 

 that light which falls upon the other lens is converged to a • 

 circular spot, which represents the snn. 



When the instrument is turned round by hand on the piii 

 which supports it, the comet revolves round the sun, the 

 head and tail of the comet alwavs keeping in the same posi- 

 tion, with respect to the sun, in every part of its orbit. 



And as no light is suffced to pass through the instrument 

 except that which goes through the glasses, and no licht 

 being in the room except one candle, the luminous pictures 

 are seen, upon the dark shadow made h\ the opaque parts of 

 the instrument, exceedingly clear and well defined. The 

 head and that part of the tail which joins to it are very bril- 

 liant ; but this brilliancy of the tail graduallv decreases to- 

 wards its end, as represented in Plate III. 



I am, sir, your humble servant, 



Ez. Walker. 



VII. yi Description of a Property of Caoutchouc, or Indian 

 Rubber; with some Reflections on the Cause of the Elas- 

 ticity of this Substance, In a Letter to Dr. Holme ". 



Middleshav/, near Kendal, 

 SIR} Nov. 16, I'M'J. 



J. HE substance called caoutchouc, or Indian rubber, j)os- 

 tjcsses a singular property, which, I believe, has never been 

 taken notice of in print, at least by any English writer : the 

 present letter contaitis my experiments and reflections on the 

 subject; and should they appear to deserve the attcniion of 

 your philosophical friends, I am certain you will take the 

 trouble of communicating the paper to the Literary and 

 philosophical Society of Manchester. 



• from Manchester Tian.trirtions, second scries, vol.i. 



C4 Tiw» 



