478 



win Rogers, Esq., of Philadelphia; and John Lewis Prevost, Esq., 

 His Majesty's Consul-general for Switzerland, of Suffolk Place, 

 Pall Mall East ; were elected Fellows of this Society. 



A paper was first read, entitled, " Notice of a Machine for regu- 

 lating high temperatures, invented by the hate Sir James Hall, Bart., 

 F.G.S.," and drawn up by Captain Basil Hall, R.N., F.G.S. &c. 



Sir James Hall, in his experiments on the fusion of granite and 

 other rocks, and on the effects subsequently produced upon the fused 

 mass by gradual cooling, conceived that the experimenter required 

 the power of regulating the temperature in such a manner as best to 

 imitate nature ; and for this purpose he invented the machine de- 

 scribed by Captain Basil Hall. 



The principle of the machine is such, that when any change of 

 temperature takes place in that part of the furnace in which the 

 material under experiment is placed, a corresponding change is 

 made in the current of air which maintains the heat. 



The furnace was about 3 feet long, 18 inches wide, and 2| feet 

 deep. From side to side extended a muffle, one end of which was 

 closed with a plug, furnished with a small disk of mica, through 

 which the subject of the experiment could be viewed ; and at the 

 opposite end of the muffle was placed the machine. 



This instrument consists of a spiral spring coiled in a vertical 

 plane, and facing the muffle. The spring is formed on the principle 

 of Harrison's balance in chronometers, of two metals joined together, 

 but of different degrees of expansibility, so that the spring will either 

 curl or uncurl, according as the heat is raised or depressed. The 

 outer part of the coil is fixed, while the inner end is united to an 

 axle, which, being free, turns round as the spiral winds or unwinds ; 

 or as the heat, radiating from the interior of the muffle, is raised or 

 depressed. 



To the further end of the axle is bitted a wheel, around the cir- 

 cumference of which is wound a string, carrying at its extremity a 

 small weight, which consequently rises or falls as the spiral curls or 

 uncurls. Under the weight is a little cup, forming one end of 

 a lever, to the opposite end of which a metal disk is suspended, 

 immediately over an aperture of rather smaller dimensions, and 

 near the end of a long iron channel, through which alone air was 

 supplied to the furnace. Directly under this aperture is another 

 of similar size, as well as another disk united to the upper disk by 

 a rod of metal, equal in length to the distance between the two open- 

 ings. The object of having two apertures is to secure the same 

 amount of current of air both above and below. If there were 

 only one opening, and one disk to close it, the air rushing in would 

 force it down and keep it closed ; but by this contrivance the rush 

 of air at the lower opening striking on the under surface of the 

 lower disk, exactly counterbalances that from above which strikes 

 upon the upper surface of the superior disk. To render the points 

 of* contact perfect, and to prevent the interference of small particles 

 of dirt, the disks, when closed, rest upon circular knife-edges. 



Besides the wheel, around which the cord carrying the small 



