490 Notices and Extracts from the Minutes of the Geological Society . 



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 circumference 

 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 sup- 

 plied 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 openings. 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 weight is 

 \vound, the axle connected with the spiral is furnished with a long hand, like 

 that of a clock, reaching to a large graduated circle in the same plane with 

 the first wheel, but lying beyond it. This hand is capable of being attached 

 to the wheel carrying the weight, and consequently of indicating changes of 

 temperature with great rapidity. 



To preserve uniformity of temperature, the spiral, and as much of the ap- 

 ])aratus as possible, were inclosed in a tin case, filled with water kept con- 

 stantly boiling; so that the only change to which the spiral was subjected, 

 was the heat radiating from the muffle. 



The action of the instrument is simply this. The heat of the furnace having 

 been raised to the required pitch, a change in tlie radiant heat from the 

 muffle effects a change in the action of the spiral, and the string supporting 

 the weight is consequently either lengthened or shortened. If the change in 

 the radiant heat be an increase, then the string is shortened, the weight is 

 removed from the cup at one end of the lever ; and the disks at the other end 

 consequently falling, the current of air is checked, as well as the heat within 

 the furnace. If, on the contrary, the change in the radiant heat be a dimi- 

 nution, then a reversed operation takes place, and the current of air being- 

 increased, the temperature of the furnace is also increased. 



