479 



weight is wound, 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 apparatus as possible, were inclosed in a tin case, filled with 

 water kept constantly 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 the 

 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 con- 

 sequently falling, the current of air is checked, and the heat within 

 the furnace is checked also. If, on the contrary, the change in the 

 radiant heat be a diminution, then a reversed operation takes place, 

 and the current of air being increased, the temperature of the fur- 

 nace is also increased. 



A letter was afterwards read from Mr. Telfair to Sir Alexander 

 Johnstone, V.P.R.A.S., accompanying a specimen of l'ecent con- 

 glomerate rock, from the Island of Madagascar, containing frag- 

 ments of a tusk, and part of a molar tooth of a hippopotamus; and 

 communicated by Roderick Impey Murchison, Esq. F.G.S. 



