196 Mr. Daniell on a New Register- Pi/romeler, 



they would have been estimated at about 20° Fahr. ; and taking 

 Mr. Wedgwood's original determination of the fusing point 

 of silver at 28° of his scale and the zero point at 1077°, the 

 former would come out about 1650°. By raising the zero 

 point a Httle, (and it is much more probable that the tempe- 

 rature of a red heat fully visible in the day-light is above 1077° 

 than below it,) we arrive at something like an approximation 

 to the truth. These wide discrepancies, and the practical 

 disuse of both Mr. Wedgwood's and M. Guyton's pyrometers 

 for a long time past, prove the expediency of further investi- 

 gating a subject of so much interest and importance. 



The pyrometer, which I shall now proceed to submit to the 

 judgement of the Society, consists of two distinct parts, which 

 I shall designate as the Register and the Scale. 



The first is a solid bar of black-lead earthenware, eight 

 inches long, seven-tenths of an inch wide, and of the same 

 thickness, cut out of a common black-lead crucible. In this 

 a hole is drilled three-tenths of an inch in diameter, and 7^ 

 inches deep. At the upper end of this bar and on one of its 

 sides about six-tenths of an inch in length of its substance is 

 cut away to the depth of half the diameter of the bore. When 

 a bar of any metal 6^ inches long is dropped into this cavity, 

 it rests against its sohd end; and a cylindrical piece of porce- 

 lain about 1^ inch long, which I shall call the index, is placed 

 upon the top of it, which projecting into and beyond the open 

 part, is firmly confined to its place by a ring, or strap of pla- 

 tinum ; which passing round the black-lead bar and over the 

 piece of porcelain, is made to press upon the latter with any 

 required degree of tension by means of a small wedge of por- 

 celain inserted between the bar and the strap on the side of 

 the former. It is obvious that when such an arrangement is 

 exposed to a high temperature, the metallic bar will force the 

 index forward to the amount of the excess of its expansion 

 over that of the black-lead, and that when again cooled, it will 

 be left at the point of greatest elongation. It may also be 

 observed, that the exact indication of this amount is not in the 

 slightest degree interfered with by any permanent contraction 

 which the black-lead may undergo at high degrees of heat ; 

 as any such contraction will take place at the moment of the 

 greatest expansion of the metal, and the index will still mark 

 its point of furthest extension upon this contracted basis. 



The problem now consists in the accurate measurement of 

 the distance which the index has been thrust forward from its 

 original position ; and although the amount can in any case 

 be but small, there is no reason why it may not be determined 

 with the same precision as is now commonly attained in similar 



fiuantities 



