MR. DAN I ELL ON A NEW REGISTER-PYROMETER 
262 
visible in the day- light is above 10 77 ° 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 investigating 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 1 ; y 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 solid end ; and a cylindrical piece 
of porcelain 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 platinum ; 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 porcelain inserted be- 
tween 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 con- 
traction 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 
quantities in astronomical and geodetical operations. For this purpose the 
