310 Daniell on a iSlev Pyrometer. 



its dimensions in proportion to the intensity of the heat to 

 which it is exposed. His experiments and results are alona 

 referred to in books of science upon this subject, but the py- 

 rometer itself has long fallen into disuse, partly from the 

 difficulty of obtaining clay-pieces of an uniform composition, 

 but chiefly from the discovery that a long continuance of a 

 lower degree of heat produces the same effect of contraction as 

 the shorter continuance of a higher degree. 



Being struck with the importance of the subject, I have 

 lately bestowed much of my time, and made many fruitless 

 endeavours to attain this desirable object. At length, however, 

 I flatter myself that I have been fortunate enough to combine 

 an instrument extremely simple in its construction, very ma- 

 nageable in its use, not liable to injury, when injured easily re- 

 paired, and which will extend the scale of the thermometer, 

 at least to the fusing point of cast iron. Its sensibility also is 

 very great, considered with regard to the extensive range which 

 it is destined to measure. A change of about seven degrees of 

 Fahrenheit's scale is distinctly indicated by it, while, on the 

 other hand, every degree of Wedgwood's pyrometer was calcu- 

 lated to be equivalent to 130° of the same thermometer. 



The results which I have obtained with this instrument differ, 

 unfortunately, very widely from those of Mr. Wedgwood, but 

 I shall, as I proceed, state my reasons for believing that mine 

 are the more accurate of the two. I shall not enumerate the 

 different steps by which I proceeded, but shall at once describe 

 the pyrometer in the most perfect state which my hitherto 

 limited experience of its use enables me to suggest. 



Plate VI., fig. 1, represents the instrument drawn to the 

 scale at the side of the plate. Fig. 2 represents a part of the 

 same of half the real dimensions. The tube a6 c is made of 

 black-lead earthen ware, and the shoulder in its centre is 

 moulded in its construction. The extremity a is close, and the 

 extremity c open ; d is a ferule of brass into which the end of 

 the black-lead tube is accurately fitted, and to which the scale 

 efg h is attached. In the inside of the tube ab c lying upon 



