48'2 PHILOSOPHICAL TRANSACTIONS. [aNNO 1754. 



quite loose. He has not sustained the smallest loss of blood since the opera- 

 tion. The pain and fever had been very inconsiderable, and he seemed to be in 

 a very fair way of doing well. 



LXXIX. Of a new Pyrometer, with a Table of Experiments made with it. 

 By Mr. J. Smeaton, F. R. S. p. 5Q8. 



This instrument is capable of receiving a bar 2 feet 4 inches long, and might 

 be made capable of receiving bars of a much greater length, of some kinds of 

 materials, but not of others, on account of the flexibility brought on them by a 

 degree of heat not greater than boiling water. 



The measures taken by this instrument are determined by the contact of a 

 piece of metal with the point of a micrometer-screw. The observation is the 

 best judged of by the hearing, rather than that of the sight or feeling. By this 

 method Mr. S. found it very practicable, to repeat the same measurement several 

 times, without differing from itself above -j-o-Fo-r P^rt of an inch. This principle 

 of determining measures by contact is not wholly new, but has been employed 

 on several occasions, as he was informed by the late Mr. Graham ; but the pre- 

 sent manner of applying it he believes is so: and the degree of sensibility arising 

 from it exceeds any thing he had met with. As the method will easily appear by 

 the draught (see pi. g, fig. 10 and 11) he avoids a further description of it in 

 this place.* 



As no substance has hitherto been discovered in nature, that is perfectly free 

 from expansion by heat, I chose to construct this instrument in such a manner, 

 that the bar, which makes the basis of the instrument, shall in each experiment 

 suffer the same degree of heat, as the bar to be measured: of consequence, the 

 measures taken by the micrometer are the differences of their expansion. The 

 expansion then of the basis between two given degrees of heat being once found, 

 the absolute expansion of any other body, by adding or subtracting the difference 

 to or from the expansion of the basis, according as the body to be measured ex- 

 pands more or less than the basis, will also be determined. 



When the instrument is used, it is immerged, together with the bar to be 

 measured, in a cistern of water; which water, by means of lamps applied under- 

 neath, is made to receive any intended degree of heat, not greater than that of 

 boiling, and so communicates the same degree of heat to the instrument, the 

 bar, and to a mercurial thermometer immerged in it, for the purpose of ascer- 



• I have lately seen an instrument at Mr. Short's, made by the late Mr. Graham, for measuring 

 the minute alterations, in length, of metal bars; which were determined by advancing the point of 

 a micrometer- screw, till it sensibly stopped against the end of the bar to be measured. This screw 

 being small, and very lightly hung, was capable of agreement within the 3 or 4000th part of an 

 inch. — Orig. 



