331 



transferred to Paris by an assumed retardation of 12', gives a length 

 differing from M. Biot's by 0-00023. Borda's agrees within 0-00079 

 with M. Biot's, and Captain Kater's, so transferred, holds very nearly 

 a mean between the two, but approaches rather nearer to Biot's than 

 to Borda's. 



On the Measurement of High Temperatures. By James Prinsep, Esq. 

 Assay Master of the Mint at Benares. Communicated by Peter 

 Mark Roget, M.D. Secretary of the Royal Society. Read Decem- 

 ber 13, 1827. [Phil. Trans. 1828,^. 79.] 



The author, after adverting to the many abortive endeavours of 

 former experimentalists to obtain instruments for the accurate ad- 

 measurement of high temperatures, and after suggesting doubts as 

 to the confidence to which Wedgwood's pyrometer is entitled, de- 

 scribes several attempts of his own to effect this very desirable object. 

 In the course of his inquiries, a remarkable fact presented itself to his 

 notice in the change which occurred in an index constructed on the 

 compensation principle, and formed by two slips of metal, the one of 

 silver and the other of gold, originally quite pure, and united with- 

 out any alloy. In the course of a few years, although it had never 

 been subjected to a heat above that of melting lead, the whole sur- 

 face of the gold became converted into an alloy of silver, the impreg- 

 nation extending gradually to a considerable depth in the gold, and 

 destroying the sensibility of the instrument to changes of tempera- 

 ture. After trying various plans, he gave the preference to one 

 founded on the following principles : namely, that the fusing points 

 of the pure metals are fixed and determined ; that those of the three 

 noble metals, namely, silver, gold, and platina, comprehend a very 

 extensive range of temperature ; and that between these three fixed 

 points in the scale, as many intermediate ones as may be required 

 may be obtained by alloying the three metals together in different 

 proportions. When such a series of alloys has been once prepared, 

 the heat of any furnace may be expressed by the alloy of least fusi- 

 bility which it is capable of melting. The determinations afforded 

 by a pyrometer of this kind will, independently of their precision, 

 have the advantage of being identifiable at all times and in all coun- 

 tries : the smallness of the apparatus is an additional recommendation, 

 nothing more being necessary than a little cupel, containing in sepa- 

 rate cells the requisite number of pyrometric alloys, each of the size 

 of a pin's head. The specimens melted in one experiment need only 

 to be flattened under the hammer in order to be again ready for use. 

 For the purpose of concisely registering the results, the author em- 

 ploys a simple decimal method of notation, which at once expresses 

 the nature of the alloy, and its correspondence with the scale of tem- 

 perature. Thus G .23 P would denote an alloy of gold with 23 per 

 cent, of platina. As the distance between the points of fusion of 

 silver and of gold is not considerable, the author divides this distance 

 on the scale into ten degrees ; obtaining measures of each by a suc- 



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