History of Mechanic ul Inventions, 4r. 339 



" In the third number of your Journal, in commenting on an in- 

 strument which bears my name, you seem to intimate that I have laid 

 claim to an invention which does not belong to me ; * it becomes my duty, 

 therefore, to explain where I rest my pretensions. 



" The principle upon which I have perfected the Pyrometer, which is 

 the subject of this letter, is the old-established principle of expansion of 

 air by heat, and is analogous to the Differential Thermometer of Sturmius, 

 claimed by Leslie, and also to the construction of the common steam guage, 

 which acts by the pressure of steam against a column of mercury. 



" The principle of the instrument, therefore, delineated by Dr Ure, is not 

 new, and consequently does not belong to him. Although the suggestion 

 of applying it to a Pyrometer unquestionably is his, I have never pre- 

 sumed to lay claim either to the one or the other. It is in the mechanical 

 construction of this instrument that I claim any merit. Many difficulties 

 presented themselves in its construction which could hardly have been an- 

 ticipated by Dr Ure, and which required some little ingenuity to over- 

 come. It was found that an air-tight screw in platinum could not be 

 made to resist considerable pressure ; it was, therefore, necessary to have 

 recourse to a stem without a joint, which, after a considerable period, I 

 accomplished. The internal diameter of the platinum bulb measured one 

 half inch, whilst the stem proceeding therefrom was not the twentieth 

 part of an inch internal diameter; and to construct this in a metal so dif- 

 ficultly fusible as platinum was pronounced an impossibility by the first 

 philosophical instrument maker of this metropolis. It was also found, 

 that, by uniting the platinum stem to a glass tube, as recommended by 

 Dr D"re, it was useless in its application, because its frangibility was 

 such as to render it liable to be broken by every trifling motion, inde- 

 pendently of which, its position could not be accommodated to a furnace. 

 It therefore became necessary to have a moveable air-tight joint, in con- 

 structing which no small difficulty arose, because the pressure of the con- 

 fined air, when intensely heated, was so great as to force itself through 

 common joints ; but that obstacle was also removed; and the instrument, 

 as it now appears, may be made to traverse in any position within the ra- 

 dius of a circle ; and without it was so constructed, would, of course, be 

 next to useless. 



■' I submit, under the circumstances above stated, that the principle of 

 this Pyrometer belongs to Sturmius, the delineation of it to Dr Ure, and 

 the execution of it to myself." 



Art. XXXI— HISTORY OF MECHANICAL INVENTIONS AND 

 PROCESSES IN THE USEFUL ARTS. 



1. Mr Ritchie's Photometer, and the Illuminating Powers of Oil and 



Coal-Gas. 



An account of this ingenious instrument, invented by Mr William 

 Ritchie, Rector of the Academy of Tain, has been read before the Royal 



We have merely pointed out the similarity of the two instruments. F,n. 



