THERMOMETER AND PYROMETER. 



17 



steel, and is that by which the expan- 

 sions of all the other metals are to be 

 compared together. Its extremity to 

 the risrht passes through a hole in the 

 upright, and is fixed to a spring which 

 may be tightened by the screw ra. Its 

 other extremity is free, and presses 

 against a snail on the axis of the lever 

 /. The other bar e e is a prism of any 

 metal, the right end of which rests on 

 the end of the screw /, while its other 

 bears on a snail on the axis of the lever 

 h. When the bars are expanded by the 

 heat of the spirit lamp g, they move 

 the levers, to each of which is attached 

 a slender watch-chain : the chain from 

 the lever / passes round a pulley \ 

 inch in diameter, fixed on the axis 

 round which the inner graduated circle 

 of the dial moves ; the chain from the 

 lever h passes round a similar pulley 

 on the axis of the index, as seen in the 

 figure ; and the expansions of this bar 

 are marked by the index on the fixed 

 outer circle. Both pulleys have a 

 thread wrapped round them in a con- 

 trary direction on each, and then pass- 

 ing over the pulleys at o to the weight 

 k, which acts as a counterbalance to 

 bring back the index and movable 

 circle as the bars cool. The index and 

 circle are both adjusted to the be- 

 ginning of their scales by means of the 

 screws /, m, at the commencement of 

 each experiment; and when the tem- 

 perature applied expands the standard 

 bar to a given degree, as indicated on 

 the inner circle, the index will show on 

 the outer circle the relative expansi- 

 bility of whatever metal is applied to 

 the instrument at e e.*- 



This instrument was chiefly intended 

 by its ingenious inventor, a chrono- 

 meter-maker by profession, for ascer- 

 taining the relative expansion of the 

 metals usually employed in the con- 

 struction of pendulums ; an important 

 object, for which many of the best py- 

 rometers have been devised. 



In this instrument the dial is about 

 three inches in diameter ; the levers 

 two inches and a half in length, and 

 the proportions of the several parts 

 such that the expansion of ,"& inch in 

 the bar will move the index wholly 

 round the circle ; or each degree will 

 mark the 75 Vjj of an inch in the length- 

 ening of the bar. From the mean of 

 numerous experiments, Ellicot ascer- 



This description is taken from an original in- 

 strument now before the author. 



tained the following to be the relative 

 expansions of seven metals . 



Steel. Iron. Gold. Copper. Brass. Silver. Lead. 



56 60 73 89 95 103 149,* 



which is more nearly in the ratio of the 

 conducting power of the different me- 

 tals, than ot any other of their physical 

 properties. 



4. n the 44th volume of the Philo- 

 sophical Transactions is a description 

 of another pyrometer by Dr. Cromwell 

 Mortimer, which, though less accurate 

 and convenient than Ellicot's, is worthy 

 of notice, especially as it may be em- 

 ployed to show the alterations of 

 atmospheric temperature. 



a, b, fg. 1 0, is a round rod of brass 

 Fig. 10. A. 



or steel, % inch in diameter and three 



* Phil. Trans, vol. xxxix. p. 297. 

 C 



