THERMOMETER AND PYROMETER. 



10387, the following formula will ex- 

 press the height of such thermometer 

 when plunged in boiling water under 

 every variation of barometric pressure. 

 99 



5o 



or, as expressed in the more usual way 

 of considering all the figures after the 

 index as decimals, De Luc's formula 

 would stand thus : 



99 x 100 . , 

 2 log y - a = T. 



De Luc's researches and his formula 

 are reduced to English measures, and 

 adapted to Fahrenheit's thermometer by 

 Horsley, in a valuable paper in the 

 Philosophical Transactions;* where a 

 table is computed for the direction of 

 artists in adjusting the boiling point. It 

 is unnecessary to give his equation of 

 the boiling point, because the later ex- 

 periments of Shuckburg, and of the 

 Committee of the Royal Society, enable 

 us to present a more complete table for 

 the direction of British artists in cor- 

 recting the height of the boiling point in 

 every ordinary fluctuation of the baro- 

 meter. 



The use of this table requires no fur- 

 ther explanation : but it is necessary to 

 remark, that it presupposes the thermo- 

 metric tube to be cylindrical, or of equal 



* Vol. liiv. parti. 



dimensions throughout, before the indi- 

 cations of the table can be received as 

 quite correct ; yet, unless the irregularity 

 of the tube be considerable, a small 

 correction will scarcely produce any 

 sensible error in the instrument. 



In proportioning the bulb to the 

 tube, the eye and experience of the artist 

 are usually judged sufficient for the pur- 

 pose ; or they are copied as nearly as 

 possible from standard instruments. 

 M. Durand has, however, thought it 

 necessary to propose an algebraic for- 

 mula for determining the proportions 

 they ought to bear to each other ; but 

 there are practical difficulties in the way 

 of its application, which render his for- 

 mula an exercise rather of his own 

 ingenuity than of utility to the artist. 



During the various improvements of 

 the common thermometer, the air ther- 

 mometer was almost wholly neglected 

 until of late years ; but the attention of 

 philosophers was directed to the changes 

 of bulk which solids undergo by altera- 

 tions of temperature, as a measure of the 

 relative degrees of heat. 



CHAPTER II. 



History and Construction of Pyro- 

 meters. 



1. THE impracticability of applying 

 the known modifications of the thermo- 

 meter to bodies much heated, induced 

 the celebrated Musschenbroek, before 

 the middle of the last century, to employ 

 the expansions of solid rods of metal to 

 indicate the temperature of such bodies ; 

 and he gave the name of pyrometer to 

 his invention. 



As the expansions of solids are ex- 

 tremely minute, it was necessary to 

 devise some method of rendering them 

 perceptible ; and the mechanism repre- 

 sented in/g-. 8 was the Dutch philoso- 

 pher's arrangement for this purpose, 

 a is a metallic prism 5.8 inches in length 

 and 0.3 in thickness, resting in a notch 

 in the upright i, where it is secured by a 

 screw, and heated by the lamp b with 

 five wicks. The prism is pinned to the 

 end of a bar c, which has twenty-five 

 teeth in one inch of its length, and forms 

 a rack sliding smoothly on the table of 

 the instrument through the two holdfasts 

 seen in the figure, and playing in the 

 six-leaved pinion d on the same axis as 

 the wheel /, which is furnished with 

 sixty teeth. This wheel plays in another 

 pinion <?, of six leaves also, which is on 

 the axis carrying the index g, which 



