THERMOMETER AND PYROMETER. 



/ on the axis of the wheel m m. This 

 wheel has round its circumference 100 

 teeth, which work in the ten leaves of 

 the pinion n, on the axis of the wheel o 

 of 100 teeth, that gives motion to the 

 pinion p of ten leaves, on the axis of 

 which the index is fixed. 



As the wheels m and n have each 

 100 teeth, and the pinions n and p ten 

 leaves, it is obvious that when the wheel 

 m has made one revolution the pinion 

 p, and of course the index, will have 

 made 100 revolutions ; as the pinion / 

 has twelve leaves, and the bar i k has 

 fifteen teeth to one inch (equivalent to 

 12^1 it is obvious that while ik moves 

 one inch the pinion/? will have moved 

 100 + i or 125 times round, and the in- 

 dex would at the same time be carried 

 125 times round the circle dd. This 

 circle is graduated into 360 degrees, and 

 beina: eleven inches in diameter, it is 

 subdivided into half degrees. Hence 

 each degree of that circle will be equi- 

 valent to an expansion of 125 x 360 

 = _ 5 _ 5 th of an inch of expansion in 

 the bar/; and as the half degrees can 

 readily be distinguished on the dial, the 

 instrument will show expansions only 

 amounting to ?5 & 3 oth part of an inch. 

 A silk thread is several times wound 

 round the axis of n and passes to the 

 slender spring s, which keeps the teeth 

 of the pinions and wheels in close con- 

 tact, and pulls back the train of wheels 

 v r hen the cooling of the bar / allows 

 the short bar i k to recede. 



The inner circle of the dial is divided 

 into eight parts, corresponding to so 

 many thousandths of an inch in the ex- 

 pansion of the bar/, or t^th of an inch 

 for each degree of the outer circle over 

 which the index has moved. Bars of 

 different metals laid in g for a given 

 time, and exposed to the same lamp, 

 afford an indication of their relative ex- 

 pansibility ; and to ensure equality in 

 the bars it is recommended to have 

 them wire drawn through the same 

 hole. There is, however, in this instru- 

 ment no accurate measure of the tem- 

 perature applied to each bar ; and, not- 

 withstanding the delicacy of the move- 

 ment, it seems inferior to Ellicot's 

 pyrometer, as it wants a constant and 

 uniiorm standard by which to compare 

 the expansions in each separate experi- 

 ment. 



10. A new method of ascertaining the 

 expansibility of different substances was 

 suggested by the late Mr. Jesse Rams- 

 den, and on his hint it was attempted 



by the ingenious and indefatigable D 

 Luc, whose researches on the barometer 

 gave this subject an increased interest 

 to his mind. The object in view was to 

 determine the relative expansion of so- 

 lids by observation with a microscope 

 furnished with a micrometer. The 

 microscopic pyrometer of De Luc is 

 seen in Jig. 17, where a b represents a 



Fig. 17. 



strong board of even-grained deal ; to 

 which the frame c c c c is firmly joined, 

 that when a b is suspended vertically 

 from a strong post, the front of the in- 

 strument bearing the microscope d d is 

 towards the operator. 



The microscope is securely united to 

 the frame by the braces e e and the 

 cross-bar /; and the whole of this part 

 of the apparatus can be moved up or 

 down by the slides g g, which fit so 

 tightly on their centres as to require 

 slight blows with a hammer on the 

 frame to cause them to move down, 

 and may be further tightened by the 

 screws Hi. The microscope is kept 

 horizontal by the cross-pieces, and by an 

 inner sliding frame not seen in the 

 figure. The microscope is so adjusted 

 that an object is distinctly seen when a 

 full inch from its lens ; and it is furnished 

 with a micrometer, movable by k, for 

 ascertaining the expansions of the rods 

 subjected to experiment. A piece of 

 thick deal I is seen at the top of the 

 frame lying horizontally from a groove 



