THERMOMETER AND PYROMETER, 



21 



the pulley h in different ways, and thence 

 go over the pullies at t, respectively, 1 

 inch and ^ inch in diameter. These last 

 are put on the common axes of the 

 indices k, I, in the same manner as the 

 hands of a clock. The face of the dial 

 is 12 inches in diameter, and from the 

 construction, the index / ranges 48 

 times, and the index k 12 times as much 

 as the bar g. The dial has on it three 

 circular scales ; the inner is divided into 

 240, corresponding to those of Fahren- 

 heit's thermometer ; the middle is di- 

 vided into 360; and the outer into 

 1080 parts, marking 18 for each degree 

 of the thermometer, and 12 for each 

 degree of the circle. 



This instrument may be used as a 

 pyrometer in low temperatures ; for the 

 bar a is removable ; and from the con- 

 struction, each division of the outer 

 circle is equivalent to an expansion of 

 755T<>th of the bar. 



Used in this way, Mr. Fitzgerald in- 

 forms us that the dilatations of metallic 

 bars 2 feet long, at the same tempera- 

 ture, were as follows : 



Divisions. 



Spelter or zinc . . . . = 1570 

 Zinc 18, copper 2 parts . . = 1550 



Brass =1120 



Iron = 785 



Steel = 695 



which agrees pretty well with the ex- 

 periments of Smeaton and Ellicot. 



When used as a thermometer the in- 

 dex k marks 74 divisions in passing from 

 the usual extremes of temperature in 

 our climate, and 212 divisions from 

 freezing to boiling water. 



Mr. Fitzgerald experienced some 

 difficulty in proportioning the strength 

 of the springs to the weight sustained 

 by the levers, and he improved the in- 

 strument by the adoption of pulleys and 

 counterpoise weights, as in fig. 14, 

 which he ingeniously converted into a 

 register thermometer, by adapting two 

 light index hands a, a, fixed to two 

 brass circles moving between friction 

 wheels, attached to a fixed circle d. 

 They were so nicely fitted as to move 

 readily by a weight of 8 grains hung on 

 them. These hands are moved in op- 

 posite directions, by a small stud in the 

 under surface of the index f, which re- 

 ceives its motion from a cord passing 

 from the pulley h round a small wheel 

 on its axis. 



This alteration of the instrument was 

 intended only to note the changes of the 

 atmosphere, which it seems to have done 



with much delicacy ; for it had a range 

 Fig. 14. 



of 72 inches from the common changes 

 of the heat of the weather in London ; 

 and it would show an alteration amount- 

 ing to 50 or 60 degrees of its scale, 

 when the pyrometric bar of the instru- 

 ment was five or six times breathed 

 upon* 



8. In Ferguson's Lectures two pyro- 

 meters, the invention of that great self- 

 taught mechanician, are described. 



Fig. 1 5 was merely intended to exhibit 

 to his audience the expansions 'of bodies 

 by heat, yet is worthy of notice. 



a a, a mahogany board, on which are 

 fixed four brass studs ; of these b sup- 

 ports a screw for adjusting the 'pyro- 

 metric bar/, which rests in notches in 

 the studs cd. The extremity of the bar 

 presses against the crooked lever g-, which 

 acts on the index i i ; the stud e holds 

 the spring h, which brings back the in- 

 dex when the bar cools. The lever g 

 (of the second order) has the portion 

 between the point of contact of the 



* Phil. Trans, vol. lii. p. 146. 



