26 



THERMOMETER AND PYROMETER. 



movable shelves, and showing only 

 their handles h h h h, when under the 

 boiler. 



The boiler contains another essential 

 part of the apparatus, viz. two brass 

 slides, composed of two cheeks, kept 

 at equal distances by cross bars as in 

 C, where a prism or bar is represented 

 as resting in the centre of the slides. 

 The long slide reaches from microscope 

 to microscope, and has its cheeks If 

 inch deep. It is attached to the boiler 

 only at the point w, and it rolls on the 

 small roller x, near the left hand of D. 

 The right hand end of the long slide is 

 shut up by a piece of strong brass y, y, 

 supporting two rings, for the part n of 

 the fixed microscope. The short slide 

 v, v, v, v, is only 14 1 inches long ; its 

 cheeks are H inch deep, kept parallel by 

 braces, as seen in D. It moves within 

 the long slide ; and its outer end rests 

 on the cylindrical surface of the last 

 brace of the long slide, fitted to receive 

 it, while a narrow longitudinal bar z 

 moves freely in the notch of a bridge 

 B, framed for it in the long slide. The 

 outer end of the short slide is shut up 

 by a similar piece of brass to that clos- 

 ing the opposite extremity of the long 

 one. The bar or rod to be examined 

 abuts against the piece of brass y, 

 it rests on the three rollers s s s, I inch 

 in diameter, and is kept in the centre of 

 the slides by three milled nuts eee> that 

 screw up so as not to press too much 

 on the sides of the bar. At / is a tube 

 and wire moving through a collar of 

 oiled leather, that by means of a helical 

 spring presses a flat piece of metal 

 attached to the wire, against the shut 

 end of the short slide and rod to be 

 measured, so as to keep the other ex- 

 tremity of the rod in contact with y. 

 On the application of heat, the rod ex- 

 pands, and overcoming the slight resis- 

 tance of the spring, carries before it the 

 short slide, and with it the tube con- 

 taining the object lens of the micro- 

 meter microscope o, p, r, a space pro- 

 portional to the temperature applied; 

 and it is this space, measured by the 

 micrometer, that determines the nume- 

 rical value of the expansion of the rod. 

 The microscope tubes are divided 

 into three pieces, for the convenience of 

 applying the instrument to measure 

 rods shorter than five feet. For this 

 purpose the centr.il screening tube of 

 the fixed microscope, supported on the 

 mahogany prism i k by a collar, may 

 be moved and damped at any part of 



that prism ; the eye-piece, in like man- 

 ner, may be moved along the eye prism ; 

 but the object lens tube was left in the 

 rings of the slide, and another lens of 

 the same focus was clamped to the 

 cheeks of the slide at suitable distances. 

 The standard prisms, during each 

 experiment, were kept at the freez- 

 ing temperature, by being sur- 

 rounded with pounded ice. The mi- 

 croscopes were then accurately ad- 

 justed to the marks, by bringing the 

 cross wires to bisect them, and until this 

 was accomplished, the rod to be mea- 

 sured was also surrounded with ice. 

 The lamps were then applied to the boiler, 

 and the elongation of the rod, at the 

 boiling heat, was ascertained by the 

 micrometer attached to the microscope 

 o, p, r. In these delicate investiga- 

 tions there were two observers, who si- 

 multaneously used both microscopes, 

 lest any alteration had taken place in 

 the fixed end of the rod ; and to ensure 

 accuracy, the experiments were twice at 

 least repeated. 



The value of the indications of the 

 micrometer, on which so much depends, 

 was previously thus ascertained : 



The head of the micrometer screw 

 = 0.9 inch in diameter, and was divided 

 into fifty equal parts, each of which was 

 reckoned two ; and they were therefore 

 numbered to 100. Fifty-five revo- 

 lutions of the head were found equal 

 to 0.77175 of an inch; it follows that 

 there are 71.27 threads of the screw in 

 one inch ; and seven revolutions and 

 nearly T ~ths move the micrometer wire 

 jyh of an inch ; consequently T5 Vh of 

 part of a revolution, or half a division of 

 the head, will answer to a motion of 

 something more than 0.00014th of an 

 inch. Having found 7.13 revolutions 

 equal to 0.1 inch at the wires, it is ob- 

 vious that the number answering to 0.1 

 inch at the mark being aiso found and 

 added to the former, their sum will give 

 the measure of 0.1 inch at the object 

 lens of the microscope o, p, r, or the 

 space through which the free end of the 

 rod has moved by the change of tempe- 

 rature. This last point was ascertained 

 by experiment to be = 24.93 revolutions 

 of the micrometer head ; which being 

 added to 7.13 = 32.06, " for the number 

 of revolutions measuring a motion of 

 0.1 at the object lens, or an expansion 

 of T \jth of an inch," or half a division of 

 the micrometer head is equivalent to an 

 expansion of the rod under examination 

 of an inch ; and \ of a division, 



