P Y R O M i: I E R. 



neter. 



I'! \ 1 I 



DCCCLXXJ. 



No. II. 

 KiB. 6. 7. 



Fig. 7. 



screw passes through two solid screwed hole* at D 

 and V. Tlu- pu.-,- Y/ is made a little springy, and 

 i-ndi-avmir- to pull the screw backwards Ir.nii tin- hole 

 at 1), ron^-qm -ntly tin- micrometer screw is constantly 

 g ;il r :nn>t its threads the same way. and thus ren- 

 ders it* motion perfectly steady and gentle. The index 

 X has divisions upon it answering to the turns of the 

 screw. This piece points out the divisions of the plate, 

 as the face of the plate points out the divisions of the in- 

 dex. In using the instrument, lay hold of the knob at 

 O with one hand, and moving the feeler up and down, 

 with the other move forward the screw I till its point 

 comes in contact with the feeler, then will the plate 

 and index V and X show the number of turns and 

 parts of a turn. In Fig. 7. is represented the instru- 

 ment when ready for use, and immerged in its CI.-UTII 

 of water AB. The cover C of the cistern goes on be- 

 tween the bar EF and the basis BC when the in- 

 strument is raised on blocks. The handle D is for 

 taking off the cover when hot; E is the mercurial 

 thermometer ; F the cock to let off the water ; Gil a 

 hollow piece of tin, which supports seven spirit lamps, 

 which are raised higher or lower by the screws I and 

 K in order to give the water in the cistern a proper 

 degree of heat. The following are the measures in 



Mr. Smeaton's instrument : 



Inches. 



From the fulcrum of the lever to the tube, 5.875 



From the fulcrum to the place of contact, 2.895 



Length of 70 threads of the screw, 2.455 



Divisions in the circumference of the index plate, . 100 



From these data it will be found that the value of 

 one division will be the 5876th part of an inch. When 

 the screw is altered one-fourth of one of these divi- 

 sions, the difference of contact will be very perceptible 

 to the slightest observer ; and consequently the23145th 

 part of an inch will be perceptible in the instrument. 



Mr. Smeaton remarks, that the micrometer is best 

 judged of by the hearing, rather than by the sight or 

 feeling ; aud that by this method, he found it prac- 

 "ticable to repeat the same measurement several times 

 without differing from itself above the 20000th part of 



an inch. 



The following are the results obtained by Mr. bmea- 

 ton from an increase of heat corresponding to 180 of 

 Fahrenheit, or the difference between freezing and 

 boiling water. 



Ten thousandths of 

 an Inch. 



1. White glass Barometer tube, 



2. Martial regulus of antimony, 



3. Blistered steel, - 



4. Hard steel, 



5. Iron, - 



6. Bismuth, 



7. Copper hammered, 



8. Copper, 8 parts mixed with tin 1, 



9. Cast brats, 



10. Brass 16 parts, tin 1, 



1 1. Brass wire, 



12. Speculum metal, 



13. Spelter solder, viz. brass 2, zinc 1, 



14. Fine pewter, 



15. Grain tin, 



16. Soft soder, lead 2, tin 1, 



17. Zinc 8 parts, with tin 1, a little hammered, 



18. Lead, 



19. Zinc or spelter, 



20. Zinc hammered one-half an inch per foot, 



100 

 130 

 138 

 147 

 161 

 167 

 204 

 218 

 225 

 229 

 232 

 232 

 247 

 274 

 298 

 301 

 323 

 344 

 353 

 373 



These results agree very well with those made by 

 Mr. Ellicott. See the Phil. Trans. 1754, vol. xlviii. 



C. Description rf Ferguton* Lever and Wheel 

 meters. 



The lever pyrometer, invented by Mr. Ferguson, is 



so simple, as scarcely to require a figure for it* illtutra- 



it a , i i> mtter. 



tion. Upon a Hat piece of mahogany are fixed bran 



studs, on which the metallic bar is placed. One 

 end of a bar bears against a lever of the second kind, 

 at a point very near its fulcrum. The other end of this 

 lever, which is bent, bears against another lever, and 

 very near its fulcrum ; and the other end of this last le- 

 ver is the index, which has a graduated arch under it. 

 The small expansion of the metallic bar is magnified by 

 the first lever in the proportion of the distances of the 

 point of pressure from its plane, and from its other 

 extremity ; and this magnified effect is again magnified 

 by the other lever, so that an expansion of the 400th 

 part of an inch corresponds to a whole inch on the 

 scale. This pyrometer is liable to the objection that 

 the distance of the points of pressure from the fulcrum 

 and extremity of each lever is variable during the ex- 

 periment. See Ferguson's Leclures,v. i. p. 14. Ed. 1824. 



Mr. Ferguson's wheel pyrometer differs from the Fer^uoo' 

 lever one in the substitution of wheels and pinions in whl and 

 place of levers. The metallic bar bears against the pinion py- 

 end of a short bar which advances between rollers, rometer. 

 This short bar has fifteen teeth on one side, which act 

 upon the leaves of a pinion of twelve teeth, on the axis 

 of which is fixed a wheel of one hundred teeth, which 

 teeth again take into the leaves of another pinion of 

 ten leaves, on the axis of which is placed another 

 wheel of one hundred teeth, which again take into the 

 leaves of a third pinion of ten leaves, whose axis car- 

 ries the index. It is evident from a slight calculation, 

 that one degree of the circular scale divided into 

 360 parts, corresponds to the 45,000th part of an inch. 

 By means of a piece of watch spring connected with 

 the second pinion by a silk thread, the wheels are 

 pulled back again when the bar contracts, and the 

 teeth of the wheels are kept in contact with the leaves 

 of the pinions. See Ferguson's Lectures, vol. i. p. 301. 

 Both these pyrometers are more fitted for the exhibi- 

 tion of the principle and effects of a pyrometer in a 

 public lecture, than for taking any nice measures of 

 expansion. 



7. Description of Ramsden's Microscopic Pyrometer. 



The pyrometer of Mr. Ramsden, which we propose Raimden'* 

 to describe, derives its name from two microscopes at- microsco- 

 tached to it, by which the expansions are measured. P'c pyro- 

 The apparatus consists of a strong deal frame, five feet meter, 

 long, nearly twenty-eight inches broad, and about 

 forty-two inches high. The bar of metal, the expan- 

 sion of which is to be measured, and which may be 

 even two feet long, is placed in a copper trough or boiler 

 more than live feet in length, and filled with water. 

 Beneath the trough are twelve spirit lamps, whose 

 flames heat the water in the trough to the boiling point, 

 and consequently the metallic bar is raised to the same 

 temperature. Parallel to the copper trough, and at a 

 little distance from it, are placed two other wooden 

 troughs full of water, in each of which is placed a cast 

 iron prismatic bar. At the extremities of the bar?, 

 and perpendicular to them, are fixed the two micro- 

 scopes above mentioned. One of these microscopes has 

 only a simple mark or point in the field of view, but 

 the other is furnished with a wire micrometer, similar 

 to the wire micrometer described under MICROMETER. 



