VOL. XLVIII.] PHILOSOPHICAL TRANSACTIONS. 485 



From the fulcrum of the lever to the feeler 5.875 inches. 



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 

 Hence the value of one division will be the ^ , \% ^ part of an inch ; but if the 

 screw be altereil ^ of one of these divisions, when the contact between the screw 

 and feeler is well adjusted, the difference of contact (if he may so call it) will be 

 very perceivable to the slightest observer; and consequently -g.-3-i-, ^ part of an inch 

 is perceivable in this instrument. 



There is one thing still remains to be noticed, and that is, the verification of 

 the micrometer-screw, which is the only part of this instrument that requires 

 exactness in the execution ; and how difficult these are to make, perfectly good, 

 is well known to every person of experience in these matters; that is, that the 

 threads of the screw may not only be equidistant, in different places, but that the 

 threads shall be equally inclined to the axis in every part of the circumference. 



The result of the experiments made with this instrument, agrees very well 

 with the proportions of expansion of several metals given by Mr. Ellicott; which 

 were deduced from his pyrometer published in the Philosophical Transactions: 

 and, considering the very different construction of the two instruments, they 

 abundantly tend to confirm each other. 



Referencti to the Figures. 



Fig. 10, pi. 9. represents the instruinent independent of the cistern in which it is used, abcd, 

 is the main bar or basis of the instrument, ef, is the bar to be measured, lying in 2 notches; one 

 fixed to the upright standard ab, the other to the principal lever hi. The end e of the bar bf, bears 

 against the point of g, a screw of use in examining the micrometer-screw. The other end of tlie 

 bar F bears against a small spherically protuberant bit of hard metal fixed at the same height as o, in 

 the principal lever hi. k, is an arbor fixed in the basis, which receives at each end the points of the 

 screws hl, on which the lever hl turns, and serve as a fulcrum to it. o, is a slender spring, to 

 keep the lever in a bearing state against the bar; and p, is a check, to prevent the lever from falling 

 forward when the bar is taken out. n, is the feeler, something in the shape of a t, suspended, and 

 moveable up and down on the points of the screws im, which, as well as lh, are so adjusted, as 

 to leave the motion free, but without shake, qr, is the handle of the feeler, moveable on a loose 

 joint at R; so that, laying hold of it at q, the feeler is moved up and down without being affected 

 by the irregular pressure of the hand. The extremity s of the feeler is also furnished with a bit of 

 protuberant hard melal, to render its contact with the point of the micrometer-screw more perfect. 

 T, is the micrometer-screw ; v is the divided index-plate, and w a knob for the handle. The micro- 

 meter-screw passes through two solid screwed holes at d and y. The piece yz is made a little 

 springy, and endeavours to pull the screw backwards from the hole at d ; of consequence keeps the 

 micrometer-screw constantly bearing against its threads the same way, and so renders its motion 

 perfectly steady and gentle, x, is the index, having divisions on it, answering to the turns of the 

 screw. This piece points out tlie divisions of the plate, as the face of the plate points out the divi- 

 sors on the index. When the instrument is used, lay hold of the knob at q with one hand, and, 

 moving the feeler up and down, with the other move forward the screw i, till its point comes in con- 

 tact with the feeler; then will the plate and index v and x show the turns, and parts. 



