200 



TABLE Continued. 



MICROMETER. 



For farther information en fixed micrometers, gee 

 Huygen's Syslema Salurni, p. 82. Cassini, Phil. 

 Trans. No. 236. Smith's Optics, Tol. ii. p. S42. Ca- 

 vallo, Phil. Trans. 1791, p. 283. Cavailo's Nut. Philo- 

 sophy, vol. iii. Pfiil. Mag. vol. xxix. p. 28. Brewster's 

 Treatise on New Philosophical Instruments, p. 48. Ber- 

 noulli, Mem. Acad. Berl. 1773, p. 193. Watt, in the 

 Edinburgh Philosophical Journal, vol. ii. p. 121. 



Wire 

 Microme- 

 ters. 



tricalscales. 



M 



ton's m 



Mr. Co- A series of micrometers, principally for the purposes 



ven'try's of microscopical .observations, were constructed by the 

 microme- l a te Mr. Coventry of Southwark, with a degree of deli- 

 cacy and accuracy which was never before equalled. 

 They consisted of glass, ivory, and silver scales, on 

 which are drawn parallel lines from the 10th to the 

 10,000dth part of an inch. 



Mr. Barton of the mint, well known for his me- 

 chanical ingenuity, has carried the art of dividing mi- 

 crometrical crometrical scales to the highest degree of perfection. 

 scales. The engine which he uses was given to him by his late 

 father-in-law, the celebrated Mr. Harrison. It was 

 constructed by Mr. Harrison himself, and its merits de- 

 pend chiefly on the beauty and excellence of the screw; 

 the apparatus for cutting which, by an excellent inclin- 

 ed plane, also accompanied the engine. The plate in the 

 screw is not divided higher than 2000dth parts of an 

 inch ; but Mr. Barton has drawn divisions on glass, so 

 minute as the 10,000dth part of an inch. In drawing 

 lines of 000 in an inch, Mr. Barton often leaves 

 out one line by design ; and one of the greatest proofs 

 of the stability of tho engine is, that after liaving taken 

 off the brass table with the work upon it, when the 

 omission is distinctly perceived, he can restore it to its 

 place, and introduce the line without its being diilingtiith- 

 ai/le from the rest. 

 6 



CHAP. II. 



On Wire Micrometers, in which the Wires or Fibres are 

 opened and shut Mechanically, 



IN the original micrometer of Mr. Gascoigne, two On wire 

 metallic edges were made to separate from and ap- microme. 

 proach one another ; and when his instrument was txts, in 

 shown to Dr. Hooke, he immediately suggested the w |" ch tlie 

 substitution of fine wires in place of the edges of me- ^J e " l 

 tallic plates, and they have accordingly been retained jpened'and 

 in almost all the subsequent forms of the instrument. shut me- 



The wire micrometer as constructed by Messrs. Au- chanicuiiy. 

 zout and Picard underwent various improvements in 

 the hands of Dr. Bradley and other astronomers, and in Trough- 

 our article ASTRONOMY, vol. ii. p. 732, we have given a ton's mi. 

 drawing and description of one of the most improved crometer. 

 kind as made by Mr. Troughton. 



In all micrometers with moveable fibres, their sepa- 

 ration and approach is effected by means of a screw 

 with about 100 threads in an inch, and as every revo- 

 lution of the screw is again divided upon a circular 

 plate into lOOdths of an inch, the 10,000dth part of 

 an inch may be perceived in the number of the wires. 

 Sometimes the screws are made with 200 or even 300 

 threads in an inch, with the view of giving additional 

 delicacy to the scale, but in cases of this kind, the threads 

 are so minute, that they have not the requisite strength 

 for a micrometer. 



The double screw of Mr. Hunter, which we have al- Hunter's 

 ready described in our article MECHANICS, vol. xiii. double 

 p. 531. furnishes us with the means of obtaining a very screw, 

 slow motion in the moveable wires by means of two 

 screws, with different numbers of threads in an inch, 

 the effect being the same as if a single screw was em- 

 ployed, having the size of its threads equal to the dif- 

 ference of the size of the threads of the two screws. 



The very same contrivance, with a slight modifica- Prony'i 

 tion, has been proposed by M. Prony, under the name nonhis 

 of the nonius screw, for the purpose of moving the wires screw - 

 of micrometers. It is represented in Plate CCCLXXV. 

 Fig. 6. where AB is an axis divided into three parts 

 ab, cd, ef. The screws upon the parts ab, ef, have the 

 same number of threads in an inch, and pass through 

 two female screws in the fixed supports C, D. The 

 axle AB moves horizontally, and any part of its axis 

 describes at each turn of the handle H a space equal to 

 the distance between the threads. The middle screw cd 

 has a greater or a lesser number of threads in an inch than 

 ab or ef, the difference between the two being as small as 

 we chuse. It passes through a female screw M, which 

 carries the moveable micrometer wire, and the piece M is 

 prevented from turning by a groove in CD, in which its 

 lower extremity is guided. The screw M therefore 

 will be carried backwards towards H by the screw cd 

 more or less rapidly than it advances forwards along 

 with the axis AB, according as the number of threads 

 in cd is less or greater than those in ab and ef. The 

 threads in the screws ab, ef, may be of any magnitude, 

 as the piece M advances or recedes in virtue of their 





