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[[Margin]] A Description of a new Micrometer, invented in the year 1761. By B. Talbot, Teacher of the Mathematics at Cannock, Staffordshire. I have considered this micrometer as p. 128.[[/Margin]]
Figure 12. is a section of it, supposed to be cut through at the [[strikeout]] [?][[/strikeout]] 
focus of the object-glass; in which [bold]ab[/bold] is a male screw, one half of whose threads are what I call a right-hand screw, and the other half a left-hand screw, and both halves having the same number of threads in an inch. [bold]cc[/bold] are two female screw, exactly fitting the male ones, and part of the brass being filed away, they make the two standards for fixing the very fine wires or hairs [bold]gg,[/bold] which are fastened by a small hole in the blocks [bold]cc,[/bold] and at top by the small screws [bold]hh.[/bold]--[bold]AB[/bold]and [bold]AB[/bold] are two brass plates (about an inch broad, and two long,) which contain, or hold, the micrometer screw [bold]ab;[/bold] and these plates are fixed, by two screws, to a brass bar [bold]dd;[/bold] in this bar is a small groove, in which the bottom of the blocks, or standards,[bold]cc,[/bold] (being filed thin) are fitted, so as to slide tight along and keep the standards from shaking. At [bold]a[/bold] is a little brass knob with a square hole in it, to fit in the small square end of the micrometer-screw at [bold]a,[/bold] and serves to turn the said screw round backwards or forwards, by which means the standards, [bold]cc,[/bold] and consequently the hairs [bold]gg,[/bold] recede from, or approach to each other with an equal velocity, and, if continued, will meet exactly in the axis of the telescope, or center of the field of view. At the bottom of the little knob [bold]a[/bold] is fixed a small steel index or finger, moving round a circle drawn on one of the plates [bold]AB,[/bold] this circle is divided into sixty equal parts, and figured 10, 20, &c. to 60/0.
And near the other end of the screw [bold]ab[/bold] is fixed into it a small stud or pin [bold]e,[/bold] which moves the wheel [bold]f[/bold] containing 30 teeth; on the axis of this wheel is fixed an index [bold]i[/bold] moving round a circle drawn on the other plate [bold]AB[/bold], and divided into thirty equal parts, and figured 5, 10, &c. to 30/0.
Now, these indices always shew how many revolutions, and parts, of the micrometer - screw the hairs are asunder, for while the screw makes a [[strikeout]]one[[/strikeout]] revolution the index [bold]a[/bold] moves over 60 divisions; but the index [bold]i[/bold] moves over but one. So by having the number of threads of the screw in an inch, and the focal length of the object-glass made use of, the angle becomes known, and so a table may be made to shew the angle answering any number of revolutions or parts thereof, almost by inspection. Or it will be easy to compute the focal length of an object-glass, that one minute may exactly correspond to one revolution of the screw; and then their indexes will always shew, by inspection,the minutes and seconds contained between the hairs of the micrometer.
[[left margin]][[strikethrough]]See p. 129.
[[strikethrough]]
In my micrometer there is very near 64 threads in an inch, viz. 30 revolutions =, 94 parts of an inch, therefore as tangent 15':,94/2:: rad.: 107,7 inches, or 9 feet, nearly, the focal length of an object-glass that will shew minutes and seconds by inspection of the indexes.
Fig. 13. contains a perspective view of the micrometer(diminished nearly to half its size,) as fixed in the frame, or eye piece, of the refracting telescope, which in mine is made of wood, and turned round at each end, but left square in the middle, as in the figure. In the middle of this square part, is sawed a notch, just wide enough to contain the brass bar [bold][underline]dd,[/bold][/underline] and standards [bold]cc[/bold] (mentioned in figure 12.) and so deep as to permit the bar to lie level with the square part of the frame, and the brass plates being of the same length and breadth as the square part of the frame, fit close to the sides thereof, when the micrometer is slipped down into the slit or notch, and are fastened by two screws one of which is seen at [bold][underline]k,[/bold][/underline] the other cannot be seen in this view. 
---The eye-piece unscrews at [bold][underline]ll,[/bold][/underline] in order to take out the eye-glass to clean, &c. there may also be a screw at [bold]m,m,[/bold]to screw it to the