But contrary to what Mr Ramsden has stated, it should be remarked, that, for unequal parts, (sines, tangents, &c.) its performance must be tedious, and of course expensive ; for after every tread, the screw would require to be put forward by hand, to the amount of at least one significant figure of the Tables for every division from which they are gathered.
Mr Ramsden states the accuracy of this engine to be equal to the 4000th part of an inch, which it certainly ought to be ; this exceeds in precision, by a small degree, what may generally be expected from Bird's standard measures ; yet, as we know that Sir George Shuckburgh Evelyn sought in vain, when he examined our national standards, for some work of Ramsden's to place in his list; and as in the apparatus for our great trigonometrical sur vey, otherwise made entirely by himself, a 42 inch scale of Bird's was used, it may be presumed he never execut ed any thing of the kind.
Soon after the above described engine was finished, and before the description was published, another right-line engine appeared. This, with the assistance of an ingenious workman, was made by the late Mr Harrison, a son of the celebrated artist, who gained the great reward for finding the longitude at sea by means of timekeepers. There is reason to believe, that this engine possesses great merit, and that, in its construction, it is materially different from Ramsden's ; but we are not sufficiently acquainted with it to point out those differences, or to offer any description of it. We know, however, that this engine is not idle ; it is in the possession of John Barton, Esq. a relation of Harrison, who, we think, is now deputy comptroller of his Majesty's mint. But the avocations of office cannot sup press his natural inclination to mechanical exercises, for which his ingenuity, as well as his nice hand and eye, so eminently qualify him.
There was a man of the name of Coventry, a glazier, who for many years rendered himself useful by dividing with wonderful minuteness and accuracy upon glass and other substances, the micrometer scales for microscopes and other purposes. His engine consisted of a very long beam compass, nicely balanced, having at one end a socket moveable by a micrometer screw, and furnished with a fine diamond point. The death of this man, which only happened a few years ago, would have been more regret ted, had not Mr Barton, with the engine of Harrison, pro duced, in similar works, still more exquisite specimens of art. The mother-of-pearl scales (Cavallo's micrometer) come from his hand almost miracles of neatness and ac curacy.
It is, however, comparatively easy, to produce equality among neighbouring divisions in short measures, to what is required in larger works, where the most distant parts would occupy their relative places as correctly as adja cent ones. The latter, indeed, is the great difficulty of
the art in all its branches, but more pal ticularly so, in that which is the subject of the next Section.
WE now come to treat of the art in its highest order; the importance of which may be inferred from consider ing with what attention it has been cultivated, during a succession of ages, by men of science as well as artists. A modern journalist has designated it, " one of the nicest operations of manual labour;" the truth of which may be evinced from the very limited number of those who have performed it with success, and the high consideration in which they are held by all who are qualified to appreciate their labours. But the exactness required in the art may be drawn from still more certain sources. In a circle of three feet radius, wlch is that of the mural circle at Greenwich, a minute of a degree is little more than the 100th part of an inch, and this quantity, respecting lati tude, is the measure of about a mile upon the surface of the earth ; but if we speak of longitude, as derived from lunar and solar tables, the minute represents upon a mean ill) less than about 30 miles; while some of the elements of astronomy, respecting their maxima, are by observation brought clown from heaven to earth under an infinitely greater disparity.
There was a time when astronomers graduated their own instruments. Tycho Brahe and Hevelius are said to have done so; but neither the methods used by them, nor by any of the more ancient astronomers, have come down to us; nor perhaps need this, excepting from mere curi osity, be regretted, if the statement of the errors of their instruments, given by Sir George Shuckburgh Evelyn (Phil. Trans. for 1793) be tolerably correct. Sir George says, " With respect to the precision of astronomical in struments in general, I may notice, by the way, that from the time of Ilipparehus and Ptolemy, before and at the commencement of the Christian xra, to the age of Wal ther and Copernicus, in the beginning of the 16th century, few observations can be depended on to less than five, eight, or perhaps ten minutes; those of Tycho Brahe, in deed, that princely promoter of astronomy, to within one minute. The errors of Hevelius' large sextant of six feet radius, towards the middle of the 17th century, might amount to 15 or 20 seconds; Flamstead's sextant to 10 or 12 seconds; and lastly, those of Graham's quadrant, of eight feet radius, with which Dr Bradley made so many observations from 1742, might amount to seven or eight seconds." It should, however, be remarked, that the above statement, being derived from inspection of the observations made with the several instruments, exhibits errors not strictly imputable to graduation ; a part may have been produced by other defects in the instruments, and also by imperfect observation.