ingenious workman, was made by the late Mr Harrison, 
‘a son of the celebrated artist, wi ed the great re- 
ward for finding the longitude at sea by means of time- 
epers. There is sepenumete that this engine 
possesses great merit, av , in its construction, it is 
Wieierialhy -Oitieeeheiebone Rameduals but we are not 
tev with it to point out those differ- 
ences, or to. any description of it. We know, how- 
ever, that this is not idle ; it is in the possession 
. a relation of Harrison, who, we 
ntry’s OT » a glazier, 
ne- who for many rendered himself ul by divid- 
ing with wonderful minuteness and accuracy upon 
glass and other substances, the micrometer scales for 
i and other purposes. His engine consisted 
of a very long beam compass, nicely balanced, having 
sat 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 regretted, had not Mr Barton, 
-with the engine of Harrison, produced in similar works, 
still more exquisite imens of art. The mother-of- 
‘pearl scales (Cavallo’s micrometer) come from his hand 
-almost miracles of neatness and accuracy. =. 
-» It is, however; comparatively easy, to uce equa- 
lity among neighbouring divisions in short measures, 
‘to what is required.in larger works, where the most 
vdistant parts would occupy their Cea as cor- 
«rectly as:adjacent ones. The latter, indeed, is the great 
‘difficulty of the art in all its branches, but more parti- 
veularly so; in'that which is the subject of the next 
Seer. III, Original Graduation. 
et 
' We now come to treat of the art in its highest order ; 
mn -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 
| _s ‘may be evinced from the very limited number of those 
«who have rmed it with success, and the high con- 
‘sideration in which they are held by al! who are quali- 
’ ‘fied to appreciate their labours. But the exactness re- 
} ‘quired in the art may be drawn from still more certain 
q ‘sources. 
| “ofthe mural circle at Greenwich, a minute of a degree 
‘is little more than the 100th — of an inch, er this 
i , quantity, respecting latitude, is the measure of about a 
})- mile mo pae Servemaataptireanery a of 
\ Jongitude, as derived from lunar and solar tables, the mi- 
‘nute represents upon a mean no less than about 30 miles; 
) ‘while some of the elements of astronomy respecting 
their maxima, are by observation brought down from 
heaven to earth under an infinitely greater disparity. 
jrono- ~*~ ‘There was a time when astronomers graduated their 
di for- -@wn instruments. Tycho Brahe and Hevelius are said 
trly di- sto have done so ; but neither the methods used by them, 
: nor by any: of the more ancient astronomers, have come 
‘own to us; nor perhaps need) this, excepting from 
! ‘mere curiosity, be regretted, if the statement of the 
‘errors of their instruments, given by Sir George Shuck- 
é 
YOL. X. PART 1. 
GRADUATION. 
on “the importance of which may be inferred from consider- - 
In circle of three feet radius, which is that - 
861 
burgh Evelyn (Phil. Frans, for 1793) be tolerably cor- _ Original 
rect. Sir says, “ With respect to the precision 
of astronomical instruments in general, I may notice, Errors of the 
by the way, that from the time of Hipparchus and Pto- oj4 astrono- 
lemy, before and at the commencement of the Christian mical in- 
era, to-the age of Walther and Copernicus, in the be- struments, 
inning of the 16th century, few observations can be 
epended on to less than five, eight, or perhaps ten 
minutes ; those of Tycho Brahe, indeed, that princely 
promoter of astronomy, to within one minute. The 
errors of Hevelius’ large sextant of six feet radius, to- 
wards the middle of the 17th,century, might amount to 
15 or 20:seconds ; Flamstead’s sextant to 10 or 12 se- 
conds ; and lastly, those of Graham's quadrant, of eight 
feet radius, with which Dr Bradley made so many ob-« 
servations 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. 
Sir George (Phil. Trans. for 1798) examined the er- Sic 
rors of division in several of the old British standard ae 
measures, and compared their length with his own, purgh’s 
made by Troughton in 1796. . The first in his list is comparison 
the standard yard of Henry VII. about the year 1490, of standard 
which is said to have been taken from the length of the "Ute 
arm of that monarch ; but as history is silent respecting 
this trait of his greatness, it may be presumed that the 
surveyors of his. Majesty’s person took. for the purpose 
half the distance between the extreme finger ends of 
out-stretched hands and arms. Upon this standard the 
greatest measured interval exceeds.the least, by no less 
a quantity than .132 of an inch; and the whole yard is 
.076 of an inch shorter than Sir George’s.. In the stan- 
dard yard of Elizabeth, about 1588, the greatest inter- 
val exceeds. the least by .185, and the entire yard is 
-015 longer. For the standard ell of Elizabeth, of the 
same date, the greatest difference of intervals is .072, 
and the whole length exceeds the modern.standard by 
.016 of an inch. 
Many of our standard measures bear. no subdivisions; 
being simply finished to the length. Of these, nothing 
but their whole ‘extent could be examined; and this 
was done by Graham in the four following instances. 
The yard-bed. of Guildhall, about 1660, too long by 
032, Ell-bed of Guildhall, same date, .018 too long. 
A standard belonging to the clock-makers’ company, 
about 1671, too short by .028 on 36 inches. And the 
standard of the tower of London, about 1720, made by 
Rowley, was .004 too long. 
Graham’s own. standard, made by Sisson, in 1742, 
with which it is supposed the above comparisons were 
madeywas itself examined by Sir George, and found to 
be as follows: line E, .0013 longer, and line Exch. 
0067 shorter, upon 36 inches than his own. 
Many of the above measures exhibit miserable work ; 
a neat hand by doubling, tripling, &c. a string, and ma- ~ 
king notches with a file, would subdivide better. .. But 
it is likely, and indeed the list indicates as much, that 
greater attention was paid to the whole length, than 
was done to the intermediate parts; yet are the yard- 
bed of Guildhall, and the standard of the cloek-makers 
company, exceptions, and: differ from-each other by no 
less than O06 of an inch. Indeed, it seems difficult to 
suppose that the makers of those measures did their very 
best.» It is more probable, that, according to their own 
judgment, or thatiof the times in which they lived, they 
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