LORD OXMANTOWN ON THE REFLECTING TELESCOPE. 
515 
ference, so great, is mainly owing to a variety of minute circumstances incidental to 
the process of polishing, which influence the result in a greater or less degree ; among 
which, the most important are, variations in the extent or relative velocities of the mo- 
tions by which the necessary friction is produced ; alterations of temperature; or some 
accidental pressure during the process. With a view of obviating these causes of un- 
certainty at a very early stage of these experiments, a machine was constructed for 
grinding and polishing, where the different motions were susceptible of separate ad- 
justment, and were all under complete control. The first trials with it were upon the 
whole satisfactory, and I sent a sketch of it to Sir D. Brewster’s Journal for Oc- 
tober, 1828, in the hope of directing the attention of practical men to the subject, and 
perhaps of raising a doubt in their minds as to the justness of a very deep-rooted 
opinion, that specula could not be polished successfully except by the hand; an 
opinion which, if unfounded, must necessarily have been a serious obstacle to im- 
provement, by precluding the use of the only means available in making accurate ex- 
periments under circumstances identically the same, or, indeed, of trying any series of 
experiments on a large scale. The machine was soon after enlarged, so as to be ca- 
pable of working a speculum three feet diameter as its maximum, and otherwise im- 
proved, and since that no further alteration has been found necessary. From an ex- 
perience of several years, during which specula have been ground and polished with it 
many hundred times, I can safely say that it fully answers the purpose, and I believe, 
in working large surfaces, a degree of precision can, with certainty, be obtained by it, 
unattainable by the hand, even by accident. The machine, in its present state, is repre- 
sented in Plate XX. fig. 6, and Plate XXI. where A is a shaft connected with a steam- 
engine ; B an eccentric, adjustable by a screw-bolt to give any length of stroke from 0 
to 18 inches ; C a joint ; D a guide ; E F a cistern for water, in which the speculum re- 
volves ; G another eccentric, adjustable like the first to any length of stroke from 0 to 
18 inches. The bar D G passes through a slit, and therefore the pin at G necessarily 
turns on its axis in the same time as the eccentric. IT I is the speculum in its box 
immersed in water to within one inch of its surface, and K L the polisher, which is of 
cast iron, and weighs about two and a half hundred weight. M is a round disc of 
wood connected with the polisher by strings hooked to it in six places, each two-thirds 
of the radius from the centre. At M there is a swivel and hook, to which a rope is 
attached, connecting the whole with the lever N, so that the polisher presses upon the 
speculum with a force equal to the difference between its own weight and that of the 
counterpoise O. For a speculum three feet diameter I make the counterpoise ten 
pounds lighter than the polisher. The bar D G fits the polisher nicely, but without 
tightness, so that the polisher turns freely round, usually about once for every fifteen 
or twenty revolutions of the speculum, and it is prevented by four guards from acci- 
dentally touching the speculum, and from pressing upon the polisher by the two guides 
through which its extremities pass. In fig. 7. this bar is on a larger scale. I have 
tried a variety of contrivances for connecting the machinery with the polisher, but 
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