“A new Instrument for comiparing Linear Measures. 129 
that the eye still supports very well, causes the strokes marked 
on the glass, bearing the divisions into 100dths of a millimeter, to 
disappear. It has sometimes happened, that I have intercepted 
apart of this light by placing my hand before the reflectitig 
mirror: and I have remarked, that in seeking by the vertical 
motion of this hand the proper position for the degree of light 
that I wanted, I made the point of collimation to vary within 
the limits of about -},dth of a millimeter, According to this 
observation it is necessary, whilst we are comparing the linear 
measures, to keep the quantity of light thrown on the divided 
glass in the interior of the microscope always in the same state. 
~ The accuracy and convenience of my new comparer has al- 
ready been submitted to frequent trials. The ratios of some of 
the linear measures on which it has been tried had been deter- 
mined with the great comparer of M. Lenoir, and the agreement 
between the results furnished by both instruments has been very 
satisfactory. Among the operations from which these results 
are derived there is one that I made with my colleagues Messrs. 
Bouvard and Arrago. One of the objects of comparison was 
the standard platina metre of the observatory. 
Seer ee 
To Mr. Tilloch. 
Dear Sir,— Ix consequence of the observations made in the 
preceding paper, on the size of the very fine platina wire so in- 
geniously contrived and made by Dr. Wollaston, I deemed it pro- 
per to inform him that I intended to present you with a transla- 
tion of M. de Prony’s Memoir for publication in your valuable 
Magazine. I have accordingly been favoured by that gentleman 
with the subjoined information on the subject, which not only 
explains the difficulty, but informs us of the manner in which 
the wire is produced, and the mode adopted for estimating its 
size. The portions of an inch called dines, which the French 
scientific gentlemen use very commonly, are rarely employed in 
this country; it was therefore very natural to conclude, that 
M. Breguet must have made some mistake, in the statement of 
its dimensions which he carried back with him to France, and 
communicated to M. de Prony, 
I remain, dear sir, 
Yours, &c. &c. 
Christ’s Hospital, Feb. 20, 1816. T. S. Evans. 
—— 
“‘ When Mr. Breguet was in London, he received from Dr. 
Wollaston a specimen of platina wire wooodth of an inch in dia- 
meter. 
Vol. 47. No. 214, Fel. 1816, I “ The 
