It is found, upon examination of the reduction of the standards of platina and iron to the freezing point, that they vary rather less than is asserted in the report, and that they agree "within a unit in the last place of the decimals expressing their magnitudes, or one ten thousandth of an inch." At the freezing point, the standard of platina becomes equal to 39.37380, and that of iron to 39.37370 English inches on the scale of brass at and the mean of these to 39.37100 English inches at 62°, the temperature constantly adopted in the comparison of English standards, and particularly in the recent trigonometrical operations. This result corresponds in a most surprising manner with Mr. Bird's determination of the lengths of the toises sent to Dr. Slaskelyne by M. Lalande, the mean of which was 76.734 inches, con sequently the metre having been proved to contain 36.9413 French inches, ap pears to be equal to 39.3702 English inches, or rather either 39.3694 or 39.3710 ; as either of the two toises may have been more correct than the other, it will there fore be perfectly safe to give the pre ference to that measuring 76.726 inches.
Admitting the French measurements of the arc of the meridian to be correct, the complete circumference of the globe amounts to 24855.43 English miles, and
its mean diameter 7911.73.
The nineteenth volume of the Bibliothe. que Britannique contains a description of Lenoir's comparer, written by M. Prony. " Its peculiarity," according to Dr. Young, "consists in the application of a bent lever, of which the shorter arm is pressed against the end of the substance to be measured, while the longer serves as an index, carrying a vernier, and pointing out on a graduated arch the divisions of a scale, which by this contrivance is con siderably extended in magnitude." It does not appear, at first sight, to be cer tain, " that the difficulty of fixing the axis of the lever with perfect accuracy, and of forming a curve for the surface of the shorter arm, or of reducing the gradation of the arc to equal parts of the right line in the direction of the substance to be measured, might not in practice more than counterbalance the advantage of this mechanical amplification of the scale over the simpler optical method employed in the English instruments." We shall conclude this article by giv ing the following useful tables, compres sed from the work already mentioned, as the most recent and valuable authority.