Page:Miscellaneous Papers on Mechanical Subjects.djvu/182

 six weeks. The measuring instruments were supported on two adjustable stands.

They were made on the compensating principle, and inclosed in a double ease of tin, to prevent, as much as possible, their being affected by changes of temperature. The ends of the instruments were of agate, one flat, the other having a knife edge. The latter was made to slide, and was connected by a bell-crank lever to a spirit level, which indicated when the end measures were in contact, according to the method employed by Bessel, in making standard measures in Prussia.