The end-plane of this cylinder receives the pressure of the micrometer screw, so that by turning the small drum-head the coincidence-reading of the movable web with the fixed web can be changed, and thus any given angle can be measured with different FIG.
The lamp c illuminates the drum-head and also, by reflection, the portions of the position-circle which come under the microscopes d and e.
D is the drum-head which gives the fraction of a revolution, d that which gives the whole number of revolutions, I is the index or pointer at which both drums are read.
One pleasant morning after a cold night, February 24th, 1850, having gone to Flint's Pond to spend the day, I noticed with surprise, that when I struck the ice with the head of my axe, it resounded like a gong for many rods around, or as if I had struck on a tight drum-head.
