346 CAPTAIN KATER’S ACCOUNT OF THE CONSTRUCTION AND 
The marble slab formerly used was employed on the present occasion. Its 
surface was examined by means of a wire, the diameter of which was one- 
hundredth of an inch, stretched by a bow with a force of about four pounds ; 
but as this wire would suffer a deflexion by its own weight amounting to about 
four-thousandths of an inch, a wire of two-hundredths of an inch diameter was 
placed at each extremity of the marble slab, and the wire of the bow resting 
upon these ; the distance of its middle point from the surface of the marble was 
found to be a little less than two-hundredths of an inch, estimated by passing 
beneath it a wire of one-hundredth of an inch in diameter. The marble slab 
being sixty-four inches long, its surface may therefore, perhaps for the extent 
of a yard, be considered as sufficiently approximating to a plane ; and I may here 
remark that no new adjustment of the slab was found to be necessary, as its 
position appeared to have undergone no change since my last measurements. 
The scale was placed upon the marble slab near the Imperial Standard Yard, 
and the comparisons were always made about nine o’clock in the morning, in 
order to ensure as far as possible an equality of temperature in the scale and 
the Standard Yard. It will be seen that seldom more than three comparisons 
were taken on the same morning, lest the proximity of the person of the 
observer might destroy the equality of temperature. 
The microscopic apparatus used on the present occasion is that which was 
employed in the comparison of various British standards of linear measure, an 
account of which is given in the Philosophical Transactions for 1821, and the 
mode pursued in making the comparisons was the same as that which I have 
there detailed. The value of one division of the micrometer is =.0000428742 
of an inch. As the microscopes invert, an increase in the readings indicates a 
corresponding deficiency in the length of the scale. 
