282 An Account of the Comparison of 
tersection can be brought to the line traced by the cutting point. 
This I consider to be an essential improvement, as no accidentat 
derangement of the cutting frame can take place without its be- 
ing immediately perceptible; and the apparatus may be con- 
veniently applied to the division of straight lines or circles, in the 
manner I have described in the Philosophical Transactions for 
1814. 
The micrometer microscopes, used in the comparison of the 
different standards, were those employed in the determination of 
the length of the seconds pendulum, the description of which 
may be seen in the Philosophical Transactions for 1818. But 
as the arrangement of Mr. Ramsden’s bar, required that the 
support to which the microscopes were attached should rest on 
its surface, some other form of the beam carrying them became 
necessary for this purpose. 
A board was prepared of well seasoned mahogany, 36 inches 
long, 3 inches wide, and 2 thick, and an edge bar of mahogany 
34 inches wide and 14 thick, was firmly fixed along the middle 
Pi it lengthwise, which most effectually prevented the possibility 
f flexure. To the extremities of this edge bar, and projecting 
hegiha them, the microscopes were fixed, their cross wires being 
about 40 inches asunder. By this arrangement, the very im- 
portant advantage was ensured, that the apparatus being laid on 
a plain surface, such as a scale, and the microscopes adjusted to 
distinct vision, on placing it on another plane scale, the object 
glasses of the microscopes would be precisely at the same di- 
stance from this last surface as they were from that to which they 
were applied in the first instance, and consequently no error 
could arise from parallax. 
A piece of very thin brass, usually called latin brass, was bent 
round the edges of the 40- inch bar, so that the upper surface 
of the bar was in perfect contact with the brass, the side pres- 
sure being just sufficient to prevent any change of position in the 
brass, unless when moved along the bar by hand. A fine line, 
about the eighth of an inch Jong, was now drawn on one of the 
gold pins at right angles to the bar, and a similar line was traced 
on the piece of brass, which was placed so as to cover the other 
gold pin. The intersection of the cross wires of the tracing 
microscope was earefully adjusted to this last line. 
Mr. Ramsden’s bar, upon his decease, became the property of 
Mr. Berge, whose successor, Mr. Worthington, kindly granted . 
me access to it, and facilitated my exatnination by every assist- 
ance in his power. The bar was placed i in his workshop on tres- 
sels, and its surface carefully brought into the same plane, which 
was ascertained by stretching a thread from end to end. 
The 
