various British Standards of linear Measure. 281 
whole length of 20 feet being laid off on those points in the tem- 
perature of 54°. 
By means of this bar, the length of the hundred feet steel 
chain was determined with which the base on Hounslow Heath 
was re-measured, and was found to be only about 23 inches 
greater than the measurement with the glass rods. 
The standard scale used by Mr. Ramsden in laying off the 
points on the iron bar, is, it seems, no longer to be found; but 
from the declared equality of both this and General Roy’s stand- 
ard with that of the Royal Society, and the near agreement of 
the two separate measurements of the base with the glass rods 
and with the steel chain, one might have been tempted to con- 
sider General Roy’s scale as precisely similar to Mr. Ramsden’s,” 
and as offering the best source from which the national standard 
yard might be obtained. 
The spirit, however, of the recommendation of the Commis- 
sioners of Weights and Measures, appearing to be, that the stand- 
ard yard should be derived from the base of the Trigonometrical 
Survey, 1 thought it preferable to proceed a step higher, and to 
obtain a distance of 40 inches from the iron bar itself, which 
could afterwards be employed in any manner that might be found 
most eligible. en 
In order io obviate the necessity of an allowance for tempera- 
ture, I caused a triangular bar of cast-iron to be made, of the 
same dimensions as Mr. Ramsden’s, except as to length. Gold 
“pins were inserted near the extremities of this bar at the distance 
of 40 inches from each other, on which were to be drawn fine 
lines, comprising one-sixth part of the length ofthe 20 feet bar. 
The apparatus used for tracing the lines on the gold pins, is 
essentially different from that commonly employed. ‘The cutting 
point is elevated by means of an inclined plane, and is then 
carried through a distance equal to the length of the line to be 
traced. On drawing back a part of the apparatus, the extremity of 
which acts upon the inclined plane, the point descends by its own 
weight until it wholly rests upon the surface of the bar; the mo- 
tion being then continued, the frame and cutting point are drawn 
along together, without the possibility of lateral deviation ; and 
the point describes a line, the length of which may, by a certain 
contrivance, be regulated at pleasure, and its strength determined 
by repeating the operation, This very neat and important in- 
vention is due to M. Fortin of Paris, and was communicated to 
me by M. Arago, whose liberal mind knows no reserve on scien- 
tific subjects. 1 have varied the arrangement of M. Fortin, so 
as to bring the cutting point under a microscope furnished with 
cross wires, having an adjustmeut, by means of which their in- 
Vol. 58, No, 262. Oct. 1821. Nn tersection 
