276 Improvement in the Construction of Bridges, Sc. 
that the relative effect of the temperature upon the two metals 
is as 3 to 5, or nearly double the expansion in brass as ina 
steel rod of similar size. The outer rods then have expanded in 
length ,', of an inch more than the inner rods. It will be appa- 
rent from a slight inspection of the drawing, that as the brass rod 
dand the steel one e are attached by a connecting pin to the 
transverse bar fh, that by d expanding more than e, that fh 
becomes a lever, & being the fulcrum, and as g A is three times 
as long as f #, consequently if d be expanded ;'; more than e@, 
the end / will be elevated ,2, of an inch, and thereby raise the 
weight o 4 of an inch more than the expansion of d has depressed 
it. This increased elevation is intended to allow that the spring 
n, the bar 6, the rod i, and the bar m, unitedly, will expand $ of 
an inch also, and if so, it must be apparent that the whole pen- 
dulum has preserved its equilibrium and remains precisely of the 
same length as if no change had taken place in any of its parts. 
Fig. 2, shows a perpendicular view of the transverse bar fh, 
arranged so as to admit the corresponding bar for the other side 
to work freely, and at the same 
time preserve the four upper 
rods upon a line with each 
other, which, as the levers in- 
trude within each other, could not be done without the recess as 
shown in the section. 'The same letters correspond to the same 
parts in Figs. 1 and 2. The dotted lines in Fig. 2, are intended 
to show the relative position of the lever which is attached to the 
dotted line d, Fig. 1, in regard to the other. 
Baltimore, Md., 1834. ‘ 
Fig. 2. 
od 7 7 
7 
Arr. X.—Some account of Irmet. Town's improvement in the 
construction and practical execution of Bridges, for Roads, 
Railroads, and Aqueducts. 
‘THE improvement which is described in the following papel 
was secured by patent in 1835, and is intended as a general syS- 
tem for the construction of bridges, whether of wood entirely, oT 
of cast or wrought iron, over rivers, creeks, harbors, &c., where 
required for any kind of conveyance ; either with wide openings 
between the piers, of two hundred to four hundred feet, or with 
