276 Improvement m the Construction of Bridges, 6j'c. 



that the relative effect of the temperature upon the two metals 

 is as 3 to 5, or nearly double the expansion in brass as in a 

 steel rod of similar size. The outer rods then have expanded in 

 length y'g 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 

 d and the steel one e are attached by a connecting pin to the 

 transverse bar /A, that by c? expanding more than e, that /A 

 becomes a lever, g being the fulcrum, and as g h is three times 

 as long as / g, consequently if d be expanded j\ more than e, 

 the end It will be elevated j\ of an inch, and thereby raise the 

 weight I of an inch more than the expansion of c? has depressed 

 it. This increased elevation is intended to allow that the spring 

 n, the bar 6, the rod «, 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 f h, 

 arranged so as to admit the corresponding bar for the other side 

 to work freely, and at the same p- g 



time preserve the four upper i :::::;z;.".:~"::'i 



rods upon a line with each j^ I T7/ f 



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 c?. Fig. 1, in regard to the other. 



Baltimore, Md., 1834. 



Art. X. — Som,e account of Ithiel Town's improvement in the 

 construction and practical executioti of Bridges, for Roads, 

 Railroads, and Aqueducts. 



The improvement which is described in the following paper, 

 was secured by patent in 1835, and is intended as a general sys- 

 tem for the construction of bridges, whether of wood entirely, or 

 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 



