loith reference to Bridges, 235 



That with reference to the strength and safety of suspended 

 hridgeSt «w all cases likely to occur in "practice, their points of 

 attachment cannot he too lofty, nor consequently the curvature of 

 the chains too great. 



The greatest span of a catenary arch, capable of being 

 formed by iron or steel, on the supposition of these metals sup- 

 porting the utmost degree of tension theoretically assigned to 

 them, may be estimated in the following manner : 



If the tenacity of iron be taken at 50,000 pounds for a 

 square inch, and the specific gravity of iron at 7.8, the modulus 

 of tenacity will be 14814 feet. Put this equal to b, in the 

 expression for a maximum, then y will be found =: 9817 feet, 

 and consequently the whole span or 2y = 19634 feet, about 

 3.7 miles, but then x z=. 6629 feet, or 1.25 miles. 



Steel, being supposed to have three times the tenacity of iron, 

 will extend all their movements threefold. 



When X and 5/ are equal to each other, they will be 1.16 

 very nearly, a being unity, and z r= 1.914. 



If a =: unity, and x, y, and z are taken indefinitely great, 

 z = 1+x 



y^hL, lH-2vf 



Art. III. Observations respecting the Geography of Plants. 



Addressed to the Editor of the Quarterly Journal of 



Science, &c. 

 Sir, Bath, \^th August, 1820. 



The accompanying Letter, in Schrader's Botanical Jour- 

 nal *, appears to me to contain rtiuch valuable information upon 

 the new and interesting branch of science to which it relates, 



* Jahrbiicher cler Gcwachskunde. Herausgegebeo von K. Sprebgel, 

 A. H. Schradcr unci H, F. Link, Ersten Baudes, erstes Heft. 1818. 



R2 



