1838.] Ow the Construction of SuspensioJi Bridges. 



117 



much more than the thousandth part of an inch. In the first trials 

 with the mahogany rod before it was varnished, the mere change in 

 the hygrometric state of the air, in the morning and evening, in fine 

 weather, caused a difference on the 1 1th February, as the foregoing 

 experiments shew, of more than four times that quantity ; making it 

 0.0045 longer in the evening than it was in the morning. The fact is 

 worth remembering, as it may bear on other points besides the reading 

 off the height of a barometer. And I am not certain whether it is 

 generally known that mahogany and fir are so much affected by the 

 state of the atmosphere, as the foregoing experiments shew them 

 to be. 



2\st December 1837. 



"VII. — 071 the Construction of Suspension Bridges, and the Materials 

 best suited for them in this Country — By Captain George Under- 

 wood, of the Madras Engineers, 



My attention having been recendy much called to the subject of 

 suspension bridges, and to the difl^iculty of properly manufacturing 

 iron bars, in places far away from large towns, or indeed in any part 

 of Southern India, I was induced to enter into a careful examination of 

 {he merits of iron wire, and feel confident that this material may be 

 more extensively used in this country than I had anticipated. The 

 famous suspension bridge of Fribourg. in Switzerland, 875 feet span, 

 the largest in Europe, being 300 feet longer than that of the Menai, has 

 wire of about 1-1 0th of an inch diameter to form its suspending cables, 

 and the method adopted by the able engineer who erected it, is simple, 

 and highly encouraging to those who may wish to follow his example 

 (vide Penny Magazine for August 1836, No. 279*). One of the greatest 

 difficulties to be overcome in suspension bridges, is, the dangerous 

 effect of vibration, and the force that the suspended mass will acquire 

 by being put in motion, increases rapidly. Mr. Drewry, in his valu- 

 able work on suspension bridges, remarks, " Hence it is an object 

 " to make it resist motion, and especially to make every part bear its 

 fair share of strain. It is a common doctrine that lightness is the 

 peculiar excellence of a suspension bridge, but that is a principle 

 whicli must be acted upon with discretion, and not taken genciaily. 



* Saturday ]\Iac;azine for November 1S3j, No. 210. 



