Mr D. Gilbert's Theory of Suspension Bridges* 169 



each other, and in this case the light just at the edges became much more 

 vivid than the rest. The intensity of light during the brightest parts of 

 the phenomenon, which continued three quarters of an hour, could scarcely 

 be inferior to that of the moon when full. 



We once more remarked, in crossing the Atlantic, that the Aurora often 

 gave a great deal of light at night, even when the sky was entirely over- 

 cast, and it was on that account impossible to say from what part of the 

 heavens the light proceeded, though it was often fully equal to that af- 

 forded by the moon in her quarter. This was rendered particularly strik- 

 ing on the night of the 5th October, in consequence of the frequent and 

 almost instantaneous danger which took place in this way, the weather be- 

 ing rather dark and gloomy, but the sky at times so brightly illuminated, 

 almost in an instant, as to give quite as much light as the full moon simi- 

 larly clouded, and enabling one distinctly to recognize persons from one 

 end of the ship to the other. We did not on one occasion perceive the 

 compass to be affected by the Aurora Borealis. 



II.— Ora the Mathematical Theory of Suspension Bridges, with Tables for 

 Facilitating their Construction, By Davies Gilbert, Esq. V. P. R. S» 

 &c. &c. From the Philosophical Transactions for 1826. Part iii. 



The learned Vice-President of the Royal Society has furnished us in the 

 present paper with a most interesting investigation of all tjie essential for- 

 mulae connected with the theory of suspension bridges, and by expanding 

 those formula into suitable and convenient tables, has conferred a great 

 benefit on civil engineers, and, through that useful and honourable class 

 of men, the public at large. 



Mr Gilbert informs us, that his attention was first directed to the con- 

 sideration of suspension bridges, when the plan for making such a com- 

 munication across the Menai Straits * was submitted to the Commissioners 

 appointed by Parliament to improve the communication by roads and 

 bridges through Wales. It appeared to Mr G. that the depth of curva- 



* We have now before us a beautiful engraving of this truly magnificent struc- 

 ture, — a structure which, as a work of art, stands unrivalled, and which reflects the 

 highest credit on Mr Telford, the distinguished engineer. The main pillars from 

 centre to centre are distant from each other 579 feet, and the span of the immense 

 catenary formed by the massy chains 570 feet. The height from low water of spring 

 tides to the road-way is 121 feet, and from high water spring tides 100 feet, thus 

 affording free and uninterrupted scope to all the purposes of navigation. The height 

 of the main supporting pillars above the road-way is 50 feet, and the total breadth 

 of the road- way is 28 feet, divided into a foot-way of 4 feet in the centre, and a 

 carriage-way of 12 feet on each side, thus separating the entire road-way into three 

 distinct parts. The number of suspending chains amounts to 16, each composed of 

 5 bars, and each bar having a section of 3\ inches of iron. The total sectional area 

 of iron is therefore 260 inches. From the division of the road into the foot-way and 

 the two carriage-ways, there are consequently four sets of suspending chains. The 

 road-ways, from the main pillars to the land, are supported by a series of magnificent 

 arches. 



