Notes on Iron Arches. 81 



greater power than the Melbourne one, has been lately con- 

 structed, and is now daily expected to arrive from England ; 

 and as the difference of longitude between Melbourne and 

 Sydney has been accurately measured by means of the 

 telegraph, it will be easy to compare its longitude results 

 with our own. At the Adelaide Observatory no special 

 observations for longitude have as yet been taken. There, 

 also, the Government is just about to order a transit circle, 

 the telescope of which will be somewhat larger than our own ; 

 and as the difference of longitude has also been telegraphic- 

 ally determined, its results will be immediately comparable 

 with our own. The acquisition of two such fine instruments 

 by the neighbouring Observatories is a matter for congratu- 

 lation, and will enable them in future to take their share of 

 the immense work to be done in the Southern hemisphere, 

 an undue proportion of which has lately fallen to Melbourne. 



Art. IX. — Notes on Iron Arches. 

 By W. C. Kernot, M.A., C.E. 



[Read 25th September, 1875.J 



The application of iron_, and especially of wrought iron, to 

 bridge-building is deservedly ranked as one of the most 

 notable of those innovations in civil engineering practice 

 that have been made in modern times. It has enabled us 

 to cross chasms of enormous width and depths and to erect 

 safe and commodious structures in situations and under 

 circumstances which would in many cases totally preclude 

 the employment of the materials known to the bridge- 

 builders of an earlier date. So long as stone and brick were 

 the only available materials, the engineer was confined in 

 his choice to small spans, and to sites wherB a thoroughly 

 sound foundation was easily attainable. The largest stone 

 arch ever constructed, as far as I can ascertain, is consider- 

 ably less than 250 feet span, while iron structures on the 

 arch or girder principle of double, and on the suspension 

 principle of three times, this span are by no means un- 

 common, and we are yet far from approaching the limit of 

 the maximum possible span in this material. Moreover, 

 iron bridges can be employed with perfectly satisfactory 



