220 Report on the Progress and State of Applied Mechanics. 



art of making roads of broken stone, with or without a rubble foundation, 

 was brought in the present century, by Macadam and Telford, to a per- 

 fection which it would be difficult to surpass. Some of the finest 

 examples of Telford's roads exist in the neighbourhood of this city ; 

 and it is worthy of remark, that that engineer never hesitated to take a 

 circuitous course when it was conducive to easy gradients and economy 

 of works. Paved carriage ways have been rendered more smooth, less 

 sUppery, and less costly, by the use of smaller paving-stones than it was 

 formerly the practice to employ, — being cubes of granite, of about four 

 inches each way, made from fragments too small to be used for other 

 purposes, and laid on a gravel foundation. Bituminous pavements have 

 already been mentioned. The art of making wooden plank-roads, so 

 essential in newly settled countries, has been much improved by experi- 

 ence. 



54. The immense extension of Railwats has been the most conspi- 

 cuous feature in the engineering of the last quarter century. During 

 the whole of that period, a continual increase has been going on in the 

 speed of the traffic and weight of the engines and vehicles ; and a cor- 

 responding increase in the strength and stability of the roadway has 

 been required. This has led to a gradual increase in the weight of rails, 

 chairs, and sleepers, and to the exertion of much ingenuity, on the part of 

 inventors, to devise a sufficiently strong, stable, and durable construc- 

 tion of permanent way. 



55. In the selection of the course of a Railway, the judgment of the 

 engineer is exercised to find the line which best combines economy in 

 construction and economy in working. Economy of construction alone 

 may require steep gradients and sharp curves, which are adverse to 

 economy of working. In the great trunk lines of Railway, which were 

 laid out at the commencement of the formation of the general network 

 of Railways, economy of working appears to have been chiefly con- 

 sidered ; in the lines now projected and in progress, economj' of con- 

 struction is more studied. The solution of the question of the best 

 combination of those two kinds of economy, depends on the amount 

 of traffic, and must be different in every different ca^e ; but experience 

 has proved, as might have been expected, that a weak and perishable 

 style of construction is never truly economical. 



56. Amongst the great structures which the construction of hnes of 

 Railway has rendered necessary, perhaps the most remarkable are 

 Viaducts. Those of stone and brick resemble, in most respects, the 

 aqueducts of the ancients, and the viaducts of tiurnpike roads, except in 

 being on a larger scale : the Ballochmyle Bridge, a semicircular stone 

 arch of 240 feet span, has already been mentioned. Those of timber 



