Recent Advances in Mechanical Science. 143 



follow the consideration of what the Americans call the "track" 

 for a few minutes, and what is involved by the construction of 

 the track. 



In the early days of railways, engineers were content to select a 

 nice level piece of country with very few hills and as few rivers in 

 the way as possible — in fact, the choice of which towns were to 

 have the honour of being connected by railways was arrived at 

 by the character of the intervening ground — but nowadays no 

 difficulty, not even the Belfast under-strata of mud, nor warning 

 accident, seems to stop the skill and determination with which 

 each new project is set about, and generally succcessfully com- 

 pleted ; and, what is quite as wonderful, the public shew the 

 confidence they have in the members of the engineering pro- 

 fession by providing the money, without which the vast works 

 proposed could not be carried out. Many people have no con- 

 ception of what the difficulties were, in an engineering way, of 

 piercing a tunnel such as the one through Mont Cenis. The 

 work was begun at each side of this great mountain, at two 

 spots about 9I miles apart. The shaft pierced was not level, 

 but rose towards the centre of the mountain from each side for 

 drainage purposes, and could not be carried in a direct line 

 owing to geological difficulties ; yet the two holes met each 

 other accurately in the middle of this mountain, a feat acknow- 

 ledged to be a triumph of human accuracy and skill. 



To bring the feat more within the grasp of our minds, we 

 will suppose the tunnel to be only the size of a mouse-hole, 

 say about f of an inch in diameter, and there are two mice 

 who want to have a short cut between their houses 

 through a big mound, to save their having to go over the 

 hill. Then each mouse would have to burrow a hole 

 100 feet long. If they met, the total length of the hole would be 

 200 feet. If this hole were dead straight, the difficulties to be 

 conquered would be great enough, but on account of the angle 

 both- in plan and elevation, the difficulties of meeting each other 

 in the centre of the mountain are vastly increased. 



I might multiply examples of curious and cleverly tackled 



