74 NOTES OF A NATURALIST. 



and ravines. Here the engineers engaged on the 

 railway first confronted the serious difficulties of the 

 undertaking. Following the line from San Bartolome 

 to Chicla, the distance is only thirty four miles, but the 

 difference of level is 7317 feet, and the fifty-one miles 

 between this and the summit-tunnel involve an ascent 

 of 10,740 feet. The gradient is very uniform, never, 

 I believe, exceeding one in twenty-six, the average 

 being about one in twenty-eight. Some of the ex- 

 pedients adopted appear simple enough, though quite 

 effectual for the intended purpose. Very steep uniform 

 slopes have been ascended by zigzags, in which the 

 train is alternately dragged by the locomotive in front, 

 and then (the motion being reversed), shoved up the 

 next incline with the engine in the rear. In one 

 place I observed that we passed five times, always at 

 a different level, above the same point in the valley 

 below. 



Among the more remarkable works on the line are 

 the viaducts by which deep and broad ravines cut in 

 the friable volcanic rocks have been spanned. The 

 iron beams and girders that sustain these structures 

 appear much slighter than I have seen used in Europe. 

 In crossing one barranca, on what is said to be the 

 loftiest viaduct in the world, I stood on the platform 

 at the end of the car : there being no continuous road- 

 way, the eye plunged directly down into the chasm 

 below, over which we seemed to be travelling on a 

 spider's web. 



For a distance of about eight miles from San 

 Bartolomd the railway keeps near to the bottom of 

 the valley, between slopes whereon a distinct green hue 



