170 LIGHT SCIENCE FOR LEISURE HOURS. 



necessary, before the work began, to survey the interme- 

 diate country, so as to ascertain with the most perfect 

 accuracy the bearings of one end of the tunnel from 

 the other. " It was necessary," says the narrative of 

 these initial labors, "to prepare accurate plans and 

 sections for the determination of the levels, to fix the 

 axis of the tunnel, and to ' set it out ' on the mountain- 

 top ; to erect observatories and guiding-signals, solid, 

 substantial, and true." When we remember the na- 

 ture of the passes over the Cenis, we can conceive 

 the difficulty of setting out a line of this sort over the 

 Alpine range. The necessity of continually climbing 

 over rocks, ravines, and precipices in passing from 

 station to station involved difficulties which, great as 

 they were, were as nothing when compared with the 

 difficulties resulting from the bitter weather experi- 

 enced on those rugged mountain-heights. The tem- 

 pests which sweep the Alpine passes the ever-recur- 

 ing storms of rain, sleet, and driving snow, are trying 

 to the ordinary traveller. It will be understood, there- 

 fore, how terribly they must have interfered with the 

 delicate processes involved in surveying. It often 

 happened that for days together no work of any sort 

 could be done owing to the impossibility of using levels 

 and theodolites when exposed to the stormy weather 

 and bitter cold of these lofty passes. At length, how- 

 ever, the work was completed, and that with such 

 success that the greatest deviation from exactitude was 



