154 LIGHT SCIENCE FOR LEISURE HOURS. 



on the mountain top ; to erect observatories and guid- 

 ing signals, solid, substantial, and true.' When we 

 remember the nature 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 con- 

 tinually climbing over rocks, ravines, and precipices in 

 passing from station to station involved difficulties 

 which, great as they were, were as nothing when com- 

 pared with the difficulties resulting from the bitter 

 weather experienced on those rugged mountain heights. 

 The tempests which sweep the Alpine passes the 

 ever-recurring storms of rain, sleet, and driving snow, 

 are trying to the ordinary traveller. It will be under- 

 stood, therefore, 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, 

 however, the work was completed, and that with such 

 success that the greatest deviation from exactitude was 

 less than a single foot for the whole length of seven 

 and a half miles. 



Equally remarkable and extensive were the labours 

 connected with the preparatory works. New and solid 

 roads, bridges, canals, magazines, workshops, forges, 

 furnaces, and machinery had to be constructed ; resi- 

 dences had to be built for the men, and offices for the 

 engineers ; in fact, at each extremity of the tunnel a 

 complete establishment had to be formed. Those who 



