52 ANNUAL OF SCIENTIFIC DISCOVEIIY. 



The old company began work dose to the river, and excavated 

 vertically down ; but the present company commenced at the 

 ends, and are approaching the river both ways. Tlie main tun- 

 nel forms a double-track passage, and a separate tunnel in the 

 same excavation a passage-way for pedestrians. A coffer dam 

 encloses half the width of the river, and ah open cut will be made 

 to the centre of the river, and the tunnel built to the end of the 

 excavation and closed. The water will then be let in, and the 

 other half enclosed in a similar way, and the remainder of the 

 cut made and the tunnel completed. 



MOUNT CENTS TUNNEL. 



On June 1st, 1868, 8,384 metres had been completed. During 

 the month 60 metres additional were finished on the southern side, 

 and 54 on the northern, making a total of 8,498 out of the whole 

 length of 12,220, leaving 3,722 metres yet to be executed. It ap- 

 pears that at the close of 1867, 4 miles and 5,035 feet of the Mount 

 Cenis tunnel had been completed, leaving 2 miles and 4,018 feet 

 still to be pierced. The distance pierced in 1867 was 5,040 feet, 

 as compared with 3,416 feet in 1866, 4,079 feet in 1865, and 1,144 

 feet in 1860. The outlay, during 10 years, upon the work, 

 •amounted, at the close of 1867, to about $8,000,000. The year 

 1871, it is expected, will witness the completion of the tunnel. 

 Its total length when finished will be 7 miles, 3,773 feet, and its 

 total cost is estimated to reach the sum of $12,000,000, or some- 

 thing more than $1,500,000 per mile. 



HEMPEN TELEGRAPH CABLES. 



Capt. Rowett, R.N., has patented a simple hemp covering for 

 the external part of telegraph cables, leaving the conductor and 

 the insulation of the cable to electricians. Iron cables, like the 

 first Atlantic, become fragile from oxidation ; the iron wires of 

 the present cable, though galvanized, and protected by a hemp 

 covering, will rust in course of time, and in so doing will destroy 

 the hemp immediately around them, just as a rust}^ nail does the 

 wood in which it is driven. Hemp alone will remain good for 

 more than 50 years. In his process the hemp is steeped in a pre- 

 serving solution, wiiich renders it safe from the attacks of teredos 

 and worms, and the attachment of marine plants. His ropes 

 are flexible, easily handled, and, being of a very light specific 

 gravity, comparatively much stronger than a cable of mixed iron 

 and hemp. An iron wire will break with 3 miles of its length in 

 the sea; the Atlantic cable will bear 11 miles of its length ; but a 

 hempen cable alone will bear nearly 20 miles of its length ; so 

 that the latter will afford ample strength to bridge any submarine 

 valley over which it may happen to be suspended. The Atlantic 

 cable cost £300 per mile; the hempen cable will cost only £130 

 per mile. 



