58 



THE SEVERN TUNNEL. 



value, it was imperative that the exact position of each 

 sounding should be positively fixed, and that the true levels 

 of the bottom should at the same time be determined. As 

 the soundings had i;o bo taken off a boat which was floating 

 on water having an over- varying level, the securing of these 

 results required some contrivance and a good deal of care 

 in carrying thein out. Thoy also took much, time, because 

 they could only bo taken at the turn of the tide, when the 

 water was comparatively quiet. During this time the 

 sounding went on rapidly for about twenty minutes; but, 

 after that, the lead was carried away by the force of the 

 current, and no bottom was felt. "No bottom" was then 

 called, and the work ceased for tlie day. Thus twenty 

 minutes only was a day's work. 



The mode of operation was as follows : First, a base line 

 had been carefnlly pegged out, with numbered iron bars 

 drilled into the rock at every thirty foot along a lino, square 

 to the line of Railway, on the rocks forming the edge oE 

 the Shoots. A lino of soundings was taken across the 

 Shoots at every iron bar — that is, at every ton yards. 



When, therefore, a particular lino of soundings had to 

 be taken, a pole was fixed at the proper iron bar, and a, 

 second, carrying a cross-bar near the top, at some distance 

 behind, in a line parallel to the line of liiiilway. Those two 

 poles gave the exact line for the soundings. Then a third 

 pole, carrying a largo globe on top, was fixed in the hase lino, 

 but eighty yards to the right of the first pole. All was then 

 ready for the boat,. 



The sounding boat, which carried four oarsmen amid- 

 ships, with two engineers at the bow and two astern, 

 carried also a sounding machine fastened to the bow of the 

 boat. This machine was made for the purpose ; it had a 

 reel, on which was evenly coiled a fine sounding wire, 



