In boring shafts in the manner described, without being able to prove in the usual way the perpendicularity of the shaft, it might be feared that the system would be open to objection on this account. It appears, however, that in all cases where Chaudron has sunk shafts by this system he has succeeded in making them perfectly vertical. This is ensured by the natural effect of the treble guide, which the chisels and the two sets of arms attached to the boring tools afford, and by the fact that if the least divergence from a plumb-line is made by the boring tool, the friction of the tool upon one side of the shaft is so great as to cause the borers to be unable to turn the instrument. Boring alternately with the large and the small instrument, the shaft is at length sunk to the point at which the lowest feeder of water is encountered. In a new district this has to be taken, to some extent, at hazard; but where pits have been sunk previously, it is not difficult to tell, by observing the strata, almost the exact point at which the bottom of the tubbing may be safely fixed. This point being ascertained, the third process is arrived at. [Illustration: Hydraulic Test Apparatus. Fig. 151.] As the object of placing tubbing in a shaft is effectually to shut off the feeders, which for water supply may have some bad qualities, and to secure a water-tight joint at the base, it is important that the bed on which the moss box has to rest should be quite level and smooth. This is attained by the use of a tool, termed a "scraper," attached to the bore-rods, the blades being made to move round the face of the bed intended for the moss box. The tubbing employed is cast in complete cylinders. At Maurage each ring has an internal diameter of 12 feet and is 4 feet 9 inches high. Each ring has an inside flange at the top and bottom, and also a rib in the middle, the top and bottom of the ring being turned and faced. The rings of tubbing are attached to each other by twenty-eight bolts 1·1 inch in diameter, passed through holes bored in the flanges. The tubbing is suspended in the pit by means of six rods, which are let down by capstans placed at a distance of 30 feet above the top of the pit. These machines work upon long screws. When a new ring of tubbing is added, the rods are detached at a lower level, and are hung upon chains, thus leaving an open space for passing it forward. Before each ring is put into the pit it is tested by hydraulic apparatus, Fig. 151. The tubbing is usually proved to one-half more pressure than it is expected to be subjected to. At Maurage, where a length of 550 feet of tubbing has to be put in, the chief particulars respecting it are;--