with two pairs of horizontal driving-wheels as well as with the ordinary coupled vertical ones, and the power of the machine is thus enormously increased; the horizontal wheels gripping the centre rail with great tenacity by being brought together, and being almost incapable of slipping, like the ordinary wheels when on even a 
THE CENTRE RAIL ON A CURVE.moderate gradient.[1]
The third rail is the ordinary double-headed rail, and is laid horizontally; it is bolted down to wrought iron chairs, three feet apart, which are fixed by common coach-screws to a longitudinal sleeper, laid upon the usual transverse ones: the sleepers are attached to each other by fang-bolts. The dimensions of the different parts will be seen by reference to the annexed cross section:—
Let us now take a run on the railway, starting from St. Michel. For some distance from that place the gradients are not of an ex-
- ↑ These engines are described in the reports by Captain Tyler to the Board of Trade.
tions of the line, and round all except the mildest curves. Thirty miles, in all, of the road have the centre rail.
