36 TECHNOLOGY. 
the valve air-tight. This heating arrangement was subsequently found 
to be ineffectual and unnecessary, and has since been discarded. 
It was doubted at first whether the trains, once started, could be stopped, 
but it was found that they were perfectly manageable with powerful brakes. 
In order that the conductor may be informed of the extent of the rarefaction 
in front of the piston a tube passes through it up into the piston-car, near 
his seat, where it communicates with a barometer, and he is thus informed 
of the amount of atmospheric pressure which he at any moment has on; 
he has also the means within his reach of regulating the speed of the train, 
and when it becomes too high of admitting air through another tube, which 
also passes through the piston and comes up near his seat, where it is 
furnished with a cock; by the admission of air through this tube the speed 
is soon checked, but the air-pump continuing uninterruptedly at work the 
vacuum is soon re-established. 
The whole length of the line is 3050 yards or nearly 12 miles, with a rise 
of 714 ft. from the commencement at Kingstown to the termination at Dalkey, 
the average rise being 1 in 140, but the last 365 yards have a rise of 1 in 
57. The line is worked only one way by the atmospheric apparatus, the 
return being effected by the force of gravity. — 
As stated above the length of the line is 3050 yards, but the atmospheric 
main is only 2400 yards long, the remainder of the way, 650 yards, being 
run by the momentum previously acquired. The diameter of the main is 
15 inches, and near its extremity branches out a pipe, c (pl. 6, jig. 2), which 
leads to the exhausting apparatus, distant 500 yds. The air-pump, which is 
double acting, is 663 inches in diameter, with a stroke of 66 inches. It 
is worked by a high pressure condensing engine with 343-inch piston and 
66 inches stoke, working expansively, the cut-off valve being regulated by 
a governor, so as to vary the speed of the engine from 4 at the lowest 
to 1 at the quickest. 
At the entrance end, and some thirty feet from it, is a kind of balance- 
valve, B (fig. 2), very ingeniously contrived to open by the compressed air 
in front of the piston; and at the other or exit end is another valve, open- 
ing outwards by means of the compression of the rarefied air, after the 
piston has passed the tube leading from the main to the air-pump. 
2. Bripge-Burpina. 
Bridge-building may, strictly speaking, be considered a branch of road- 
building, for a bridge is merely a road over a river or a ravine; still it 
appears to be of sufficient importance to merit a chapter by itself. 
The oldest bridge of which we have any information is that over the 
Euphrates at Babylon, and described by Diodorus, Herodotus, and Philo- 
stratus. According to Diodorus it was built by Semiramis, but Herodotus 
ascribes the building of it to Nitocris, about five generations later, and the 
probability is that it was repaired or completed by him. The length of the 
bridge was near 3000 ft.; the piers stood 12 ft. apart in the clear, were of 
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