COMPARATIVE SKILL OF THE ANCIENTS AND MODERNS IN MECHANICS.

Many persons suppose, and maintain, that the grandeur of the monuments of the ancients, and the great size of the stones they employed for building purposes, prove that they understood mechanics better than the moderns.  The least knowledge in mechanics, however, will show this opinion to be erroneous.  The moderns possess powers which were unknown to the ancients, as the screw, and the hydraulic press, the power of which last is only limited by the strength of the machinery.  The works of the ancients show that they expended a vast deal of power and labor to gratify the pride and ambition of kings; but the moderns can do all these things much easier, and in far less time, whenever they deem it proper.  There was nothing in ancient times to be compared with that daring, ingenious, and stupendous monument of engineering skill—­the Britannia Tubular Bridge, across the Menai straits—­projected, designed, and built by Robert Stephenson, the famous English engineer.  He had previously built a similar but smaller structure—­the Conway Tubular Bridge.

THE BRITANNIA TUBULAR RAILWAY BRIDGE.

Had this stupendous fabric existed in ancient times, it would have been regarded as the first of the seven wonders of the world.  Greater and more expensive structures have been raised, but none displaying more science, skill, and ingenuity, and none requiring such tremendous mechanical power to execute.

The Britannia Tubular Bridge was built to conduct the Chester and Holyhead Railway across the Menai Straits, to the island of Anglesea, in the Irish Sea.

The difficulties which the engineer had to overcome, were greatly augmented by the peculiar form and situation of the straits.  Sir Francis Head says, “The point of the straits which it was desired to cross, although broader than that about a mile distant; preoccupied by Mr. Telford’s suspension bridge—­was of course one of the narrowest that could be selected, in consequence of which the ebbing and flowing torrent rushes through it with such violence, that, except where there is back water, it is often impossible for a small boat to pull against it; besides which, the gusts of wind which come over the tops, down the ravines, and round the sides of the neighboring mountains, are so sudden, and occasionally so violent, that it is as dangerous to sail as it is difficult to row; in short, the wind and the water, sometimes playfully and sometimes angrily, seem to vie with each other—­like some of Shakspeare’s fairies—­in exhibiting before the stranger the utmost variety of fantastic changes which it is in the power of each to assume.”  The Menai Straits are about twelve miles long, through which, imprisoned between the precipitous shores, the waters of the Irish Sea and St. George’s Channel are not only everlastingly vibrating, backwards and forwards, but at the same time and from the same causes, are progressively rising and falling 20 to 25 feet, with each successive tide, which, varying its period of high water, every day forms altogether an endless succession of aqueous changes.