with the ocean, and in the heart of the country, where it can be best protected from any invasion. In an island of such limited extent as Great Britain, the rivers must necessarily be small, but the tidal wave compensates for the defect, and gives us the advantages of river navigation. The mechanical power of the tide is made available by means of the tide mill. The rise and fall of the tide can be utilised as well as the fall of a river. This source of power has not been very generally turned to account, though there is no mechanical difficulty in applying it.
One of the most useful purposes served by the moon is that of a time-ball. When the time-ball falls at Greenwich, all within range of seeing, have the opportunity of knowing that it is precisely one o'clock at Greenwich. But its utility would be vastly increased, if the mariner far out at sea, could also see the signal, for it would at once give him his longitude. He has the means on shipboard, of ascertaining the local time, and at sea the clock in the saloon is put right every day at noon. If the ship is sailing westwards, the hands must be put back in proportion to the speed of the ship. The longitude of a ship is simply the difference between her own clock and a clock at Greenwich. If, for example, the ship's clock shews twelve o'clock at the moment the time-ball falls at one o'clock at Greenwich, her longitude is one hour west. The great object, then, is to see the time-ball at Greenwich, or something equivalent to it. The
