In this chapter we have dealt with a few only of the more notable examples of river improvement in modern times. The list might be almost indefinitely extended. There is hardly a brawling mountain torrent between Land’s End and John o’ Groat’s that has not been reclaimed, deepened, widened, or otherwise improved upon by the art and the genius of the engineer. Nor has the work been confined to modern times. The Romans are known to have constructed embankments for the control of British rivers during the period of their occupation, although for something like 1000 years afterwards their example was not followed. The engineers and the local authorities of the nineteenth century have done much to redeem this reproach. The improvement and conservancy of rivers have now been reduced to a science, founded mainly upon the following general principles[37]:— 1. That the freer the admission of the tidal water, the better is the river adapted for all purposes, whether of navigation, drainage, or fisheries. 2. That its sectional area and inclination should be made to suit the required carrying power of the river throughout its entire length, both for the ordinary flow of the water and for floods. 3. That the downward flow of the upland water should be equalised as much as possible throughout the entire year; and 4. That all abnormal contaminations should be removed from the streams. Our tidal rivers are undoubtedly one of the chief sources of our maritime supremacy. For this reason it is of the utmost importance that they should be kept in good repair, free from unnecessary obstructions, and well adapted to the purposes of navigation. As it is, however, this is not always the case. The chief reason for existing maladministration, where it exists, is the absence of a uniform system of control. The Thames, for example, has been hitherto controlled partly by the Thames Conservancy and partly by the Metropolitan Board of Works. The Great Sluice, at Boston, in Lincolnshire, was constructed in 1764 by Smeaton, for the purpose of stopping the flow of the tide in the river Witham, and converting the upper part of the river into a fresh-water canal as far as Lincoln. As, however, the control of the river is divided—one body dealing with the tidal part from the Grand Sluice to the sea, and the other with the canal and drainage of the land above—each opposes the schemes of the other, and the navigation has been ruined.[38]