Cresy has pointed out that Pliny’s correspondence with the Emperor Trajan proves the importance attached to the subject of waterways. “The consul in a letter points out such designs as were worthy the glorious and immortal name of Trajan, ‘they being no less useful than magnificent.’ He describes an extensive lake near the city of Nicomedia, upon which the commodities of the country were easily and cheaply transported to the high road, and thence were conveyed on carriages to the sea coast at great charge and labour. To remedy this inconvenience, he recommends that a canal should be, if possible, cut from the lake to the sea, observing that one had already been attempted by one of the kings of the country, but whether for the purpose of draining the adjacent lands, or making a communication between the lake and the river, was uncertain. These useful works, in common with all others, fell into decay with the decline of the Roman empire. During the disastrous period which succeeded, until the time of Charlemagne, Europe is deficient in any examples of similar undertakings: this sovereign commenced the projects of uniting the Rhine to the Danube, and of opening a new communication between the German Ocean and the Black Sea.” The Romans were great canal-makers. They were, indeed, as their extant works in Italy, Spain, and other countries show to this day, very capable hydraulic engineers. But in Roman times, canals were constructed for irrigation and water-supply purposes, rather than for purposes of navigation. It was not until some centuries after the decline of the Roman power that navigation canals began to attract attention. Previous to the time when locks, sluices, and other works of engineering art became general, canals could only be carried through comparatively level territories. Hence we not unnaturally find that some of the earliest canals for navigable purposes were constructed in Holland, where the configuration of the ground is specially adapted to their construction.