Otho Müller used a dredge (fig. 13) consisting of a net with a square iron mouth, each of whose sides was furnished with a thin edge turned slightly away from the dredge's centre. As any one of these everted lips could act as a scraper it was a matter of indifference which struck the bottom when the dredge was lowered. The chief defect of the instrument was the ease with which light objects could be washed out on hauling, owing to the size of the mouth. However, with this instrument Müller obtained from the often stormy Scandinavian seas all the material for his celebrated _Zoologia Danica_, a description of the marine fauna of Denmark and Norway which was published with excellent coloured plates in 1778; and historical interest attaches to the dredge as the first made specially for scientific work. _Ball's Dredge._--About 1838 a dredge devised by Dr Ball of Dublin was introduced. It has been used all over the world, and is so apt for its purpose that it has suffered very little modification during its 70 years of life. It is known as Ball's dredge or more generally simply "the dredge." [Illustration: FIG. 14.--Ball's Naturalist's Dredge.] Ball's dredge (fig. 14) consists of a rectangular net attached to a rectangular frame much longer than high, and furnished with rods stretching from the four corners to meet at a point where they are attached to the dredge rope. It differs from Müller's dredge in the slit-like shape of the opening, which prevents much of the "washing out" suffered by the earlier pattern, and in the edges. The long edges only are fashioned as scrapers, being wider and heavier than Müller's, especially in later dredges. The short edges are of round iron bar. Like Müller's form, Ball's dredge will act whichever side touches the bottom first, as its frame will not remain on its short edge, and either of the long edges acts as a scraper. The scraping lips thicken gradually from free edge to net; they are set at 110° to the plane of the mouth, and in some later patterns curve outwards instead of merely sloping. All dredge frames are of wrought iron.