ing through the moving water of the surface. Moreover,
it sometimes fell on its back and was then useless, although when the
apex or towing point was weighted no great skill is needed to avoid
this.

Otho Muller 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 Muller 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 Muller'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 Muller's,
especially in later dredges. The short edges are of round iron bar.

Like Muller'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 deg. to the plane
of the mouth, and in some later patterns curve outwards instead of
merely sloping. All dredge frames are of wrought iron.

The thick inner edges of the scrapers are perforated by round holes at
dist