sounding line. At the bottom of the accumulators another block is hooked with the sounding line rove through. To the end of the sounding line is attached the sounding rod or tube. The sounding line is marked at every 25 fathoms, and lengths of 3000 fathoms are kept on each reel. The sounding rod consists of a long cylinder of brass tubing about 2 inches in diameter, and fitted with a pair of butterfly valves at the lower end, opening inwards. The water that enters the lower end of the tube, while it is sinking, passes out through holes in the upper part. Round this tube

2

3

2

3

a

sinkers of iron, each 56 lbs. in weight, are attached, I cwt. being usually allowed for each thousand fathoms. These sinkers are so fastened to the tube about 18 inches from its lower end that they are detached on reaching the bottom, and left there. Owing to the enormous pressure at great depths, it would be impossible to draw them up. It is known when they have reached the bottom and fallen off by the sudden change in the rate at which the line has been running out. On striking the bottom the sounding tube is forced into the mud, and the valve prevents the materials thus obtained from falling out. In this way not only is the depth at a particular place obtained, but specimens of the ocean floor are brought up. In Globigerina ooze the tube sinks 8 or 9 inches, and in clay about 2 feet.

Figs. 165 and 166 give a view of two of the sounding rods used during the voyage of the Challenger, with four weights or sinkers in position round the tube. These sinkers are cylindrical pieces of iron with a hole through the centre, through which the sounding tubes pass. They rest on an iron disc or washer, which is held in position by a wire fastened to an arrangement at the upper end of the rod, so that on reaching the bottom the wire is set free, and the sinkers slide off.

But there are also other instruments attached to the sounding line when it is sent down. These are a specially constructed and protected self-registering thermometer to measure the temperature, a pressure gauge to register the pressure of the superincumbent column of water, and a water-bottle to bring up a specimen of water from the depth reached. The thermometer used in deep-sea sounding has to be specially constructed to resist the enormous pressure to which it is subjected. Hence the bulb which contains the mercury, or other fluid by whose expansion and contraction the temperature is measured, is enclosed in a strong glass case, so that its indications are not affected

FIG. 165. -The Hydra sounding rod.

FIG. 166. -Baillie's sounding rod.

by the external pressure. Thermometers are sent down to different depths,

1

I mile.

I fathom 6 feet; and 880 fathoms, or 1760 yards, or 5280 feet =

and the temperature ascertained both at the surface and at various levels. The water-bottle is really a metal cylinder having a stopcock at each end connected by a rod, and so constructed that the water flows freely through it while descending, but when an upward movement begins the rod shuts the stopcocks and encloses a specimen of the water. On leaving the ship the sounding line with the instruments attached may be represented by the diagram (Fig. 167), though it should be noted that, through fear of damage from the motion of the ship, the

FIG. 167.-Diagram to illustrate method of sounding on board H.M.S. Challenger. A, block secured to fore yard; B, accumulators; C, block through which sounding line passes; F, pressure gauge and thermometer. E, water-bottle; D, sounding

rod and sinkers.

sounding rods and sinkers were lowered into the water before the waterbottle and other instruments were attached. At first the line is let out slowly, but on reaching about 400 fathoms it is allowed to run out freely, the ship being kept over the place where the sinkers entered the water. The interval of time between the marking of every 100 fathoms is noted. These intervals gradually increase, owing to the sinkers being retarded in their descent by the friction of the increasing amount of line passing through

the water. But the intervals are sufficiently regular to show that any sudden lengthening of the time indicates that the bottom has been reached. The bottom having been reached, the line is carefully drawn up by means of a donkey engine. When the thermometer, water-bottle, and sounding tube have reached the surface they are taken on board and detached from

FIG. 168.-Sea-bed around Britain. The darkest shading indicates land above 600 feet high; the next shading shows the present outline of the land; the lightest shading indicates the sea floor at depths of 100 fathoms and less, and shows what would become land were the bed elevated this height.

the line. The temperature indicated by the thermometer is read off and entered into a book; the water and the contents of the sounding tube are carefully preserved for examination.

For measurements where there was no reason to expect a depth of over

SURFACE OF THE ATLANTIC OCEAN.