A better plan has been proposed, and being practically applied, has been found to work very well. This consists in incasing the full bulb in an outer covering of glass, so that there shall be a coating of air between the bulb and the outside coating, and that this air being compressed by the pressure of the water outside, shall thus protect the inside bulb. Observations taken in 1869 with thermometers constructed in this way, as deep as 2,435 fathoms, in no instance gave the least reason to doubt their accuracy. A modification of the metallic thermometer, invented by Mr. Joseph Saxton, of the United States office of weights and measures, for the use of the coast survey, may be thus described. A ribbon of platinum and one of silver are soldered with silver solder to an intermediate plate of gold, and this compound ribbon is coiled round a central axis of brass, with the silver inside. Silver is the most expansible of the metals under the influence of heat, and platinum nearly the least. Gold holds an intermediate place, and its intervention between the platinum and silver moderates the strain and prevents the coil from cracking. The lower end of the coil is fixed to the brazen axis, while the upper end is fastened to the base of a short cylinder. Any variation of temperature causes the coil to wind or unwind and its motion rotates the axial stem. This motion is increased by multiplying wheels, and is registered upon the dial of the instrument by an index, which pushes before it a registering hand, moving with sufficient friction to retain its place, when pushed forward. The instrument is graduated by experiment. The brass and silver parts are thickly gilt by the electrotype process, so as to prevent their being acted upon by the salt water.
The box in which the instrument is protected is open to admit the free passage of the water. This instrument seems to answer very well for moderate depths. Up to six hundred fathoms its error does not exceed a half degree, centigrade; at 1,500 fathoms it rises however to five degrees, quite as much as an unprotected Six thermometer, and the error is not so constant. Instruments which depend for their accuracy upon the working of metal machinery cannot be depended upon when subjected to the great pressure of deep soundings.
For taking bottom temperatures at great depths, two or more of the thermometers are lashed to the sounding line at a little distance from each other, a few feet above the sounding instrument. The lead is rapidly run down, and after the bottom is reached an interval of five or ten minutes is allowed before hauling in. In taking serial temperature soundings, which are to determine the temperature at certain intervals of depth the thermometers are lashed to an ordinary deep sea lead, the required quantity of line for each observation of the series ran out, and the thermometers and lead are hove each time. The operation is very tedious; a series of such observations in the Bay of Biscay, where the depth was 850 fathoms and the temperature taken for every fifty fathoms, occupied a whole day. In taking bottom temperatures with a self-registering thermometer, the instrument of course simply indicates the lowest temperature to which it has been subjected, so that if the bottom stratum is warmer than any other through which the thermometer has passed, the result would be erroneous. This is only to be tested by serial observations; but from these it appears, wherever they have been made, that the temperature sinks gradually, sometimes very steadily, sometimes irregularly from the surface to the bottom, the bottom water being always the coldest.
Several important facts of very general application in physical geography have been settled by the deep sea temperature soundings which have been recently made, and the theories formerly held on this subject shown to be erroneous. It has been shown that in nature, as in the experiments of M. Despretz, sea water does not share in the peculiarities of fresh water, which, as has been long known, attains its maximum density at four degrees, centigrade; but like most other liquids increases in density to its freezing point; and it has also been shown that, owing to the movement of great bodies of water at different temperatures in different directions, we may have in close proximity two ocean areas with totally different bottom climates, a fact which, taken along with the discovery of abundant animal life at all depths, has most important bearings upon the distribution of marine life, and upon the interpretation of palaeontological data.
Mr. Wyville Thompson, who conducted the series of important deep sea soundings undertaken in the Porcupine, says very truly, "It had a strange interest to see these little instruments, upon whose construction so much skilled labor and consideration had been lavished, consigned to their long and hazardous journey, and their return eagerly watched for by a knot of thoughtful men, standing, note-book in hand, ready to register this first message, which should throw so much light upon the physical conditions of a hitherto unknown world."
Up to the middle of the last century the little that was known of the inhabitants of the bottom of the sea beyond low water mark, appears to have been gathered almost entirely from the few objects thrown up on the beaches after storms or from chance specimens brought up on sounding lines, or by fishermen engaged in sea fishing or dredging for oysters. From this last source, however, it was almost impossible to obtain specimens, since the fishermen were superstitious concerning bringing home anything but the regular objects of their industry, and from a fear that the singular things which sometimes they drew up might be devils in disguise, with possibly the power to injure the success of their business, threw them again, as soon as caught, back into the sea. Such superstitions are dying out, and in fact so singular are many of the animals hid in the depths of the sea; their forms and general air are so different from anything which the fishermen were used to see, that we can hardly wonder at the fear they excited. When, however, the attention of naturalists was turned toward the sea, they used the dredge such as was used by the oyster fishermen, and all the dredges now in use are simply modifications of this.
The dredge for deep sea operations is made with two scrapers, so that it shall always present a scraping surface to the bottom, however it may fall. The iron work should be of the very best, and weighing about twenty pounds. The bag is about two feet deep, and is a hand-made net of very strong twine, the meshes half an inch to the side. As so open a net-work would let many small things through, the bottom of the bag, to the height of about nine inches, is lined with a light open kind of canvas, called by the sailors "bread-bag." Raw hides have been used for making the dredge bag, but, though very strong, they are apt to become too much so to another sense than touch. It is bad economy to use too light a rope in such operations, and best to fasten it to only one arm of the dredge, the eyes of the two arms being tied together with a thinner cord. In case, then, the dredge becomes entangled at the bottom, this cord will break first, and thus releasing one of the arms of the dredge, may so change the direction of the strain upon the rope as to free the dredge itself.
Dredging in deep water, that is, at depths beyond 200 fathoms, is a matter of some difficulty, and can hardly be done with the ordinary machinery at the disposal of amateurs. The description of the apparatus used in the Porcupine, in 1869 and '70, on her dredging cruise in the Bay of Biscay, will show what is necessary. These arrangements are also shown in the cut. This vessel, a gun-boat of the English navy, of 382 tons, was fitted out specially for this work. Amidships she was furnished with a double cylinder donkey-engine, of about twelve horse-power, with drums of various sizes, large and small. The large drum was generally used, except when the cord was too heavy, and brought up the rope at a uniform rate of more than a foot a second. A powerful derrick projected over the port bow, and another, not so strong, over the stern. Either of these was used for dredging, but the one at the stern was generally used for soundings. The arrangement for stowing away the dredge rope was such as made its manipulation singularly easy, notwithstanding its great weight, about 5,500 pounds. A row of some twenty large pins of iron, about two feet and a half long, projected over one side of the quarterdeck, rising obliquely from the top of the bulwark. Each of these held a coil of from two to three hundred fathoms, and the rope was coiled continuously along the whole row. When the dredge was going down, the rope was taken rapidly by the men from these pins in succession, beginning from the one nearest the dredging derrick, and in hauling up a relay of men carried the rope from the drum of the donkey-engine and laid it in coils on the pins, in reverse order. The length of the dredge rope was 3,000 fathoms, nearly three and a half miles. Of this, 2,000 fathoms were hawser-laid, of the best Russian hemp, 2 1/2 inches in circumference, with a breaking strain of 2 1/4 tons. The 1,000 fathoms next the dredge were hawser laid, 2 inches in circumference. Russia hemp seems to be the best material for such a purpose. Manilla is considerably stronger for a steady pull, but is more likely to break at a kink.
