THE BULLDOG SOUNDING MACHINE.
The apparatus known as the bull-dog machine is an adaptation of Sir John Ross' deep-sea clamms, together with Brooke's idea of disengaging the weight. It was invented during the cruise of the English Navy vessel, the Bull-dog, in 1860, and the chief credit for it belongs to the assistant engineer during that cruise, Mr. Steil. A pair of scoops are hinged together like a pair of scissors, the handles represented by B. These are permanently fastened to the sounding rope, F, which is here represented as hanging loose, by the spindle of the scoops. Attached to this spindle is the rope, D, ending in a ring. E represents a pair of tumbler hooks, like those used so generally. C is a heavy weight, of iron or lead, hollow, with a hole large enough for the ring upon D to pass through. B is an elastic ring of India rubber, fitted to the handles of the scoops, and designed to shut them together as soon as the weight, C, which now holds them apart, is removed. When the bottom is reached, the scoops, open, are driven into the ground, the tension on the rope ceases, the tumbler hooks open and release the weight, which falls on its side, and allows the elastic ring to shut the scoops, inclosing a portion of the bottom in which they have been forced. The trouble with this apparatus is its complicated character; pebbles may get in the hinge and prevent the scoops from closing. In all apparatus to be used for such a purpose the greater the simplicity the better, and an invention, which shall at once be simple and effective, capable of bringing up a pound or two from the bottom at a depth of 2,000 fathoms or more, without fail, and without too much trouble, is still a desideratum, and its invention is well worth the attention of the ingenious.
Another arrangement, called the Hydra sounding machine, is intended to bring up portions of the bottom and water from the lowest strata reached. It consists of a strong brass tube, which unscrews into four chambers, closed with valves, opening upward, so that in the descent the water passes through them, freely; but when it is commenced to haul up, the pressure of the water closes the valves. This apparatus is also furnished with weights to sink it, which are released, on reaching the bottom, by a similar method to those described. This instrument was used during the deep sea sounding cruise of the Porcupine, and never once failed. Its faults are its complication, and that it brings up only small samples of the bottom. Captain Calver, who used it, could always, when at the greatest depths, distinctly feel the shock of the arrest of the weight upon the bottom communicated to his hand.
MASSEY'S SOUNDING MACHINE.
Various attempts have been made to construct instruments which should accurately determine the amount of the vertical descent of the lead by self-registering machinery. The most successful and the one most commonly used is Massey's sounding machine. This instrument, in its most improved form, is shown in the accompanying cut. It consists of a heavy oval brass shield, furnished with a ring at each end of its longer axis. To one of these a sounding rope is attached, and to the other, the weight is fastened at about a half fathom below the shield. A set of four brass wings or vanes are set obliquely to an axis, so that, like a windmill or propeller wheels, it shall turn by the force of the water as it descends. This axis communicates its motion to the indicator, which marks the number of revolutions on the dial plate. One of these dials marks every fathom, and the other every fifteen fathoms of descent. This sounding machine answers very well in moderately deep water, and is very valuable for correcting soundings by the lead alone, where deep currents are suspected, as it is designed to register vertical descent alone. In very deep water it is not satisfactory, from some reason which it is difficult to determine. The most probable explanation is that it shares the uncertainty inherent in all instruments using metal wheel work. Their machinery seems to get jammed in some way, under the enormous pressure of the water, at great depths.
To ascertain the surface temperature of the water of the sea is simple enough. A bucket of water is drawn up, and a thermometer is placed in it. With an observation of this kind the height of the thermometer in the air should be always noted. Until very recently, however, very little or nothing was known with any certainty about the temperature of the sea at depths below the surface. Yet this is a field of inquiry of very great importance in physical geography, since an accurate determination of the temperature at different depths is certainly the best, and frequently the only means, for determining the depth, the width, the direction and general path of the warm ocean currents, which are the chief agents in diffusing the equatorial heat; and more especially of those deeper currents of cold water which return from the poles to supply their places, and complete the watery circulation of the globe. The main cause of this want of accurate knowledge of deep sea temperatures is undoubtedly the defective character of the instruments which have been hitherto employed.
The thermometer which has been generally used for making observations on the temperature of deep water is that known as Six's self-regulating thermometer, inclosed in a strong copper case, with valves or apertures above and below, to allow a free passage of the water through the case and over the face of the instrument. This registering thermometer, consists of a glass tube, bent in the form of a U. One arm terminates in a large bulb, entirely filled with a mixture of creosote and water. The bend in the tube contains a column of mercury, and the other arm ends in a small bulb, partly filled with creosote and water, but with a large space empty, or rather filled with the vapor of the mixture and compressed air. A small steel index with a hair tied round it, so as to act like a spring against the side of the tube, and keep the index at any point it may assume, lies free in either arm, among the creosote, floating on the mercury. This thermometer gives its indications only from the expansions and contractions of the liquid in the large full bulb, and consequently is liable to some slight error, from the variations of temperature upon the liquids in other parts of the tube. When the liquid in the large bulb expands, the column of mercury is driven upward toward the half-empty bulb, and the limb of the tube in which it rises is graduated from below, upward, for increasing heat. When the liquid contracts in the bulb, the mercury rises in this arm of the tube, which is graduated from above downward, but falls in the other arm. When the thermometer is going to be used, the steel indices are drawn down in each limb of the tube, by a strong magnet, till they rest, in each arm, upon the surface of the mercury. When the thermometer is drawn up from deep water, the height at which the lower end of the index stands in each tube indicates the limit to which the index has been driven by the mercury, the extreme of heat or cold to which the instrument has been exposed. Unfortunately, the accuracy of the ordinary Six's thermometer cannot be depended upon beyond a very limited depth, for the glass bulb which contains the expanding fluid yields to the pressure of the water, and compressing the contained fluid, gives an indication higher than is due to temperature alone. This cause of error is not constant, since the amount to which the bulb is compressed depends upon the thickness and quality of the glass. Yet, as in thoroughly well-made thermometers, the error from pressure is pretty constant, it has been proposed to make a scale, from an extended series of observations, which might be used to correct the observations, and thus closely approximate the truth.