Page:Popular Science Monthly Volume 19.djvu/75

Rh instrument is to be used, the end is closed, and the line let go; when bottom has been reached it is brought up again, and we find that a certain amount of water has lodged in the lower part of the tube. It is evident that, as the instrument descends and the air in it is compressed, the water forces its way in through an orifice, and past the spout. This spout is so formed that it delivers the water against the walls of the tube, down which it runs, and collects at the bottom. When the motion of ascent begins, the air, by its elasticity, tends to recover its original volume, and expands in the direction of greatest freedom. Now, all the water which has entered has collected below the spout; consequently, in reëxpanding, this water will be left undisturbed.

Assuming that the volume of the mass of air in the instrument varies inversely with the pressure to which it is subjected, we require, in order to be able to construct a scale for our instrument, and so to interpret its results, to know the total volume of the tube, the volume of the part which I call the vestibule, the dimensions and volume of the narrow tube, and of the wide one.

Fig. 4 represents an instrument modified so that it can be used either for great or small depths, according as either end is closed. Mr. Hunt, of the United States Coast Survey, has invented an apparatus consisting of an air-tight bag, made of flexible material, with a long, flexible tube attached to it. The bag, being filled with air, is sunk to the bottom (in a moderate depth of water), while the other end of the flexible tube is connected with a Bourdon's pressure-gauge in the ship or boat, the observation of which gives an exact profile of the bottom as the bag is towed over it.

Bottom temperatures may be measured by common thermometers protected so as to be uninfluenced in coming up through the warmer upper strata of water, by bringing the water to the surface and taking its temperature, or by self-registering thermometers, such as Cavendish's and Six's. A great amount of ingenuity has been displayed in the invention of machines for registering the actual temperature of the water at any given depth, independently of that of the water above it, all of which require some assistance from