dead reckoning and observation, it forms a kind of check on those two methods.
I will now enumerate some corroborating circumstances that must be considered in connection with the difference between the position by observation and that by account, ere this difference—its set and velocity—be tabulated as one of the permanent, ever-flowing currents of the ocean:
I. Temperature.—Of two contiguous bodies of water—one hot, the other cold—the latter, being specifically heavier, will displace the former, and hence a permanent current is established,
II. Evaporation.—Since no salts are taken up in the vapor, a body of salt water from which great evaporation takes place will be specifically heavier than an adjoining one that gives off less vapor, and so a continuous flow from the dense to the light fluid will be maintained.
III. Winds.—In a gale, the waves roll one after another in huge volumes toward the point to which the wind blows; and the friction of the wind upon the water produces a temporary surface set to leeward.
IV. Difference of Barometric Pressure.—In gales of wind, it is common for the barometer to fall from, say, 30·20 to 29·70—half an inch—in less than a day, and while the ship is passing over a comparatively small extent of ocean. Take a very extreme case, merely for illustration. Suppose two contiguous square miles of ocean, the barometer standing 30·20 over one of them, and 29·70 over the other. This difference of half an inch in the barometer is equivalent to a difference of about one quarter of a pound pressure per square inch of surface, or 36 pounds per square foot. Taking 6,086 feet as the side of a square mile, it will contain 37,039,396 square feet; each square foot sustains a difference of pressure of 36 pounds, so that there are in all 1,333,418,256 pounds more pressure on the square mile over which the barometer stands 30·20 than on the one over which it stands 29·70. It is evident that, in order to attain an equality of level, a very decided temporary set must take place from the former square mile toward the latter.
Instead of confining the case to the impossibly small area of two square miles, let us suppose a gradual fall of the barometer from one part of the ocean to the other—such a fall, in fact, over such an area as often comes within the experience of every naval officer—and it stands to reason that waves of the ocean, like those of the air, only smaller and more sluggish, are consequent upon every change of the barometer.
V. Rotation of the Earth.—From being at rest, suppose the earth to begin to revolve, as now, from west to east. On starting, the water of the ocean would, owing to its inertia, recede from the western shores of all the continents, and, as the earth continued to revolve, it
