jectile that distance through either air or water. The power that conveys the waters of the Gulf Stream across the ocean is acting upon them (§ 95) every moment, like gravity upon the current of the Mississippi river; with this difference, however, the Mississippi runs down hill, the Gulf Stream on the dead level of the sea. But if we appeal (§ 80) to salt and vapour, to heat and cold, and to the secreting powers of the insects of the sea, we shall find just such sources of everlasting changes and just such constantly acting forces as are required (§ 108) to keep up and sustain, not only the Gulf Stream, but the endless round of currents in the sea, which run from the equator to the poles, and from the poles back to the equator; and these forces are derived from difference in specific gravity between the flowing and reflowing water.
147. The true cause of the Gulf Stream.—The waters of the Gulf as they go from their fountain have their specific gravity in a state of perpetual alteration in consequence of the change of saltness, and in consequence also of the change of temperature. In these changes, and not in the trade-winds, resides the power which makes the great currents of the sea.
CHAPTER III.
§ 150-191.—INFLUENCE OF THE GULF STREAM UPON CLIMATE AND COMMERCE.
150. How the Washington Observatory is warmed.—Modern ingenuity has suggested a beautiful mode of warming houses in winter. It is done by means of hot water. The furnace and the caldron are sometimes placed at a distance from the apartments to be warmed. It is so at the Washington Observatory. In this case, pipes are used to conduct the heated water from the caldron under the superintendent's dwelling over into one of the basement rooms of the Observatory, a distance of one hundred feet. These pipes are then flared out so as to present a large cooling surface; after which they are united into one again, through which the water, being now cooled, returns of its own accord to the caldron. Thus cool water is returning all the time and flowing in at the bottom of the caldron, while hot water is continually flowing out at the top. The ventilation
