When a fire is kindled on the hearth, we may, if we will observe the motes floating in the room, see that those nearest the chimney are the first to feel the draught and to obey it,—they are drawn into the blaze. The circle of inflowing air is gradually enlarged, until it is scarcely perceived in the remote parts of the room. Now the land is the hearth, the rays of the sun the fire, and the sea, with its cool and calm air, the room; and thus we have at our firesides the sea-breeze in miniature.
When the sun goes down, the fire ceases; then the dry land commences to give off its surplus heat by radiation, so that by nine or ten o’clock it and the air above it are cooled below the sea temperature. The atmosphere on the land thus becomes heavier than that on the sea, and consequently there is a wind seaward, which we call the land-breeze.—Maury.
SUPERIOR SALUBRITY OF THE WEST.
All large cities and towns have their best districts in the West;[38] which choice the French savans, Pelouze, Pouillet, Boussingault, and Elie de Beaumont, attribute to the law of atmospheric pressure. “When,” say they, “the barometric column rises, smoke and pernicious emanations rapidly evaporate in space.” On the contrary, smoke and noxious vapours remain in apartments, and on the surface of the soil. Now, of all winds, that which causes the greatest ascension of the barometric column is the east; and that which lowers it most is the west. When the latter blows, it carries with it to the eastern parts of the town all the deleterious gases from the west; and thus the inhabitants of the east have to support their own smoke and miasma, and those brought by western winds. When, on the contrary, the east wind blows, it purifies the air by causing to ascend the pernicious emanations which it cannot drive to the west. Consequently, the inhabitants of the west receive pure air, from whatever part of the horizon it may arrive; and as the west winds are most prevalent, they are the first to receive the air pure, and as it arrives from the country.
FERTILISATION OF CLOUDS.
As the navigator cruises in the Pacific Ocean among the islands of the trade-wind region, he sees gorgeous piles of cumuli, heaped up in fleecy masses, not only capping the island hills, but often overhanging the lowest islet of the tropics, and even standing above coral patches and hidden reefs; “a cloud by day.” to serve as a beacon to the lonely mariner out there at sea, and to warn him of shoals and dangers which no lead nor seaman’s eye has ever seen or sounded. These clouds, under favourable circumstances, may be seen gathering above the low coral island, preparing it for vegetation and fruitfulness in a very striking manner. As they are condensed into showers, one fancies that they are a sponge of the most exquisite and delicately elaborated material, and that he can see, as they “drop down their fatness,” the invisible but bountiful hand aloft that is pressing and squeezing it out.—Maury.
BAROMETRIC MEASUREMENT.
We must not place too implicit a dependence on Barometrical Measurements. Ermann in Siberia, and Ross in the Antarctic Seas, have demonstrated the existence of localities on the earth’s surface where a permanent depression of the barometer prevails to the astonishing extent of nearly an inch.
GIGANTIC BAROMETER.
In the Great Exhibition Building of 1851 was a colossal Barometer, the tube and scale reaching from the floor of the gallery nearly to the top of the building, and the rise and fall of the indicating fluid being marked by feet instead of by tenths of inches. The column of mercury, supported by the pressure of the atmosphere, communicated with a perpendicular tube of smaller bore, which contained a coloured fluid much lighter than mercury. When a diminution of atmospheric pressure occurred, the mercury in the large tube descended, and by its fall forced up the coloured fluid in the smaller tube; the fall of the one being indicated in a magnified ratio by the rise in the other.