time to time as occasion may require. Every cistern intended to hold water for drinking should have a filter. For this purpose a chamber can be parted off on one side the cistern, within which the suction-pipe takes the water for use, and the filtering material placed so as to pass the water through holes in this partition. Sometimes soft or half-burned bricks are used for this partition, through the pores of which the water passes freely, if sufficient surface be used. If, however, it be desired to remove any dissolved impurities from the water, these mechanical filters, whether brick, gravel, sand, or sponge, are useless, and recourse must be had to a charcoal or spongy iron filter, which acts chemically upon various substances in solution, burning them up, as it were, by the oxygen within the filter.
These filters, however, require such frequent cleansing for the renewal of their efficiency, that they can only be used effectively in a portable form, and cannot therefore be built into the tank.
Products of Coal-Gas Combustion.—From a lecture by Mr. Thomas Mills, before the British Association of Gas Managers, we take the following remarks on the products of combustion of coal-gas: "Of such gas, supposing its specific gravity to be ·5, or half that of air, a cubic foot contains about half its own weight of carbon; and, if this cubic foot of gas be burned, it will give a little more than half a cubic foot of carbonic acid, or, in weight, 488 grains. Again, a cubic foot of coal-gas, of ·5 specific gravity, contains about 41 grains of hydrogen, and this hydrogen in burning will produce 372 grains of water. If we regard the quantity of air necessary to supply the requisite quantity of oxygen to a cubic foot of gas, it lies between five and six feet of air. For every cubic foot of gas burned we require the oxygen of between five and six cubic feet of air, and this will give half a cubic foot of carbonic acid as a result. When we come to estimate the total products of gas-combustion in such a place as London, the figures are really startling. The quantity of gas consumed in London annually may be taken approximately at about 15,000,000 cubic feet. The amount of carbonic acid given off during a year by the combustion of these 15,000,000 of cubic feet is 433,000 tons. The amount of water produced by the combustion of this quantity of coal-gas is 360,000 tons, or 80,000,000 gallons. One of the largest, if not the largest gasholder-tanks in London, is at the Phoenix Works at Kennington. That gasholder-tank, supposing it had no internal cone, and were perfectly flat at the bottom, would hold 10,000,000 gallons of water, if filled to the brim. You might empty that tank and fill it eight times over in a year with the water produced by the burning of the coal-gas consumed in the metropolis during that time."
How the Penguin rears its Young.—In the southern part of the Indian Ocean—about latitude 40° south, longitude 80° east, or about half-way between Africa and Australia—are the two islands, St. Paul and Amsterdam, both of recent volcanic origin, and both the favorite resort of the albatross. But they are most of all remarkable for the number of penguins which have here their permanent residence. According to a writer in Chambers's Journal, these penguins form a rude sort of commonwealth among themselves. In the rearing of their young they exhibit considerable dependence on one another. The hens lay one or two eggs, never more, in a hollow of the ground or on a little grass. The task of incubation is performed by both parents, the one "off duty" going to the sea to procure food for itself, and when the young are hatched bringing a supply for the family. "Where tens of thousands of nests are collected together so closely that the visitor cannot walk without demolishing new-born nestlings or eggs at almost every step, it is difficult to understand how each bird knows its own nest, eggs, or nestling, as it appears to be the case until the young are able to walk about for themselves. Then the latter form into 'infant school,' presided over by several matrons, and ask and receive food from any charitable passer-by, and the social system, so far as it goes, has attained its highest point. There is no longer any recognition of meum and tuum, but a determination on the part of each adult to do the best for the rising generation, without regard to the petty rights of property so
