7. The following is a very simple, expeditious, and cheap method of purifying the waters of a lake, a swamp, or a muddy river; viz. by filtrating them through a bed of sand, enclosed between two rows of large boards, or planks, perforated with holes, and which will suffer the water to transude into a small reservoir, hollowed out upon its margin. This contrivance is represented in fig. 7, and was alluded to at pages 56 and 169.

On the borders of a lake or rivulet (A) choose a proper spot, in the form of a neck, as little muddy as possible: places in a

linės

parallel with the shore, three or more large piles, six feet in length and eight inches square; drive them in with heavy strokes of a large hammer, after having disposed them in such a manner as to leave two feet in the ground, two more between the ground and the surface of the water, and two feet above the water.

To the fore part of these piles nail large boards, perforated with several holes, an inch. in diameter, observing to bury the lower edge of the first board well in the mud, before fixing it on the piles, and to join its upper margin as accurately as possible to the succeeding boards. These boards or planks must be very strong, and of sufficient length for their extremity to rest upon the solid earth; and they must be elevated, one above another, to the height of half a foot above the surface of the water.

At three or four feet distance from the first piles, similar ones must be fixed, exactly parallel with them, (driven into the ground upon the same principles and in the same proportions with the first,) in a small ditch four feet deep, previously dug for receiving them; observing to leave full two feet breadth

of earth between the margin of the rivulet and the beginning of the ditch, in order that the men may be able to work in it, without danger of the water flowing in upon them.

When the piles last mentioned have been strongly driven into the small ditch, let there be fixed upon the surface facing the rivulet, very long planks, likewise perforated with holes, and extending from the bottom of the ditch to half a foot above the level of the water we may even dispense with having these planks perforated, and only join them loosely together, in order to leave room for the water to filtrate through them.

The second planks being firmly fixed, it is to be supposed that a quantity of sand has been procured, and kept in the vicinity. That which is found at the bottom of running streams, is the best for this purpose. The labourers must be made to work rapidly in removing the two feet of muddy earth which have been left between the sides of the ditch and that of the lake, and immediately fill up the inside of the ditch with the sand, in such a manner that the whole box or space formed by the planks is entirely occupied by the sand therein accumulated.

This operation being finished, a small basin

(B) must be dug for receiving the water which filtrates through the planks and the sand; observing to begin digging opposite the middle of the planks, and go on progressively towards each extremity. This basin may be made eight or nine feet in length, and four or five in breadth. When only one foot of earth remains against the planks, to be removed, and it is seen that the water begins to filtrate easily into the small basin (B), we may cease to remove the earth, and leave that which still remains against the piles.

By this method we may establish one or more filtrating springs on the borders of a lake, a swamp, or a muddy marsh; the waters of which oozing through a full foot of earth, left between the basin and the piles, will deposit in its more compact texture the fine powder which it might carry along with it notwithstanding the thickness of the sand and by this operation, which is easily executed, an army will obtain, in three hours, several sources of wholesome limpid water; without being obliged to waste a considerable time in collecting the water in buckets, and filtrating it through funnels with spunges, or by the use of filtering stones. These last-mentioned

means, which an officer may use with the aid of his domestics, are frequently out of the reach of the common soldiers, who have no leisure for the tedious operations they require. The wells or fountains here proposed, when once established, will continue day and night in purifying their waters, without demanding the smallest labour; and the troops will be ensured against falling victims to the distressing effects of corrupted water.

This description is principally translated from a French work, which I have repeatedly had occasion to quote; and the plan itself corresponds, in principle at least, with that suggested by Dr. LIND on a smaller scale: namely, as follows-Let a quantity of clean sand or gravel be put into a barrel placed on one end, without the head, so as to fill one half or more of it; and let another barrel, with both ends knocked out, of a much smaller size, (or let it be an open cylinder of any kind,) be placed erect in the middle of it, and almost filled with sand or gravel. If the impure water be poured into the small barrel or cylinder, it will rise up through the sand of both barrels, and appear pure above the sand of the large one in the interval between it and the small one.