Putting aside public water supplies, the usual sources are wells, rivers and springs. Of these, the first are, in most parts of the world, the most common form of private water supply, and, speaking generally, they are by far the most reliable, for save in most exceptional cases, the pollution of a well always takes place from above. It is, of course, most desirable that the upper part of the well tube should be lined with impervious cement, but provided a reasonable amount of care be taken to prevent the surface of the ground near the well becoming fouled, little danger is to be apprehended from dirty surface water gaining admission to the well, for there are few better filters than a few feet of ordinary soil. The ordinary filter, employed in large waterworks, consists of nothing more than a few feet of sand, and it is well known that, when in good working order, such filters rival even the Pasteur biscuit porcelain filter in their power of excluding germs. Now to reach the interior of a well by any other route than through its mouth, water must needs pass through a much greater thickness of soil than is ever used for the filtration of water on a large scale, and hence, provided the mouth of a well be protected, its water may be used with the greatest confidence. At first sight, it appears that nothing should be easier than to provide the well with a water-tight cover of some sort, and draw all water by means of a pump; and wherever the water lies sufficiently near the surface, and the means of keeping a pump in repair are at hand, there can be no doubt that there can be no better plan. Very often, however, wells are so deep as to necessitate the use of a force pump, in which case, unless long connecting rods are used, which are very apt to get out of order, the pumper has to work half-way down the well, a necessity which introduces new difficulties and dangers. In places where pumps cannot be readily repaired, it is useless to attempt the adoption of this method of raising water, and one must trust to other means of protecting the supply. The mouth of the well should be raised a foot or two above the level of the ground by building a masonry drum, wide enough for the person drawing water to stand on, and well sloped, so that slopped water runs off, and not back into the well.