We give an engraving of a simple form of filter which may be of use to some of our readers, as we receive frequent inquiries upon the subject. A represents half a hogshead barrel; B a porous stone basin about 18 inches deep and 3 inches thick—or a double-wall box, having the space between the walls filled with clean sand and charcoal, and the walls finely perforated, may be used—through which the water has to pass, and fastened to the bottom of the barrel. C is a piece of thin lead pipe, which passes through the water to introduce air into the porous basin ; D is the cross- piece to support the lead pipe ; E is a tap to draw off' the pure water, screwed in the bottom of the barrel. A small bunghole may be made in the side of the barrel to , let off the refuse water when it requires cleaning. When the porog.s stone vessel is used it may be cemented to the bottom. The wooden box, which will answer equally well, may be nailed fast. is a ques- How to Choose a SteamlSngine. “Which is the most economical steam engine? tion otten asked in these days of steam power. What ismeant by this question is, of course, which will take the least fuel ? As the steam engine is quite simple in its best estate, there are but few points to consider in making the choice. It is not, however, the engine which is constructed in the most simple manner, or with the fewest parts, that is the most economical; for if this were the case, the best piston engine would be the one with a single slide valve like our locomotives. Such engines involve considerable waste of steam on account of the large passages between the valve and the piston; they involve also the necessity of exhausting through the inlet passages. Thpse are grave objections when economy is the object sought, and it has been found far better to submit to a little complexity and have these objections removed; consequently the most economical engines are now made with four valves, viz., two inlet and two exhaust valves. The exhaust passages are made more than twice the capacity of the inlets, so that the piston is at once relieved of , all counter-pressure, and receives the full value of the acting steam. Besides this, the valves are placed close to the ends of the cylinder so as to shorten the passages as much as possible. The loss of steam in some of tlie present locomotives amounts to some ten per cent. The , boiler should be of such capacity and construction as to generate abundance of steam without a blower or extra draft, and the fire should be surrounded, except at the bottom, with generating surface. If wood is the fuel, the boiler ought to be longer than when coal'is used. In either case the draft passages in and around the boiler should n()t extend longer than the heat maintains its generating power. The locomotive boiler may be considered one of the best type, but it must be of the best material and workmanship, else it will give much trouble. It should be surrounded with brick-work if used for stationary engines.—RailWay Times.