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We find the following remarks in the editorial columns of the London Engineer: Reverting for a few lines to the superheating of steam, it is tolerably evident that the difficulties opposed to the successful introduction of superheated steam are to be found chiefly in leakage and deposit of sediment in the superheating tubes, and the erosion to which metallic surfaces, thus exposed to high temperatures, are liable. For the prevention of deposited sediment or other impurities in the stearnpipes, we must either have recourse to pure water or to surface condensation and distilled wate1- the prevention of sedimentary deposits, of course, instigates the external erosion, as the overheating of pipes, which is the cause of such action, is in a great measure prevented by the comparative freedom with which heat is conducted through an unobstructed metallic medium. Another cause of difficulty in the superheating apparatus is the unequal expansion and contraction to which the parts of the apparatus are sub ject. It appears that, on board the Woolwich steam^ boats, the saving of fuel by superheating the steam is found to average from 15 to 20 per cent; but it is considered upon the whole questionable, whether the extra cost of mamtenance, due to the application of the superheating process, is materi.ally less than the saving. It is, however, without doubt, established that, with a working pressure in the boiler of 20 lbs. or 21 Ibs. per .square inch above the atmosphere, the proportion of fuel saved by superheating the steam is from 15 to 20 per cent on board those steamboats. That is one point gained, and it is for engineers to secure the ntire saving, by im proving their details, simplifying the apparatus, rendering every square inch of superheating surface more effective than before, and superseding the additional maintenance heretofore expended. With respect to feed water heaters operating by the means of the exhaust steam, they are either surface-heaters or heaters by direct intermixture ; and the latter must be allowed to be, in all respects, the most convenient, the most compact, the most efficaciousin sh0rt, the best surface-heaters by waste steam, like those by the waste gases, require a large extent of surface for the absorption of a sufficient quantity of heat, involving an arrangement of concentric pipes, or faggots of tubes, or similar expedients for eking out the needed surface, to the painful violation of the principles of taste in design, and to the grievance of the manufacturer who studies simplicity in all his works. Some inventors will work ten times as hard as others, and the fruit of their labors will be a design ten times more complex. Givenany quantity of tubes, a rather extensive supply of c0cks, in various shapes, with from one to four passages in and out, a few dozen union-couplings, and a gross of bolts and nuts, and inventors are to be found who will not only scheme a feed water heater out of these materials, but who. will use them all up, or "bring them all in." But as the cost of making, maintaining and working an engine, may be taken to be in the ratio ot its weight, it is lio doubt better to dismiss complications, or to seek for permanent arrangemen ts in things .which ^ simple, direct and to the point.
