J. S. B., of Md., asks an explanation for the stoppage of an exhaust steam pipe only three inches from the valve, occurring about four years after it had been in use. The deposit was as hard to chip as the iron itself and resisted the action of acids. We have not met with precisely a case of this kind, but presume it was a gradual accumulation of scale until the exhaust became reduced so much as to interfere with the working of the engine when it was discovered. It is a mistake to suppose such deposits may not be formed In the exhaust. Boilers which prime and carry wet steam into the cylinder, are liable to form a scale in the exhaust pipe; or even to throw out a fine floury deposit from the mouth of the exhaust pipe, consisting of an impalpable powder of carbonate of lime. See articleon " Formation of Deposits in Steam Boilers,' page 282, current volume. W. C, of Mass.—Perfect exhaustion ought to reduce the pressure on the exhaust side of a piston to atmospheric pressure in a non -condensing engine, worked non-expansively, or to about fifteen pounds per square inch, during the greaterpart of the stroke. Practically however, there are circumstances connected with the working of steam engines, which make the mean pressure throughout the stroke on the exhaust side somewhat more than than this. It takes time for the steam to escape sufficiently to reduce the pressure to tnis point, and when compression is used by closing the exhaust before the completion ot the stroke ; or, when lead is uge'd, the pressure will be increased at the latter part of the stroke. To compute the Imean pressure, therefore requires the knowledge of many data, none of which you supply, and which you probably cannot obtain in the case specified. W. E. S., of Conn.—The oxide of lead is, as explained in the paragraph referred to, litharge, or protoxide of lead. We do not know the proportion in which this is mixed with concentrated glycerin to make the cement referred to on page 235, current volume, but we presume it need only be mixed to give the proper consistence. If you make a trial of this cement, we should be glad to learn how it succe eds with you. ?. R., of -. We do not credit the assertion made by teamsters, that wagons with wooden axles—all other things being equal-have a lighter draft through mud, sand, or up an inclined plain. It will be time enough to look for a reason when the fact has become established by accarate experiment. E. R. K., of 111.—We are informed that shellac dissolved in alcohol will stick paper labels to tin and hold them, and we see no reason to doubt the statement. We think, however, the cans ought to bewarm when the labels are applied, to speedily evaporate the alcohol, still the latter is only an opinion. H. C, of Pa.—li the description of the art of graining on paper from the natural wood, given on page 309, Vol. XX, does not give you a sufEcient idea of the process, it must be obvious to you that no " recipe" will enable you to apply it. W. R. T., of Miss.—Toucan make a beautiful mirror, which will withstand a high degree of heat without injury, of platinum. It is the only thing we can recommend you for the purpose you specify. It is quite malleable, and from your evident skill in working metals you will have no difficulty in making it for yourself. F. A. B., of 111.—We have already discussed the irregularity of piston movement on crank engines ad nauseam. You will find the whole thing explained in back numbers, or in Auchincloss' "Link and Valve Motions," published by D. Van Nostrand, 23 Murray street, New York. J. T. S., of Pa.—The front flue sheet ought to'be taken into account in determining the heating surf ace of your boiler. T. J. B., of Wis.—There is no possible danger of bursting in the pipe which supplies your factory with water ,from the greathead ueed. It will stand at least twice that head. A cast iron pipe, fifteen inches in diameter and three quarters of an inch thick, will sustain 600 feet head if the iron be of best quality. C. L. M., of Texas.—Sand is the best material for molding for brass casting. You will not succeed with plaster-of-Paris. It is not sufficiently porous to allow the gases to escape. H. C, of Ca.—Unless the peculiar exigencies of the case require it, experience has shown a direct connection of crank and piston to be better than intermediate gearing. Ve cannot here enter into a discussion of the reasons tor this, but you willfind the subject fully treated in various works on steam enginsering.—Fine paper may be made impervious to air by coating it with gums. So may; cloth. Whether'either of these will " aneiPer your purpose " we cannot say, as you forgot to mention wat that purpose was. S. and-C, of Mass.—We can see no reason why the cmente 1.00V of one part of a cellar should be wet, while another part, made more recently, should be dry, unless it be that the composition of the older portion is different. It probably contains something which attracts moisture. The eand employed might have been beach sand not properly washsd to free it from salt. O. W., of Md.—To prevent the formation of dandruff on a healthy scalp, wash the head daily in pure cold water, and weekly with water containing a little borax in solution, and use as little oil in dress-inethehairas possible. Above.all, keep the general health good by proper diet and exercise, avoid late hours, and you will have little trouble either from dandrulf or dyspepsia. C. S. K., of Pa.—No septum, solid, fluid, or gaseous, has yet been discovered that, placed between a magnet and its armature, will overcome the attraction of the former for the latter. It has been long sought by would-be inventors of electro-magnetic motor engines, and we receive very often queries similar to yours. There is no such substance.