CORRESPONDENTS who expect to receive answers to their letters must, in all cases, sign their names. We have a right to know those who seek information from u?: beside, as sometimes happens,we may prefer to address correspondents by mail. PECIAL NOTE.—This column is designed for the general interest and instruction of our reader s,not for gratuitous-replies to questions of a purely business or personal nature. We will publish such inquiries, however-when paid for as advertisemets at $1*00 a line, under the head of "Business and Personal." %W~AH reference to back numbers should be by volume andpage. S. M. P., of Minn.—Callan's compound cast-iron battery, about which you ask, is fully described in the Philosophical Magazine, Vol. XXVIII, pas;e 49. As you may not be able to get access to that work, we will say that it consisted of 300 cast-iron cells, each containing a porous cell and a zinc plate fourinches square, 110 cast-iron cells with porous cylinders and zinc plates6 inches by4 inches, and 177 cast-iron cells, each containing a porous cell and zinc plate 6 inches square. There were, therefore, 577 elements containing 96 square feet of zinc and 200 square feet of cast iron. The porous cells contained dilute nitro-sulphuric acid, and the cast-iron cells containedstrtmg'nitro-sulphuric acid. It was, of course, a powerful battery. Animals were instantly killed by its discharge. J. W. S., of Mass.—The ups and downs in a water pipe running from a distant spring to a pump, unless one or more of them exceed the hight to which water can be raised by an atmospheric pump in the locality named, will not afltect the working of the pump except to add to the frictitfh of the water column, thus absorbing power. The nearer you can place the pump d,own to the level of the spring the less power will "be required to work it. You are right in supposing you will Ret the effect of a siphon in the case you mention. J. GK M., of Ala.—The shaft for driving your looms should make as nearly as may be, the same number of revolutions per minute as the loom makes picks. In other words, the pulleys on the line shaft and the loom pulleys should be of the same diameter. It is considered better practice to speed up as near to the prime mover as possible than contra-wise, as that allows lighter shafting and so reduces not only first cost but subsequent friction. A. B., of N. J.—The cement of which you inquire is made by dissolving enough gutta-percha in a mixture of ten parts of bisulphide of carbon and one of oil of turpentine to form a thick compound. It is a strong cement and holds leather very firmly provided all oil is removed from the surfaces to be united. The pieces, after they are put together with this cement, should be held firmly together until perfectly dry. P. R., of Ohio.—The chemical composition of urea is expressed by the formula C H O N tliatis 2 equivalents of carbon, 4 equivalents of hydrogen,2 equivalents of oxygen, and 2 equivalents of nitrogen. As the equivalent of carbon is 6, that of hydrogen 1, that of oxygen 8, and of nitrogen 14, the proportional weights of each element are, of carbon U, hydrogen 4, oxygen 16, nitrogen 28. GK H. W., ofN. H.—The authorities to which you refer give the generally accepted theory of the action of the injector. To understand the action of that ingenious device, a good understanding of elementary principles is necessary. We could hardly make it plainer to you even by an extended essay, much lgs in the space we can give you in this column. H. C. C, of Ind.—There are several processes now on trial for preserving and transporting meats from South America, and other localities where they are now wasted, to places where they can be used as food. Some of these promise well. You will find numerous allusions to them in back numbers of our paper. A. B., of Va.—You cannot successfully melt iron with the common appliances used for melting the more fusible metals and cast it, neither can you, in our opinion, spin a piece of common plate tin into the shape desired in a lathe. With some of the very best and heaviest qualities you might perhaps succeed but we think it doubtful. D. T. T., of N. Y.—The data given for computing the diameter of the small piston to your hydraulic press and its length of stroke are insufficient. You should in addition to the diameter of the large piston, and the resistance it must overcome at each stroke through a distance of one half an inch, also give the power applied to the smaller piston. D. P. R, of Pa.—There are very few structures which possess greater strength in proportion to weight than paper tubes laid up with good glue. An exterior coat of shellac will protect them frofl* the action of moisture. The interior may be protected by stopping the encla with good corks and coating the ends of the corks with sealing wax. R. H. D., of Tenn.—You can extract the moisture from the air under a glass receiver, by placing therein a small open vessel containing strong sulphuric acid ; or you may dry air, by passing it over lumps of quicklime. The choice of the methods must depend upon the circumstances of the case, which you do not give us. R. S., of Mass.—Your method for binding schoolbooks is undoubtedly new, and from the somewhat vague idea we get from your description of it, we think it would succeed. A method sufiieiently cheap and more substantial than the present is imperatively demanded. R. GK, of N. Y.—Good glue is the best material to fix emery to cloth or leather. It should be used freely and allowed to get very dry and hard previous to UBing. Emery belts ought not to be run on too small pulleys, as they crack the glue muCh more than large ones. F. C. B., of N. Y.—We have not met with either the alloy or the liquid you mention. We are therefore unable lo give you the information you seek. J. S., of Pa.—We havereadyour communication with interest and agree with you on many points, yet we do not think it best to give place to communications on such subjects. R. C, of Me.—The specimen of peroxide of manganese sent is in our opinion too impure for use in glass mannfacture. It contains iron in considerable quantity, W. J. B., of Ohio.—Case-hardened iron expands more by the action of heat than steel, and less than iron not case-hardened, by a very slight quantity. J. S. P.,of S. C.—To make a good whitewash for inside "work, use only lime and water with a little good white glue or isinglass. S, K. Van D., of Iowa.—Mottled iron is a mixture of white and gray irons. It takes its name from its spotted appearance. E. E., of N. C.—The periodicity of the occurrence of the great meteoric showers was determined not by calculation but by observation. W. C, of N. Y.—If you will daub some gas tar about the holes of rats they will vacate the premises. J. F., of Ky.—The flow of rivers is more rapid in high water than in low water. D. &., of 111.—The term "Macadam," applied to roads, is the name of the inventor of the road. It was invented about 1783, and subsequently elaborated in practice in th3 roads of Bristolin England where it was first used. S. N., of Ohio.—You can obtain brass rods alrealy formed for small pinions of six teeth at dealers' in such articles. All you will have to do will be to turn down the bearings.