F_ B.-The best water fence we have ever seen to be used in small streams was made as follows: A gate sliding in upright ways at the ends, like an old-style turnpike Iate, has attached to the bottom board (a scantling is hetter as not so likely to be broken in high water) crutches which rest upon common empty barrels or casks. The uprights at the ends of the gate are provided with friction rollers, so that the gate slides up and down easily in the ways. Two or three casks will generally support the weight of the gate, so that it descends nearly to but does not enter the stlrtace of the water. A Iate thus constructed will rise and fall with the water, and if everything Is properly made, is not liable to be washed away in high water. A good water gate for fenCing pnrposes is in request. C. S., of Va.-A siIple test will enable you to distinguish between the pulverized carbonate of soda and the chlorate of potash, the crystals of which are so broken as to render them dlfcult for you to distingnlsh. Taste would be enough to a person famillar with these salts but premising that you are not suffic1.etly posted, you can detect the difference by adding to a little of each in the soUd state a little sulphuric acid. With carbonate of soda there wlll at once ensue " great disengage ment of colorless gas (carbonic acid) with mnch frothing, With chlorate of potash the action will be,slow while the, materials are cold; but when a gentle heat is applied the mixture becomes very yellow and a green. sh irrltatlng and suffocating gas (chlorine) is cvolved. C. B., of N. Yo-Time _will remedy the disagreea1e taste of the water in your newly cemented cistern. We know ofnothifig ; ou can do but possess your soul in patience. R. S. M., of Mass.-4 c eap attractive device in the way of signs for shop windows is always salable, and the'e is no d(ubt that yours i. patentable. YOI would do well to prosecute the case at once Its amusing character WOuld be sme " to draw." T. L. B., of Vt.Your method of preparing plumbago as a stove polish Is old, except the form in which it is packed for use. You could probably secure a patent on that as its convenience is obvious. R. C. D .. of Minn.The inventor of logarithms was John Napier, a Scotchman; born 1550, died 1617. He was a man of great inventive talents, and displayed them in mechanics as well as in mathematics. R. S., of Ca.The pressure per square inch in fr esh water at a depth of ten thousand feet is 4,334 pounds nearly. E. B. R., of N. Y.Sand molds for brass castings are faced with either meal dust, flour, powdered chalk, linely-sifted wood ashes, pulverized charcoal, rotten stone, and sometimes with the soot of cork- shavings, or other carbonaceous material. Among these various substances you will undoubtedly find the one which you suppose is the secret of the founders you refer to. R. L. J., of Ohio.There is no rule by which the proportions of any one part of a marine engine can be computed independently of the other parts. Details vary greatly in different engines, accordingto particular circumstances. Your question, therefore, as to the propp.r hight of gallows frame above cylinder, is, in the absence of all other data, unanswerable. T. D., of La.The project of the Nicaraguan canal dates back to 1551, at which time the historian Gomara indicated four routes for such a canal. These routes have ever since been considered as the most favorable ones. A survey of one of these routes was made under the orders of the Spanish Government in 1781. 'V. W., of N. Y.The various kinds of self-raising flour in market are, we believe, all patented. If you wish, you can obtain the specifications of these patents from the Patent Office on payment of small fee. C. L., of Ky.An instrument for measuring distances passed over in traveling upon roads by carriages, velocipedes, etc., may be properly called an odometer or perambulator. Your device is probably patentable. Z. W., of Ind.While we are aware that practical men are often led to use various solutions for tempering steel, their practice varies so much that many will be found to C0nclemn what others praise in the highest terms. Scientific men have failed to detect any better results from solutions than from simple water. There is an amount of tradition. ary practice transmitted from father to son, and each workman gene rally favors that to which he has been bred. We do not believe in solutions, and therire do not recommend any of the numerous salts which have been employed for this purpose
This article was originally published with the title "Answers to Correspondents"