The. EMlors are not responsible for the Opinions exprewBd by their Cor~ respondents. Generous Inventors MESSES. EDITORS :—Generosity is a noble virtue, and a supply of the " milk of human kindness is a good thing to liave in the house." Tour correspondent, A. B. C, seems to have been gushing and running over with both these when he so magnanimously gives to the world a valuable discovery because he is unable to patent it, and advises other poor and unfortunate inventors to go and do likewise. The feeling is certainly commendable, and if the world, which he proposes so generously to benefit, was possessed of an equal amount of the same feeling, the plan would no doubt work admirably, j No one having any right to interfere with his self-immolation upon the altar of philanthrophy, if he is so moved—but after he has done his best for this needy and deserving world, and has himself, perhaps, fallen into direful distress, let him appeal to the world to aid him to retrieve a position, and see how the world will respond to him. I have five or six patentable things which I am unable to patent. Many of them have cost me years to mature, and nearly all of them, I believe (what inventor don't beHeve this of his bantlings ?) to be valuable ; but if the dear, deserving, and neglected world wants the benefit of them, it must pay for them, because it is vastly richer and more able to make sacrifices than I am. Inventors have plenty of examples of the bad policy of giving their ideas to the world, and among the most conspicuous is that of Archer, the discoverer of the "collodion process" in photography, which was one of the most valuable discoveries of the century. This he generously and nobly surrendered to the public—died poor—and an appeal made to the civilized world in behalf of his family, who were in need, procured for them (in all the world) perhaps two thousand (dollars (I believe much less than that), certainly in the United | States not one thousand, and, I think, only about one-third that warn! Magnificent and splendid generosity of this same unfortu nate and very needy world, for one of the most splendid sac-jifices ever made by mortal man. Sic euntfata liominum! V. M. G. Peekskil, N. Y. [Our correspondent is entirely right. An inventor who j generously bequeaths his invention to the public is liable, for all the public cares, to die in poverty and neglect, while those who hold on, and succeed in making the public pay, are hon -ored and respected.—EDS. Lightning Rods MESSBS. EDITORS :—In a late No. of the SCIENTIFIC AMBHI- I CAN there is a communication on the above subject from Mr. John Wise, of Lancaster ? In the spirit of Mr. Wise, I propose to answer some of his questions and propositions, and in so | doing, shall state nothing new, but something useful to the !? owners of those splendid barns in Lancaster county. 1. Lightning rods are conductors in proportion to thickness, " sectional area," and shortness or " inversely in proportion to their length," and to be effective must be connected in the ground to a metallic plate, and this metallic plate must jbe imbedded in the moist earth. This is the outlet of the bolt of electricity that first reaches the rod, and to be effective must have the capacity to conduct off the rod itself. Not exactly must the plate have the capacity to conduct off the rod, but it must go further ; it must spread to dimensions of damp earth until the earth in contact with the plate has ! the capacity to conduct off the rod itself. Now, how great ; must this plate be ? We will suppose the plate went into a body of water, a large cistern. Pure water conducts in the ! proportion to iron as 6 to *6,754,000,000. It is safe to say that this metallic plate, if immersed in water, should have a surface equal at least to the surface upon which the barn stands or covers. Again, as the earth conducts in proportion to the moisture contained,we will saythe earth has five parts earth to one of moisture, where the plate is buried. Then the plate should be six times as large as if it were immersed in water, and until this rod is so fixed and secured it is insulated ; not absolutely | insulated, but insulated in jn'oportion as its outlet has not the I capacity to conduct off the rod itself. It is not necessary that the plate should be of the dimensions stated to protect the building from every bolt of electricity, but it is liable to be struck by a bolt of sufficient force to fly from the rod unless these conditions are first complied with, and if complied with, it would be physically impossible to fly from the rod and produce injury. In the country the rod should go the ground and from that j to the spring or well, and be connected to as much metallic surface in contact with water or moist earth as possible. If there was a spring convenient, I would run to the spring and have five hundred feet of the rod buried in the channel of the spring, the rod could be lessened in diameter for economy as it enters the moist earth or water. In cities, the lightning rods should lead direct to the water pipes—the iron pipes in the streets—then there is no possible danger to the building, as lightning rods are usually constructed and applied. Again, as to covering the rods with paint, and having the projections bright and pointed. Atmospheric electricity is the same as produced by zinc and copper plate in water, but a different degree of tension. Two cells have twice the tension of one, and a thousand—a thousand times the tension of one, and as the tension is increased, so is its nature to jump and crack, nntil we reach the effect produced by the Iloltz machine and Euhmkorff coil; lightning being the most extreme case of tension with which we are acquainted. Glass/wood,nor any substance ismuchof a barrierinthe way ofthedischargeof electricity of high tension, no substance—nothing being an absolute non-conductor except a perfect vacuum, and any of these things as insulators a.bout a lightning rod are of no earthly use—a metallic roof, connected to earth by an ordinary rod in the manner stated is as secure protection to a building as is possible to mp.ke, and perfectly secure. PHILADELPHIA. [The above communication is from a gentleman whose practical experience as an electrician is probably not surpassed by any other individual.—EDS. How to Clean Files MESSES. EDITOES:—I have just tried a very effective way of cleaning files filled with work. Simply holding them in a jet of steam under forty pounds pressure. In one minute the files come out " as good as new." This may accommodate some of your numerous readers. Mt. Lebanon, N. Y. JAS. F. SMITH. Finishing Taps and Reamers MESSRS EDITORS.—In ths SCIENTIFIC AMERICAN, page 99, current volume, I find a plan for tempei'ing taps, etc. I think it may answer in some cases, but I will give one which I have used for a number of years with great success. The f orgings are got out in the usual way, left to anneal, then centered and turned just sufficient to remove the scale. Then anneal again and turn down to within a thirty-second of an inch, or less, of finishing size. Anneal once more and finish in the lathe. If not sprung in turning, the tap or reamer will come out all right when hardened. I have tried this process with taps inch diameter and 3 inches long up to those 1 inch diameter and 2 feet long and found it always safe and sure. Providence, R. I. D. B. K How the Scientific American is Appreciated From a private letter written by a prominent engineer in neighboring city wo extract the following : " Your favor is at hand, and I have now, by me, completely finished and bound, all the volumes, new series, of the SCIENTIFIC AMERICAN from I to XIX, and many numbers of the old series. I intend to preserve each number and to pick up, at whatever reasonable cost/the earlier numbers. The SCIENTIFIC AMERI CAN is a library in itself." T. P., Jr Providence, R. I. A RECENT analysis of Croton water by Professor Chandler, shows that the city of New York may congratulate itself upon having a supply of as pure water as is furnished to any city in the United States. ~*~CnlJ65%