The Editors are no responsible for the Opinions expressed by their correspondents. Crank vs. Pulley Messrs. Editors :The subject of the crank and pulley have at various times received attention in the columns of your journal, and always to the disadvantage of the pulley. Not I think because of the intrinsic advantage of the crank over the pulley, but from the manner in which the subject has been presented by the advocates of tho pulley. These advocates have always laid their great stress on tho pulley by the advantage in leverage, which they affirm it has over the crank, while every mechanician will see at once that they are mistaken; but instead of meeting tho crankites with argument and facts to sustain their side of the question, they call hard names and endeavor to ridicule the crankites out of their true position. Now while I am not certain that I am right, not because I am not positive in the correctness of my position, but because I find no one who agTees with me on the subject, I will, with your permission, endeavor to show wherein the crank loses power in its operation, and wherein it would be saved by the pulley, or some other device, could one be found that would work as practically as the crank. In the first place, there is a difference in the" travel of the piston from the center of the cylinder, to the ends of the cylinder, according to the length of the pitman; with a short pitman more and with a Ions' pitman less. When the piston is at the outer end of the cylinder and moving in toward the crank at half stroke, the crank has not made a quarter revolution; a;id when it travels the other half of the stroke, or comes to the other end of the cylinder, it makes just as much more of the quarter revolution as it fell short of in the first half of the stroke'; and when it turns the corner and returns, the first half of the stroke makes more than a quarter of a revolution, and the last half of a stroko less than a quarter revolution; so that there is a constant antagonism between the travel of the piston in the first and last half of the stroke; and not uniform either, for the first half of the outgoing stroke is the longest, and the first half of the ingoing stroke is the shortest. Now this being the case, just as much steam is used in one end of the cylinder as the other; either the piston must make unequal time in its travel, or the crank and fly wheel must make unequal time in its motionand there is a constant antagonism between them, and it seems to me an effort of power to keep up an equilibrium. But it may be said, which is true, there is just an equal amount of leverage, in the two halves of the stroke, and consequently, an equal amount of power exerted, on the crank, the whole stroke. That is all true, but it does not help the matter any, it does not change tho time taken to pass through, or over a given space, for tho leverago on the (short travel of the crank being just tho name as that os. the long tmvsl, it lutS a teMeaey to mate iiw nrtwik ttaM fisfet1 tmt tha ahm ** frf the t%pM" than it does over the long, while the requirement to keep up steady and uniform travel of the crank and fly wheel, would require the quickest amount of travel over the long part of the stroke. And that is the reason in my opinion, why a large and heavy fly wheel, is required for an engine. It is to give regularity of motion and not to pass the centers. Milwaukee, Wis. J. B. Smith. How to Get Patents Extended. Patents granted in 1855 can lie extended, for seven years,under the genera law, but if is requisite that the petition for extension shonld be filed with the Commissioner of Patents, at least ninety days before the date on which the patent expires. Many patents are now allowedto expire which could be made profitable under an extended terra. Applications for extensions can only be made by the patentee, or, in the event of his death, by his legal representative. Parties interested in patents about to expire, can obtain all necessary instructions how to proceed, free of charge, by writing to MD5U CO., 37 Park Bow, New York.