In the Scientific American of Nov. 27, I find an article by Mr. W. G. Hudson, in which he takes considerable pains to convince the public that the cup eccentric, used on my truck, is anything but a scientific remedy to make a locomotive engine track square, and says that " if required to move much, to make,. the driving wheels track, it would cause the truck to run to one side and mount the rail, thereby causing . the result it is meant to avoid," amp;c. I am, perhaps, as well aware as Mr. H. of the difficulty that would ariseif the forward end of the locomotive should be moved very much to one side. I am also aware that it would not be prudent to carry a pressure of five hundred pounds to the square inch in a locomotive boiler ; but it does not follow that because five hundrfd pounds pressure would tend to burst the boiler, that ninety or one hundred pounds pressure to the square inch may not be used with safety. I never intended to move the forward end of the locomotive much to one side, and would prefer to have the locomotive built so that it would track perfectly square, but I know that locomotives frequently run to one side in consequence of their being out of line, and it is frequently the case that they have wedges only on one side of the driving or pedestal boxes, and often have no wedges in the pedestals whatever. In cases like the above, a contrivance by which the head ol the locomotive can be slightly moved without moving the stationary centre of the truck, must appear to every practical man to be of value, insomuch that when the flanges of the driving wheels are found to be wearing to one side more than to the other, the moving over ot perhaps one-fourth of an inch to one side, which may be effected by the cup eccentric in a few minutes, would save the necessity of taking the locomotive into the shop to make an alteri, and thereby miss one or more trips, causing a loss of from one to perhaps five hundred dollars. The arrangement of my truck is such, that the centre plate or axis on which the forward end of the locomotive rests, is not movable, and is never out of the centre of the truck frame, the distance always being the same, from the sides of the frame to the centre of said plate, whether the eccentric be turned to one side or not. The lower wearing surface or part of the centre-joint that comes in immediate contact with the truck frame is bolted firmly to the centre of said Irame, and cannot move, consequently the truck would not run to one side, as predicted by Mr. Hudson. When used for eight-wheeled cars, or tenders, the eccentric would be useless, and in such cases I do not use it. It adds nothing to the self-adjusting or flexible qualities of the truck, and is only, as before said, a convenience for locomotives when the driving wheels do not track square. Mr. Hudson, no doubt, built a locomotive for the Buffalo and Attica Railroad ; I have understood that he did, and it may have had a centre-bearing truck ; but since reading his article on my truck, I have talked with a gentleman who is well acquainted with both Mr. H. and his truck alluded to, and he says that it differs very much from mine. JOHN L. WHITE. Corning, N. Y., Bec. 15,1852. [We have also received a letter under the signature of Hiram W. Bostwick, Esq., President of the Corning and Blossburg, and of the Buffalo, Corning, and New York Railroad companies, who says he " has used White's Equalizing and Self-adjusting Truck for about three years, under the engines of the Corning and Blossburg Railroad, and the Buffalo, Corning, and New York Railroad, and he is well satisfied that they are the best trucks in use.'* Betre he used them the engines were frequently getting off the track, but during three years using they have not run off the track once, while the cars have done so a number of timesthe locomotive still keeping the track. They carry, he says, li the forward end ol the locomotive finely, and turn curves in a beautiful and easy manner. He is going to put them under every engine on the railroads of which he is President." _JKaiiayamp;alsQ received a letter from W. M. Mallory, of Corning, frJzj-*rh.~mttB the objection of Mr. Hudson about the " eccentric," and says it is only there of a necessity, to be used when builders of locomotives neglect to make them as perfect as they should bewhen they do not centre in the proper place, which any one, engaged on railroads, knows to be a not uncommon occurrence. " In such cases," he says, " the engine man, by a slight movement, can do in a few minutes what it would take some hours to do with the men in the shop." We present the rest of his letter entire : " But the eccentric cup part is by no means the most important part of Mr. White's valuable improvement, it is so arranged as to give an equal bearing upon each journal, under all circumstances, and it adapts itself to any un-evenness of the road, and I have known this truck used for nearly three years upon the Corning and Blossburg Railroad, which, at the time, was very uneven, and it was never thrown from the track, while locomotives, with trucks like those in common use, wer orten thrown off. I have been engaged in the practical part of the railroad business for the past twelve years, and consider this one of the most important improvements in railway carriages that has ever come to my knowledge."