[The editors are not responsible for statements made in the correspondence column. Anonymous communications cannot be considered, but the names of correspondents will be withheld when so desired.] A Correction To the Editor of Scientific American: In my article on the Diesel engine, on page 314, next to the last paragraph, the statement is made: “On the other hand, in an engine with eight cylinders, one cylinder may not become inoperative without affecting the power of the engine to a very great extent.” The word not in this sentence is an error, as the point to be made was that an engine having as many cylinders might run with one cylinder inoperative without affecting the power, to any serious extent. Haynek H. Gordon. October 7th, 1911. Roots in Sewer Pipes To the Editor of Scientific American: I noted with interest to-day the illustration and article on page 281 of the Scientific American, referring to roots in sewer pipes, and a shield to prevent their entrance. In passing, I should like to remark that so far as my experience goes, sewer pipes are cemented together, “baby” or pad of cloth or burlap being dragged through by the mason to smooth the interior of the joints. Returning to the roots, I wish to say that while quartermaster at Fort Assinni-boine, Montana, I found a main sewer pipe clogged, and a basement flooded with sewage, one day. Upon digging up the pipe (it was about 10 feet down) it was found that a mass of fine, hair-like roots, bunched together, completely closed it. This pipe was 8 inches in diameter, but just at this point was a junction, below the main was 10 inches in diameter, and this, too, was found almost entirely dosed by root masses. In all, 36 feet of pipe (about equal portions of each size) were replaced as the roots could he cleared out only by breaking up the pipe, and pulling the masses of fibers out. The largest mass was about 6 feet long and 3 inches in diameter, and when washed out and dried, was very similar in appearance to a bunch of oakum. The pipe was of the best quality of glazed tile obtainable and the joints were tightly cemented; but these root-fibers had grown through the pipe at all points, and so thickly that fragments of the pipe in some cases seemed to support a thick growth of hair. The roots came, apparently, from some small cottonwoods planted as shade trees along the roadway, and the nearest were about 20 feet laterally from the pipe line. A friend of mine told me that he had had a similar trouble with eucalyptus roots in a California post, and had cured it by packing the pipes in salt. I therefore purchased all the coarse rock salt (not lump, but broken pieces) that could be obtained in the small neighboring town of Havre, Montana, and packed it about 3 inches thick below, on each side, and above the replaced pipe, completely inclosing it. I n the dry soil of that country this mass of salt should not soon dissolve; and the earth immediately around the pipe would remain heavily “salted” for a long time to come. The eost was only a few cents per foot, and it is not easy to see how any roots could pass such a barrier. Durham, N. H. G. W. Edgekly, A Curious Meteor DR. MAX WOLF, director of the observatory of Heidelberg, reports in UAstroraomie, tha ton the night of M ay 22nd, he observed a faint meteor moving from east to west across the constellation Aquila. Its diameter was about 15 minutes of arc. It passed rapidly over the star Gamma Aquilffi, and disappeared, leaving behind it a faint trail which vanished almost immediately. The remarkable feature of the observation was that the. star Gamma remained invisible for fully 3% seconds after the meteor and its train . had disappeared. The obvious explanation is that the meteor left behind it a trail of opaque and non-luminous matter, dense enough to eclipse the star. 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