[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.] The Technique of Clam Digging IN our issue of December 16th, on page 564, we published a letter from Mr. Hugh N. Johnson criticising our advertisement headed the “Technique of Clam Digging.” The advertisement referred to appeared in the same issue on page 566. Our reply to Mr. Johnson's letter, which was inadvertently omitted last week, is as follows : Your criticism of “The Technique of Clam Digging” is just—from the viewpoint of the Pacific Coast and some parts of the Atlantic Coast. The species of Tapes and which flourish on the Pacific coast, and the Mya Arenaria, which multiplies in long-armed softness on the north Atlantic coast, are harvested with a fork and bucket at low tide. Plainly the writer of the advertisement had in mind only the Venus which, despite its suggestive name, is the aristocrat of the clam family, the only one having real gastronomic distinction. It is true, the soft, long-fingered clam dug out of the mud appeals to many tastes when steamed and that it is also used sometimes for chowder, but it never ventures to raise its long, humble neck in the presence of its relative, the little neck. With the assumption of superiority familiar in the New Yorker—anybody who manages to exist in the metropolis for six months calls himself a New Yorker—the writer of the advertisement calmly took for granted that the onl clam worth talking about is the lordly Venus Mercenaria. In New York it is “the clam”—the long-necked variety being invariably designated as “the soft clam.” And the Venus Mercenaria or hard clam, or little neck, or quahog, is harvested exactly as described in the advertisement. The mistak the writer of the advertisement made was in assuming that this clam, more or less local to a small part of the seaboard, is the only clam. He is a victim of insular prejudice rather than of ignorance.—Editor. A Mechanical Evolution and Smokeless Cities To the Editor of Scientific American: No smoke on our rivers, no smoke on our lakes, no smoke from our factories, in fact smokeless cities, is the immediate boon in store for us through the most meritorious invention of the compressed-air internal-combustion engine. Before briefly describing the principle of this beautifully simple motor, it may be well to give a short resume of the development of the marine engine. The oldest marine engine and boilers I ever had charge of were fitted in the steamer “Windermere” by John Penn&Sons of Greenwich in the year 1857. It was in this vessel that the first surface condenser, that most important of all adjuncts to the marine engine for salt-water service, was fitted by Samuel Hall. The working pressure of steam was but 15 pounds per square inch, speed of piston slightly below 200 feet per minute, and the combined weight of engines, condenser, pumps, boilers and water therein equaled one gross ton for each indicated horse-power. Now the reduction of weight for power developed is the great desideratum for marine propulsion at least, and this is certainly true as regards propulsion in air. The reduction of weight for a given power is most largely due to augmented piston speeds. This speed has advanced during half a century from 200 feet to 1,000 feet a minute for reciprocating engines, and to 20,000 feet in rotary engines. The other factors contributing to efficiency are higher steam pressures, greater vacuum, and stronger materials. Though the continuous development of the tri-compound and quadruple-expansion engines was inaugurated by the construction of the “Aberdeen,” built in the year 1882 by R. Napier&Sons, the first triple-expansion engines were fitted in the The invention of Dr. Diesel. “Propontis” by John Elder&Co., in 1874, their dimensions being 23 + 41 + 62 X 42 inches. The combination in this vessel delayed the advancement of the triple or multiple expansion system solely through the failure of the Rowan boilers. This, however, is no exception to the rule; the boilers have always been the troublesome and expensive element of marine propulsion, and their elimination will tend wonderfully to simplify the work and responsibility in the engine room. Fifty years ago the average consumption of fuel was about 5 pounds per indicated horse-power per hour. To-day this consumption is reduced to one-half pound of fuel per horse-power hour. The following table shows in chronological order I over half a century: The Reduction of Fuel Consumption per Horse-Power Hour. Fuel Consumption, Pounds. . ssr 15 . 5 1860. Steam pressures 25 pounds with surface condensation........... 3 1870. Compound engines ........... 2% 1880. Triple-expansion .............. 1, 1890. Quadruple-expansion (increased pressure) ...................... 1901. Internal-combustion engine .... lh As it may appear somewhat remarkable, I may point out here that the consumption of crude oil fuel works out about the same, whether it be used explosively or to generate steam. The space which could be saved for the of pas- sengers and cargo in a 20-knot steamer by substtuting the compressed-air, crude-oil, internal-ombustion engines for the familiar steam engine or turbine instalment, measures fully 5,000 cargo tons. Now I venture to assert that the greatest mechanical invention since Samuel Hall invented the surface condenser, or probably since Watt devised the steam engine, is now afloat on our Great Lakes, and is being duplicated in that locality for coast service. This is the Diesel improved internal-combustion motor or engine. As this engine is beautifully simple and works with crude petroleum, of course without steam, it seems destined to quickly displace the steam turbine, and with it, the use of coal and steam, for propulsion at least, will no longer be required. An engineer cannot witness the retirement of those good and faithful old servants without feelings of regret, especially since the advent, on a magnificent scale, of the steam turbine was so recent; still, it seems quite apparent that their day of usefulness is almost over. The adoption of the steam turbine was too long delayed to admit of a long lease of life, the principle of the De Laval impulse turbine having been clearly illustrated more than two thousand years ago. The internal-combustion engine of to-day does not weigh more than one-fifth of that of a marine engine and boilers of equal power; and when it is considered that there are gasoline engines on the market weighing not more than 2112 pounds a horse-power, who shall say that instead of being below one-fifth of the weight of the steam engine and boiler, the crude ol combustion motor may not be 'reduced to one-tenth or less of the weight of the steam boiler and engine instalment? To me it appears that the sterling merit of this new invention is the beautifully simple means of firing the charge. Instead of the fickle and complicated electric spark system, the crude oil is ignited by a sudden and high degree of air compression within the cylinders. This device is not only reliable, but is very simple, which is of the utmost value for marine propulsion at least. Consider a steamer with large boilers, coal bunkers, coal chutes, coaling hatches, and all the paraphernalia for operating, cleaning, and repairing, and then try to imagine all of this weight and complication absolutely eliminated, and a space six times as large as the engine room thrown open for the stowage of cargo or the accommodation of passengers. Well, this is just what the adoption of the BUSINESS BOOKS Bant Bookkeeping Department Store Accounts Business Management, Part I Factory Accounts Business Management, Part II Insurance and Real Estate Corporation Accounts and the Accounts Voucher System Practical Bookkeenlng Not old reprints or careless compilations, but new, up-to-date material, prepared especially for us. Size of books is 6713-in. x 9%,-in. ; they are printed on a fine grade of super-calendered book paper, profusely illustrated with fine drawings and half tones, durably and artistically bound in handsome red cloth. 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It will be best in ordering, to name one or two additional books as alternatives, in case our stock of the one you particularly want is exhausted. But better still, to insure gettingwhat you want, sit right down this minute, cut out the coupon, and mail to us, with currency, stamps, silver or money order. TECHNICAL WORLD MAGAZINE 5760 Drexel Avenue. Chicago. I1L, U. S. A. ........... MONEY BACK COUPON ............ TECHNICAL WORLD MAGAZINE 5760 Drexel Ave., Chicago, Ill. Enclosed herewith is j................, for which send me TECHNICAL WORLD MAGAZINE for one year and the following handbooks, prepaid : December 23, 1911 '“ITiD” Large L!ne of tj 1AR Attachments For Foot I ITIirC or Power LAinES Saltnble for fine siceuriite work in the repair shop, garage, tool room :ind nmcliine shop. Sen:; fo rCatiilogue 1! SENECA FALLS MFG. CO. 695 Water Street Senses Falls, N. Y.. U.S.A. 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Don't delay. .; HEATH FOUNDRY&MFG. CO., Plymouth, 0. V crude-oil motor will accomplish. Bearing this in mind, and with many concrete examples of this great transformar tion in evidence all the world over, I may venture to predict that no marine boilers and steam engines, for the propulsion of ships, at least, will be seen anywhere under construction, after the Panama canal has experienced five years' service. Chicago, 1ll. . Joseph R. Oldham. A Fatal Omission To the Editor of Scientific American : I want to compliment you on your editorial under the heading of “A Fatal Omission.” I have no interest whatever 'n the Pennsylvania Railroad, but I do feel a most serious blunder has been made by New York city in not arranging subway connection with their new station. I would like to see the press unitedly agitate the matter so that the convenie might be speedily brought aoout, and general public receive a benefit that they are entitled to. When we consider that ten million passengers were carried through the station the first year of its existence, it is a disgrace to the city that an immediate effort for relief is not put under way. W. F. Habtbanft.' Jersey City Heights, N. J. The Current Supplement 1, HE front page article of the current issue, No. 1877, of our Supplement, is devoted to an illustrated description of the 3,000-horse-power Hydros Electric Power Plant at Cannon Falls, Minnesota. —Prof. Woodworth's article on the Psychology of Light reaches its second installment.—Fremont's new test for steel rails, which promises to become a standard method, is described.—An article by P. V. Vernon deals with the influence of composition and previous history of steel on the power required to cut it.—An article on “Developing a Small Water Power Plant” should prove of special interest to farmers and others located at a distance from city centers.—G. C. Sawyers, cheese expert. writes on the “Manufacture of Cheddar Cheese."—At the opening of the winter season an article on “Skis, Their Construction and Use,” will be welcomed by many of our readers. —A very scholarly account of “Wheels, Ancient and Modern,” from the times of primitive man to the day of the steel-spoke automobile, is given by H. L. Heathcote.—Condiments and Stimulants have their use, as well as abuse. They are discussed from a scientific standpoint in an article derived from Prometheus.