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gable passes of movable dams vary in width from 500 ft. in the Herr isl- and dam to 600 and 700 it, in the Ohio «river: dams. © Ordinarily. the pass wickets alone are maneuvered from boats, while 'the weirs are equipped above with trestle service bridges from which the wickets are operated in much the same manner as are the pass wickets, save that hand power is. frequently substi- tuted for steam power, and the weir wickets have chains attached and lead- ing to bridges to facilitate their maneuver and tto obviate the difficulty of grappling for them with pole wicket hooks. The wicket weirs are used in conjunction with the _ bear- trap gates for regulating the pools and initially controlling rises until the stage compels lowering of naviga- ble passes. In the order of arrange- ment the navigable pass almost in- variably adjoins the lock, followed by the bear-trap gates, the wicket weir closing the remainder of the river. But the most valuable and import- ant feature of the movable dam is its automatic weir without which its op- eration would be very difficult. For that purpose in this locality the bear- trap gate is used. That type of dam is not a new device, having been in- vented by Messrs. White and Hazard about 1818. These men were interest- ed in the anthracite coal business on the Lehigh river, in the eastern part of the state. Being unslackwatered at that time the Lehigh, like most streams in this section of the coun- try, often became too low for trans- portation purposes for long continued periods of time. For overcoming this inconvenience and loss White and "Hazard set about to devise as a sub- stitute for a lock and dam _ system, a scheme comprising an inexpensive fixed dam equipped with a weir that could be opened and cYosed automat- ically, that is in the sense that the head produced by the dam would furnish the power, properly applied, for maneuvering gates turning on horizontal axes upward or downward without the assistance of extraneous forces. Thus with weir closed a pool would be formed for loading and storing coal vessels, and when open a chute would be provided for navigation through the dam with suffi- cient water from the pool, augmented perhaps by a small rise, to float the product to a pool farther down and finally to market. Having con- ceived the idea of an automatic dam, and doubtless prepared a working model for testing it, they proceeded THE Marie REVIEW to construct a crude wooden dam for practical use. To be sure, their in- quisitive neighbors were anxious to discover the mysteries of the new and unfamiliar construction on which they were engaged. Doubtless not being overconfident of its success, and the structure resembling in general ap- pearance, weight, ¢tc., a primitive deadfall bear-trap, they informed them it was a bear trap. After its comple- tion and _ successful operation the name adhered to it and has since remained. unchanged. Of all 'well known movable: dams for practical uses this is the pioneer, and with the exception of a few unimportant de- signs is also the only strictly Ameri- can movable dam, nearty ail others that have been thoroughly tested in a practical way, such as the Boule, Drum, Poiree Needle, Curtain, Chan- oine wicket and_ others, French invention. Although for many years neglected, and once tried and condemned by French egineers, it has finally come to the front and that to stay. It is ideal in principle, extremely simple in de:tgn and con- struction, loses little water in opera- tion, requires almost no _ attention or protection against injury and is by long odds the easiest of all movable dams to operate. BEAR TRAP GATES FORMERLY OF WOOD. In their earlier construction, and in- deed until within a decade, bear-trap gates were built of wood as far as possible, usually the very wood consistent with the necessary strength, in order to render them more buoyant. This buoyancy was not so essential, it was believed, ex- cept as an aid in securing the initial upward movement of the gate. The head for producing this movement was often difficult to obtain because of circumstances tending to diminish the low water discharge. Hence, owing to the use of such material the leaves of the gate were frequently weak structurally, a very serious de- fect in such a device, causing them to warp and twist, and not infre- quently to jamb between their piers. Also the lightness of the gates ren- dered them buoyant at inopportune times, permitting them to rise when it was desired to have them remain down. In consequence of these struc- tural and working defects many modi- fications of the original developed, retaining, however, in them all the main principle of the gate invented by White and Hazard. Some of the modified bear traps were fairly suc- cessful, but involved objectionable being of. lightest "3l complications. _On the whole they were comparatively satisfactory for logging dams and minor navigation works, but were impracticable for important slackwater improvements, being in great measure unreliable and undependable. Not until the erection of two, bear-traps in Herr island dam, in 1901, was there any radical departure from the old wooden gates tending to overcome their physi- cal and operating defects. The Herr island gates are of the simplest form of that type of dam--the lower leaf hinged at its downstream edge and the upper at its upstream edge, the latter lapping over nearly one-half the upstream portion of the former when down. When first contemplated of wood the usual difficulty of no initial head at time to affect a rais- ing presented itself. | Several proposi- tions 'were entertained, among them that of securing water from Alle- gheny city through the supply mains. However, when it was. discovered that more than 1,000,000 gallons would probably be required within ten minutes for the occasional raising of only one gate the idea was aband- oned. A continuation of the search for other means for overcoming the gigantic obstacles previously men- tioned eventually resulted in the de- termination to construct a_ hollow structural steel lower leaf, which, by means of air for expulsion of the water contained therein, might be made even more buoyant than a wood- en gate, and at the same time pro- duce a structure of any desired rigid- ity. Thus, if successful, air would always be available at much less cost than any other means of initial rais- ing and all objectionable features of the old wooden gates would be successfully overcome. ADVANTAGE OF STEEL GATES. These gates have now been in use more than six years, have been man- euvered hundreds of times under ev- ery possible condition and have met the highest expectations of their de- signers. They have been duplicated on the Ohio river with only two or three minor changes, two having been constructed at each of dams 3, 4 and 5, which have been operated during the present season with even greater success. They can be lowered in less than a minute, and raised in from 5 to 8 minutes, under full pool con- ditions, and under some condi- tions in even less time. With suf- ficient water head they rise without the use of air, but air can always be introduced when needed at a trifling cost. Never have the steel bear- Sst Sy, ages Re ea AB 28 ae