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New measuring flume
416
CURRENT TOpiCS.
[J. F. I.
placed by silvering. The latter method is, however, by no means perfect; the very thin film deposited by reduction of a solution of nitrate of silver is neither as white nor as brilliant as that of tin amalgam; it is deficient in strength, being detached by the slightest friction, and tarnished on exposure to sulphurous fumes. These faults are only very imperfectly overcome by a backing of varnish, of itself often a source of deterioration. Long ago an electrolytic deposit had been considered as a substitute, but experiments in this field yielded only mediocre results, whose irregularity was mainly due to the extreme thinness of the silver coating. Indeed, in the electrolytic method, it is this metallic film which consitutes the kathode, whose very minute cross-section is a poor conductor. When the contact is made around the periphery of the silvered glass, the electrolytic deposit takes place almost entirely in its immediate vicinity, and at points remote from the edges the thickness of the deposit diminishes rapidly, so that at the centre there is scarcely any deposit. This drawback has been cleverly overcome by Delere, Gresy, and Pascalis, whose method is in actual use in Paris by the Compagnie des Glaces de Saint-Gobain. This process, styled " silvered protector" (French patent No. 444,71o), consists in multiplying the number of current-carrying points of contact on the silver kathode by means of flexibly supported metallic combs. These combs have brass teeth, to each of which is attached a globule of tin, a soft metal which avoids scratching the silvering. All parts of the electrode immersed in the electrolyte are covered with an insulating layer of .paraffin, with the exception of the points in contact with the silverrag. The anodes consist of bands of pure copper supported between the contact points. The mirror to be coated is laid, silvered side up, in a shallow tray containing the electrolyte, the latter being kept of uniform strength by forced circulation. During the plating the current is interrupted every few minutes and the contact points shifted a short distance. The electrolytic action is thus uniformly distributed over the entire surface, resulting in a very homogeneous deposit of copper. Mirrors so protected resist the action of sulphurous fumes under the most unfavorable conditions and possess the unexpected advantage of increased reflecting power. New Measuring Flume. A~,'ON. (Engineering News, vol. 76, No. 6, August IO, I916.)--Experiments made in the hydraulic laboratory at Fort Collins, Colo., by the United States Department of Agriculture have resulted in the development of a measuring flume which the experimenters believe promises to overcome the chief difficulties experienced in measuring water for irrigation purposes. The action of the device depends upon the adaptation of the Venturi principle to the flow of water in an open channel. The vertical sides of the flume for a short distance converge to a throat, and from that point diverge to its normal breadth,, making it in
Sept., 1916.]
CURRENT TOPICS.
417
plan similar in contour to a Venturi tube. Gauge boxes are provided at the throat and at a point down stream for a measurement of the head. The laboratory tests thus far made indicate that the device is quite accurate in its measurements. The Venturi flume is inexpensive and easily constructed and maintained. It automatically takes care of the velocity of approach and requires but little loss of head in the ditch. The increased velocity in the throat greatly reduces the danger of trouble from silt, sand, aquatic growth, or floating trash. The discharge through the flume is dependent upon t'ne depth of water in the throat or the diverging section, as well as upon the difference of depths. The Principles of Filtration. D. l~.. SPERRY. (Metallurgical and Chemical Engineering, vol. xv, No. 4, August 15, I916.)--The process of filtration consists in separating the solids from a mixture of solids and liquid by causing the liquid to flow away from the solids through a porous mass, the openings of which are too small to allow passage of the solids. The porous mass, called the filtering medium, is composed of two parts: that which was provided in Order that filtration might begin, called the filter-base, and that which was formed during filtration, called cake, consisting of solids which were too large to pass through the openings of the porous mass. At the beginning of filtration the filter-base comprises the entire filtering medium. As the flow proceeds the solids are deposited upon the filter-base, and the filtering medium now consists of the filterbase and deposited solids or cake. The flow of liquid through the porous mass and the building up of the porous mass are constantly changing because the liquid, as it flows into the cake, is continually leaving its solids behind, thus constantly increasing the thickness of the porous mass, while, simultaneously with this action, the liquid is continually decreasing its own rate of flow, due to the increasing thickness of solids through which it has to pass. It is evident that if an cxpression can be derived showing the relation between the flow of the liquid and the building up of the porous cake and the laws of each determined, the two can be combined in a known relation, thus expressing the fundamental laws of filtration. Such an analysis of the two processes and a determination of their relationship leads to a formula for the total quantity of filtrate discharged in terms of time, pressure, and other known physical quantities of the materials employed. The Dry Powder Fire Extinguisher. ANON. (Quarterly of the National Fire Protection Association, vol. IO, No. I, ~[uly, 1916. ) --Recent analyses made for the British Government show that dry powder fire extinguishers generally contain as their main constituent bicarbonate of soda, the amount of which varies from about 46 per cent. to 56 per cent. in the different samples examined. It is
CURRENT TOpiCS.
[J. F. I.
placed by silvering. The latter method is, however, by no means perfect; the very thin film deposited by reduction of a solution of nitrate of silver is neither as white nor as brilliant as that of tin amalgam; it is deficient in strength, being detached by the slightest friction, and tarnished on exposure to sulphurous fumes. These faults are only very imperfectly overcome by a backing of varnish, of itself often a source of deterioration. Long ago an electrolytic deposit had been considered as a substitute, but experiments in this field yielded only mediocre results, whose irregularity was mainly due to the extreme thinness of the silver coating. Indeed, in the electrolytic method, it is this metallic film which consitutes the kathode, whose very minute cross-section is a poor conductor. When the contact is made around the periphery of the silvered glass, the electrolytic deposit takes place almost entirely in its immediate vicinity, and at points remote from the edges the thickness of the deposit diminishes rapidly, so that at the centre there is scarcely any deposit. This drawback has been cleverly overcome by Delere, Gresy, and Pascalis, whose method is in actual use in Paris by the Compagnie des Glaces de Saint-Gobain. This process, styled " silvered protector" (French patent No. 444,71o), consists in multiplying the number of current-carrying points of contact on the silver kathode by means of flexibly supported metallic combs. These combs have brass teeth, to each of which is attached a globule of tin, a soft metal which avoids scratching the silvering. All parts of the electrode immersed in the electrolyte are covered with an insulating layer of .paraffin, with the exception of the points in contact with the silverrag. The anodes consist of bands of pure copper supported between the contact points. The mirror to be coated is laid, silvered side up, in a shallow tray containing the electrolyte, the latter being kept of uniform strength by forced circulation. During the plating the current is interrupted every few minutes and the contact points shifted a short distance. The electrolytic action is thus uniformly distributed over the entire surface, resulting in a very homogeneous deposit of copper. Mirrors so protected resist the action of sulphurous fumes under the most unfavorable conditions and possess the unexpected advantage of increased reflecting power. New Measuring Flume. A~,'ON. (Engineering News, vol. 76, No. 6, August IO, I916.)--Experiments made in the hydraulic laboratory at Fort Collins, Colo., by the United States Department of Agriculture have resulted in the development of a measuring flume which the experimenters believe promises to overcome the chief difficulties experienced in measuring water for irrigation purposes. The action of the device depends upon the adaptation of the Venturi principle to the flow of water in an open channel. The vertical sides of the flume for a short distance converge to a throat, and from that point diverge to its normal breadth,, making it in
Sept., 1916.]
CURRENT TOPICS.
417
plan similar in contour to a Venturi tube. Gauge boxes are provided at the throat and at a point down stream for a measurement of the head. The laboratory tests thus far made indicate that the device is quite accurate in its measurements. The Venturi flume is inexpensive and easily constructed and maintained. It automatically takes care of the velocity of approach and requires but little loss of head in the ditch. The increased velocity in the throat greatly reduces the danger of trouble from silt, sand, aquatic growth, or floating trash. The discharge through the flume is dependent upon t'ne depth of water in the throat or the diverging section, as well as upon the difference of depths. The Principles of Filtration. D. l~.. SPERRY. (Metallurgical and Chemical Engineering, vol. xv, No. 4, August 15, I916.)--The process of filtration consists in separating the solids from a mixture of solids and liquid by causing the liquid to flow away from the solids through a porous mass, the openings of which are too small to allow passage of the solids. The porous mass, called the filtering medium, is composed of two parts: that which was provided in Order that filtration might begin, called the filter-base, and that which was formed during filtration, called cake, consisting of solids which were too large to pass through the openings of the porous mass. At the beginning of filtration the filter-base comprises the entire filtering medium. As the flow proceeds the solids are deposited upon the filter-base, and the filtering medium now consists of the filterbase and deposited solids or cake. The flow of liquid through the porous mass and the building up of the porous mass are constantly changing because the liquid, as it flows into the cake, is continually leaving its solids behind, thus constantly increasing the thickness of the porous mass, while, simultaneously with this action, the liquid is continually decreasing its own rate of flow, due to the increasing thickness of solids through which it has to pass. It is evident that if an cxpression can be derived showing the relation between the flow of the liquid and the building up of the porous cake and the laws of each determined, the two can be combined in a known relation, thus expressing the fundamental laws of filtration. Such an analysis of the two processes and a determination of their relationship leads to a formula for the total quantity of filtrate discharged in terms of time, pressure, and other known physical quantities of the materials employed. The Dry Powder Fire Extinguisher. ANON. (Quarterly of the National Fire Protection Association, vol. IO, No. I, ~[uly, 1916. ) --Recent analyses made for the British Government show that dry powder fire extinguishers generally contain as their main constituent bicarbonate of soda, the amount of which varies from about 46 per cent. to 56 per cent. in the different samples examined. It is