CURRENT TOPICS.

Caliche.--D. G. RUNNER in an article in Roads and Streets for June 1935 presents information on the origin, properties and use of caliche. The material is a calcareous formation of varying thickness widely found in the semi-arid regions of the southwestern United States. It is being used in road construction in the southwest due partly to the lack of other road materials of suitable quality, and to excessive cost of shipped-in aggregate.

In western South America, the name caliche is applied to the deposits of crude soda-niter. In southwest United States it is essentially a lime carbonate, containing in addition, magnesium, alumina and calcium silicates, and is in decided contrast with the true caliche of Chile. The material possesses the property of hardening when wet, which feature is commonly utilized in con- structing pavements. However, the scarcity of water in the arid sections made it necessary to obtain the desired compaction by other methods. It was found that crushing the caliche to smaller sizes aided somewhat in the dry compaction, and the finer the material was crushed the greater the ease of compacting when in a dry state. Usually the specifications for a caliche base require the material to be put down in two courses, the first course to be throughly wet and compacted before the second course is begun. The loose material laid at least 9 in. in depth, compacted to a thickness of 496 in. The caliche, after wetting and rolling is allowed to “set up” before the second course is placed. Before putting on the top course, the base section is bladed even and brought to grade. The top course procedure is essentially the same as that employed in constructing the bottom course.

IX. H. 0.

Automatic Steam Turbine. ---R. W. P. LEONHARDT (Power, Vol.

79, No. 6). In the new power house of Mitteldeutsches Kraftwerk, Magdeburg, Germany, house service energy is supplied from the 10,000 volt main station bus bars through 3 transformers which step down to 380 volts. For emergency service a 3,000 r.p+m., 400 lb. pressure, 770 degree turbine connected to a 700 Kva generator is connected in parallel with the transformers. It is arranged for automatic starting so that it is possible to have the unit supplying energy to important auxiliaries within 30 seconds. When the voltage fails across the 10,000 volt bus bars an under voltage relay

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282 CURREXT TOPICS. [J. F. I.

trips. A retarding device then interrupts the process of switching for a duration of 5 seconds so that the turbine does not start up unnecessarily should full voltage reappear across the bus bars. If voltage fails to reappear, a master relay switches into the circuit, causing the automatic starting process to continue. A series of relays operating in their proper order starts the turbine and when the generator voltage has reached 420 the three house transformers are disconnected and the main breaker of the emergency generatro is closed.

R. H. 0.

The Moscow Subway.-(Transit Journal, Vol. 79, No. 6.) On April 22, the first section of 7.1 miles of the Moscow, Russia subway was completed. It connects two of the most populous suburbs through the heart of the business district. When the entire system is completed it will be about fifty miles long and it is laid out radially, twelve lines running outward from the central part of the city with a belt line joining the ends of the radiating subway lines. The diameter of the tunnels is 18 ft., permitting use of wide and com- fortable cars. No two stations are exactIy alike, which enhances architectural interest and aids passengers in identifying them, They are finished in marble, polished granite, majolica and non- ferrous metals.

In tunneling, the natural difficulties were so great that in 1932, after 6,000 workers had unsuccessfuIly attempted a number of tunnels, several experts advised the abandonment of the project as impossible. Practically all known construction methods were tried out including the British shield system, the French caisson system, the Belgian double passage, artificial freezing and the American “flying arch.” For tunneling through solid ground the latter was demonstrated to be cheaper and quicker than the Belgian method. Late in 1932, a method was discovered of treating the subsoil with chemicals which held the tunnel wall together until the concrete linings could be completed.

The cars are 61% ft. long and built entirely of metal. Automatic devices stop them in the event the track ahead is occupied. It is estimated that the subway will carry about 128 million passengers this year.

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Standardized Rupture Disks.+ Chem. & Met., 42, 260.) The development of the frangible disk as a protective device for unfired pressure vessels has been given considerable impetus by the work of M. E. BONYUN. Briefly the tests consisted of fastening metal