Instantaneous Roentgen Ray Photography T HE sharpness of Roentgen ray skiagraphs of parts of the body is often impaired by voluntary or involuntary movements of the patient. This difficulty would not exist if “instantaneous” or exceedingly short exposures could be substituted for the usual “time” exposures. In order to obtain Roentgen rays of sufficient intensity for use in instantaneous exposures, the Roentgen tube must he excited by a single electric wave or pulse, produced by a very large induction coil. In the well-known Veifa apparatus this result is accomplished by melting a fusible plug in the primary circuit by an overload of direct current. The Erlangen firm of Reimger, Gebbert&Schall has recently put upon the market an apparatus in which the single wave or “un i-pulse” is produced by means of a specially constructed interrupter. This device consists of an amalgamated copper rod, surrounded by a closely fitting sheath of insulating materia., and dipping into a vessel which contains mercury, covered by alcohol. When the rod is drawn out of the mercury it draws some of the latter after it into the insulating sheath. The heat developed in the slender column of mercury by the strong current employed causes the column to break with explosive violence, and drives the mercury downward with great velocity, which is increased by the heat of the self. induction spark which immediately follows the rupture of the mercury column. The primary circuit is consequently broken with extreme suddenness, independently of the speed with which the rod is raised, and a spark of corresponding intensity passes between the secondary terminals. Fig. 1 shows the appearance of this spark in ordinary conditions and Fig. 2 sbows the deformation produced' by placing the spark gap in the strongest part of the coH's magnetic field. A Roentgen tube excited by this single induced wave, or “unipulse,” emits Roentgen rays of sufficient intensity to make strong skiagraphs of thick parts ot the body (Fig. 3). in 1/200 second. Why Fog Signals Fail IN a chart. published by the United States Weather Bureau, and reproduced here in abridged form, Prof. A. G. McAdie discusses some of the phenomena presented by fogs and the difficulties and dangers which arise therefrom to navigating vessels, especially near shore. The fogs of the Pacific, and especially those on the coast of California, Oregon and Washington, present some characteristic features of their own. They are low-lying, dense and of frequent and regular occurrence, and have been the cause directly and indirectly of a large per cent of marine disasters. In the vicinity of San Francisco, owing to the general movement of the air from the sea toward the land, and the climate of the great interior valley, fog is frequent and well marked. In summer the afternoon sea-fog varies in depth from 100 to 1,700 feet, but it rarely reaches far inland. On some afternoons the veloeity of the wind at San Francisco rises with almost clocklike regularity to about twentyctwo miles per hour, and a solid wall of fog, averaging 1,500 feet in height, comes through the Golden Gate, causing a fall in temperature to about that of the sea, namely, 55 deg. F. The upper level of the fog can be plainly seen from the hills in the vicinity, and it is interesting to note that above the fog level the air is cloudless, and the afternoon temperature ranges from 80 deg. F. to 90 deg. F. While the Pacific fogs occur with peculiar regularity, those of the North Atlantic Ooast, though at times persistent, are irregular both as to the time of their occurrence and their duration. The North Atlantic Coast fogs are probably due to thin strata of warm moist air passing over the cold water surface. The summer afternoon sea fogs of the Pacific are also quite different from the winter morning fogs. The laUer lie low, close to the surface of the water, and do not average more than 100 feet in depth. It is nearly always possible, by sending out a look-out, to get above the level of the fog and thus obtain proper bearings. With the summer afternoon sea-fogs this is out of the question. The distance which the fog extends seaward is not definitely known, but it is thought that an average would be about 50 miles. There are instances when a fog has been reported several hundred miles off shore. Whether a fog appears for a few hours at certain seasons, as on the Atlantic Coast, <r regularly through the summer afternoons and the winter mornings, as along the Pacific Coast, whether it forms sharply defned streaks and strata, as at San Francisco, or lies in undefined banks, as off Newfoundland, in either case it is due to a cooling of the air and consequent condensation of water vapor. The cooling may be brought about by elevation and expansion or by rapid radiation or by mixture with a cooler mass of air or contact with a cooler surface. The water vapor condenses on minute nudei which may he exceedingly fine dust or possibly ions. The morning winter fogs are low lying banks at condensed vapor which, as a rule, move from the land seaward and are probably formed by a cooling due to radiation and contact, the land surfaces being much cooler .in the early morning hours than the water surfaces, owing to the high specific heat of water. The summer afternoon fogs are probably due to cooling caused in part by elevation and expansion and in part by mixing. Fogs, as a rule, form when cool air passes over warm, moist surfaces, but in the case of the fogs near the Golden Gate, San Francisco, where the surface temperature is 55 deg. F., and the air temperature, at a height of 700 feet, 80 deg. F., condensation is more probably due to a mixing of air currents having different temperatures, humidities and velocities. The seemingly unaccountable failure of fog signals at the critical moment has been a source of much perplexity and serious disaster. It not infrequently happens that the master of a vessel will testify that the fog whistle could not be heard, while light-house officials will maintain with equal positiveness that there was no failure to give the proper signal. It is now known that both sides may have been correct in their statements. If sound travels through a medium, such as air, the density of which varies more or less from point to point, it suffers a refraction, or, in other words, the line of propagation is not a straight line. As a result of this it may occur that a sound wave, starting from some point on the surface of the earth, is deflected upward, so that a person stationed at some distance on the surface of the earth will receive no indication of the sound wave, which passes over him, above his head, leaving him unconscious of the disturbance. Another possible cause through which sound signals may become inaudible at certain points is the reflection of sound from sharply defined clouds or banks of dense fog. Such reflection may have the result that at certain points the direct wave and the reflected wave just neutralize and the sound becomES inaudible. The troubles to which air signals are subject are completely overcome when water is used as the transmitting medium. The success of submarine bell signals has been so marked that in time the siren signais through the air will prob;bly become of secondary importance. Where both signals can be employed and used simultaneously, a careful determination of the interval between the receipt of the two signals enables the mariner to obtain some indication of his distance from the point of starting of both signals, for obviously the time which elapses between the receipt of the two signals is the difference between the time taken for the air signal and the water signal to reach him. Now the velocity of sound in air is about 1,100 feet per second; in water about 4,700 feet per second. From these data the distances may be computed. The same principle can be applied with still more satisfaction if one of the signals is given by wireless telegraphy and the other through the water. It is claimed that the bells used as submarine signals can be heard farther, under ordinary conditions, than the siren, and furthermore, by means of a special telephone apparatus, it is possible to determine the direction or the origin of the sound. The receiving apparatus consists of two tanks placed in the hold of the vessel below the water-line. These tanks contain microphones immersed in liquid and connected to the pilot house. An indicator box shows the side on which the responding telephone is connected, and the master is thus able to ascertain the direction from which the signals come. The Rubber Supply of the Future THE Ohemical Engineer publishes an article by Mr. Walter Freudenberg of Bremen on the subject of the future rubber supply. The estimated planted acreages of rubber as given by three of the leading trade publications are as follows: India Rubber Trades Diary, 776,000 acres; India Rubber Journal, 980,000 acres; Gummi-Zeitung (Berlin), 1,310,000 acres; Mr. Freudenberg takes the second estimate as the basis for future yields, and gives the futures supply as follows: From Malaya, 70,000 tons; Ceylon, 19,000 tons; Dutch Indies, Borneo, South Mora, and Burma, 20,000 tons; total annual supply of plantation rubber in 1916-17, 109,000 tons. To this must be added the probable supply of wild rubber, which until recently has been about 70,000 tons annually. He considers, however, that the supply from this source may decrease, and estimates only 35,000 tons, making a gross total of 144,000 tons in 1916-17. Statistics show that for the year ending June 30th, 1910, a total of 76,000 tons was used, showing an increase of 5 per cent annually during the past 10 years; if this rate continues, about 107,000 tons would be required by 1916-17.