sounds like hailstones beating against a sheet of tin. Again, it may sound like short hisses from a steam pipe, or periodic discharges of coal down a chute. The tone of static is generally very low, while that of wireless signals is generally higher, particularly at present.

Of the various means of mitigating static, perhaps the earliest was better means of tuning. By tuning is meant the bringing of the transmitter and receiver into sympathy or tune, so that the receiver responds to the signals of the selected transmitter to the more or less total elimination of other signals. Tuning has greatly aided in the combatting of static. From the simple circuits of the early wireless receivers, there have come those systems in which several circuits are arranged in series, all inductively coupled. That is to say, the current in the serial circuit induces a secondary current in a second circuit; and this current, reacting on a third circuit, induces a third current which is brought to bear on the detecting device which in turn operates the telephone receivers worn by the operator. Such an arrangement permits of elaborate tuning, and the inductive coupling may be made either tight or loose; so that by a process of elimination one train of signals after another can be cast out until the desired one alone remains.

But the great thing to bear in mind with regard to elaborate tuning circuits is that there is a loss in transferring currents inductively from one circuit to the next. This, however, has been overcome by the powerful transmitters of to-day, which assure the transmission of a sufficiently powerful train of waves to withstand the "weeding out" process at the receiving end.

Elaborate tuning circuits have done much to master static, although static, being of a very wide wave length and therefore not susceptible to delicate tuning as are the transmitted waves, has often come through all tuning circuits along with the desired signals. Most of the systems in which the conquest of static has been claimed, have been based on intricate tuning systems which have sometimes worked and sometimes not.

Another method of handling static has been by means of the recent "beat" type of receiving sets. In these sets a local wave generator, usually of the vacuum valve design, can be adjusted to produce an undamped current of any desired frequency. This current is introduced in the same circuit as the received signals, so that by having a slight difference in frequency between the incoming signals and the locally generated waves, the two currents alternately help each other and oppose each other, producing "beats" or periodic signals. Such systems have proved comparatively effective in handling static, since they permit of a fine distinction between all signals received with a view to selecting just one of them. However, the "beat" receiver responds best to those signals known as undamped waves, as compared with the damped waves generally employed. Still another method has been to employ a tuned reed in place of the usual telephone receiver at the receiving station. Thus the tuned reed responds only to a signal of the correct pitch, to the exclusion of all others. Static is generally pretty well eliminated by such harmonic forms of receivers-and so are all other signals for that mater, proving disadvantageous at times.

The skilled operator can generally read signals through static disturbances unless the static is too loud. With modern transmitters which emit highpitched signals of whistle-like sound, the signals can be readily distinguished above the static din the greater part of the time. It is only in long-distance communication that static forms the greatest obstacle, and that accounts for the frequent and long shut-downs in some of the trans-oceanic wireless stations.

So it is evident from what has been said in the foregoing that there have been numerous ways of removing static under certain conditions. But what has been wanting all these years is a way that will eliminate static under all conditions. Perhaps Mr. Weagant has discovered some new and totally different way of handling received signals. If so, then there is much promise for the future of radio communication.-Scientific American, 7/12.

WILSON APPROVES MAKING WIRELESS A NAVY MONOPOLY.-Permanent government control of all radio communication through the acquisition and operation by the Navy Department of all shore wireless stations in the United States used for commercial purposes is planned by the administration under a bill now before Congress.-Washington Evening Star.

DRIFTING MINES.-It is stated that 25 mines were sighted by a steamer recently off the Atlantic coast. There will probably be many floating mines for months to come and doubtless some marine accidents will result. Of course, measures will be taken to pick up and destroy these menaces to navigation, but it is almost impossible that all should be found, and ship captains must run a certain degree of risk. After the Russo-Japanese war ships were sunk for a long time after the cessation of hostilities, as mines were carried out into the Pacific, some of them torn from their moorings and others being drifting mines. It is not known how many free mines the Germans set afloat, but it is believed a great number were thus started forth on missions of destruction. Fortunately the trend of the currents in the North Sea is into the gulf stream, flowing toward Spitzbergen. There is a return current west of Iceland which might bring mines back by the Labrador current into the navigation zone between the gulf stream and the coasts of Newfoundland, Nova Scotia and northeastern United States, which is the principal steamer lane. Mines that chance to work their way through the English Channel might get down into the Canaries current, and so flow into the gulf stream from the south. Were the current conditions otherwise than they are the danger from this source would be extremely grave. Mine sweeping must be continued with unremitting diligence until the seas are made safe again.-Washington Evening Star, 10/11.

VARIATIONS IN THE DIP OF THE HORIZON.-W. J. Peters, of the Carnegie Department of Terrestrial Magnetism, has recently published the results of extensive observations made during the cruises of the Galilee and the Carnegie on the variations in the dip of the horizon due to refraction. This subject has previously been investigated from time to time, especially by the Germans. An official German text-book of navigation states that the horizon has been observed as much as 15 minutes above and three minutes below its normal position. Bowditch's American Practical Navigator gives an even wider range. The subject is of practical importance, since each minute of abnormal refraction means an error of a mile in the determination of the ship's position. The observations described by Peters appear to have been taken with special care, and number no less than 3031 determinations. In all of these measurements the horizon was never raised by refraction more than 2.4 minutes nor depressed more than 2.0 minutes below the position in which it would have been seen (i. e., the normal dip, due to the elevation of the observer above the sea) if no refraction had existed. Most of the measurements were taken with a Pulfrich dip-of-the-horizon measurers made by Zeiss, of Jena. Mr. Peters thinks that the extraordinary values that have been occasionally reported may be peculiar to certain regions, where the navigator should be ready to detect them either by observing stars in different azimuths or by special instruments or attachments to the sextant. He adds that when aerial navigation across the oceans is realized, if astronomical methods of navigation are used, some simple means of measuring the dip of the horizon will become highly desirable.-Scientific American, 16/11.

ENGINEERING

THE SEMI-DIESEL OIL ENGINE.-By James Richardson, B. Sc., A. M. I. C. E.-Introduction.-On several recent occasions, authorities, when forecasting the lines of development of the oil engine, have expressed the

'Paper read before the Diesel Engine Users' Association, on Thursday, October 24, 1918.

opinion that the so-called semi-Diesel engine would play no inconsiderable part. It might be matter for surprise that publications of technical matter dealing with the semi-Diesel engines are extremely rare in comparison with the vast amount of available data relating to the Diesel engine.

Definition of the Semi-Diesel Engine.-The variously-named semi-Diesel, hot-bulb or surface-ignition engine may be defined as an internal-combustion engine, using oil fuel, which has an uncooled portion of the combustion chamber, normally at high temperature, serving to augment the heat generated by the compression pressure and to assist in the vaporization and ignition of the fuel injected at the ignition point of the cycle. From this class should rightly be excluded those oil engines which are not called by their makers Diesel engines, but which rely for ignition, as completely as the Diesel engine, upon the heat generated by the compression of the air charge, and therefore should be so named. The means of injection of the fuel with such engines may vary from the standard air injection system.

000000

REARAMORE TWO STROKE CYCLE

BOUNDER

PETTER

TWO STROKE CYCLE

THE STROM CYCLE

CAMPBELL

TWO STROKE CYCLE

"KROMHOUT" TWO STROKE CYCLE

FOUR STROKE CYCLE

FIG. 1.-Nine Representative Types of Semi-Diesel Engines.

Nomenclature. In describing and classifying engines of the semi-Diesel type, it will be necessary generally to adopt the nomenclature and technical expressions familiarized by the literature dealing with the Diesel engine, and convenient often to describe by means of comparison with the betterknown Diesel engine.

Classification. The many types of engines of the type under review vary considerably (see Fig. 1) and can be classified according to the cycle of operation upon which they work, whether the two-stroke or the four-stroke cycle, and according to the extent to which the heat of compression is relied upon for the vaporization and ignition of the injected fuel. In the present stage of development, the chief claim of the semi-Diesel engine to be considered in the forefront of internal-combustion prime movers, is its marked simplicity. Development along probable lines may reasonably and in the near future reveal qualities to gain which a certain degree of simplicity may well be sacrificed. Primarily for reasons of simplicity, the great majoritymore than 90 per cent-of these engines at the present time are designed on the two-stroke cycle principle, limited to its simplest application, and are generally confined to relatively low powers per working cylinder, 125 brake

horsepower per cylinder being the maximum attained up to the present time. Fig. 2 shows cross-sections of a Beardmore two-cycle semi-Diesel compression engine and Fig. 3 of a low-compression engine, with references. Fig. 10 shows an external view of a four-cylinder engine of the same make. On the same page are external views of single-cylinder engines by Messrs. Robey and Co., Limited, of London, and by Messrs. Petters,

FIG. 2.-A. The "Beardmore" Two-Cycle Semi-Diesel Engine.

High Compression Engine.

Limited, of Yeovil. The former are made in sizes from 6 to 50 horsepower with single cylinders, and from 24 to 100 horsepower with two cylinders. Fig. 12 shows Messrs. Petters' latest type, which is made in 35 and 50 brake horsepower sizes.

Some of the earlier semi-Diesel engines used air injection of the fuel, but this type was not developed, due primarily to the disadvantage of the extra

complication of compressors and their attendant gear. All modern engines of this work with solid" or "mechanical" injection of the fuel, because of the simplicity of this system, considered especially in conjunction with the hot bulb for assisting the vaporization and ignition of the injected fuel. Air compressors are again making their appearance on semi-Diesel engines, although not for the purpose of air injection in the ordinary accepted meaning of the term. An air jet is used to cool the combustion chamber and

FIG. 3.-B. Low Compression Semi-Diesel Engine.

piston, and so to take the place of the water-drip (mentioned later), to increase the efficiency of the scavenging of the main cylinder, and so to make possible engines of relatively high powers, i. e., over 100 brake horsepower per cylinder, without having recourse to such expedients as separate scavenging pumps, elaborate cooling systems for the main pistons, and so forth.

Compression Pressure. With all internal-combustion engines, theory teaches that the higher the compression pressure the less the fuel consumption, the less the heat required from an outside source to attain to the