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512 orders, codes, books, papers of any kind, even private letters, into the front line; wearing of regimental, brigade or divisional badges and markings, which reveal identifications, and many analogous ways. All these ' require special treatment. In many cases this demands merely vigilance to see that these things are not done, and that orders are constantly issued to the troops on the subject. Careless conversations on field tele- phones require " police " listening sets to record them, so that action may be taken against the offenders. Gossip about operations can be dealt with by the punishment of the offenders when caught; otherwise it is best coped with by the deliberate circulation of false rumours by Intelligence (B) through their police and agents. It is therefore one of the essential duties of the I(b) personnel that they should know what is going on amongst the men in their own army; from this it is an easy stage that they should keep the higher command informed of the moral, the grievances and the current rumours, not only of their own army, but also of the civil population and of the allied army and even amongst allied official classes. Every case of sus- pected espionage reported by the troops, even the most obvious cases 'of spy fever, must be investigated, and the results reported and circulated among the troops.

Finally, Intelligence (B) should advise the operations section of the precautions necessary in connexion with contemplated operations. This implies the closest cooperation with " O " ; but it also logically involves the allocation to Intelligence (B) of camouflage, and similar mechanical methods of preventing the acquisition of information by the enemy. This was not the case in the late war; and this omission was, in the writer's opinion, a fault in organization.

The other sub-section of the contre-cspionnage section is responsible for the control of the civil population. This in- volves a division of the area occupied by the army into zones for the purpose of the control of circulation, and control of the use of telegraph, telephone and other methods of communica- tion, which might be of use to enemy agents. These restric- tions vary in strictness according to their proximity to the fighting front, e.g. in the forward or army zone no access would be permitted to civilians of any sort; in the less forward zones civil liberties are less and less interfered with, until in the rear- most zone life may be almost normal for war-time.

The same sub-section is responsible for the drawing up and issue of all necessary regulations; the placing of the necessary port, frontier, railway, road and other controls at points of entry into the various zones. Here would take place the interroga- tions of persons entering or leaving the zone, the issue and visa of passes and the general supervision of civilian traffic. The form of all necessary passes (laissez-passers, sauf -conduits, protecting certificates, etc.) would be drawn up by the sub-sec- tion, in consultation with the provost-marshal's branch, and all arrangements made to fit in with his controls.

Card indexes must be maintained of doubtful persons, as full details as possible being given, to ensure their detention. Contre- espionnage summaries and instructions must be issued from time to time, and provision must be made for the rapid circula- tion of such information to all controls and I(b) personnel. Amongst the other duties of this sub-section are evacuations of undesirables, prostitutes and suspects; supervision, licence and withdrawal of telephonic and telegraphic facilities to civilians, in accordance with military exigencies; interrogation of rapa- tries; preparations of lists of guides, notables and persons who may be useful in territory occupied by the enemy, in case of an advance; lectures to troops on precautions against leakage of information, even after capture; and all general precautions against espionage where it is a case of dealing with the popula- tion in the aggregate, as opposed to the individual.

Both sections must keep adequate and up-to-date records, carefully cross-indexed. In the case of the offensive Secret Service it is undesirable that the names, tasks and whereabouts of agents should be known to anyone but the officer under whom they actually work. Any such records kept must be in safes or strong-boxes.

As regards the machinery employed, both sections require large numbers of intelligence police. Their allocation in the zone of the armies is usually on an army, corps or divisional basis, i.e. certain detachments of officers and men (I. P.) are allotted to the headquarters of armies, corps and divisions. On the L. of C., or even in the army zone in the case of pro- longed stationary warfare, they are best allotted on an area system, to ports, bases and areas. This has the advantage of acquainting the personnel intimately with the areas, the inhabi- tants and the special duties called for by special conditions.

One final duty remains to be mentioned the conveyance of false information to the enemy. This cannot, unfortunately, be dilated upon. It is best undertaken by the head of I(b) himself, in consultation with as few persons as possible. The process, if it is to be usefully employed, involves the complete confidence in him of the higher command and a foreknowledge of their plans and dispositions. It requires, therefore, that he should be above all else a person of solidity and discretion. Apart from other qualifications there is indeed no room for the employment in Intelligence (B) of any person in any grade who does not possess these characteristics. (R. J. D.) INTERNAL COMBUSTION ENGINES It is usual now, as a matter of terminology, to deal comprehensively with gas engines and oil engines under the general description of " internal combustion engines," and the present article gives some account of the new developments that have been made between 1910 and 1021; in the earlier volumes of this work (nth ed.) a full account will be found (under GAS ENGINE, 11.495 se 1-> an( I O IL ENGINE, 20.35 Se 9-) f the history of these very important engines up to 1910, together with a statement of the engineering thermodynamics of the subject.

Gas and oil engines in 1921 might conveniently be grouped as follows:

GROUP i. LARGE GAS ENGINES

(a) Of horizontal, slow-speed, double-acting type, of both 4- stroke and 2-stroke cycles.

(b) Of horizontal and vertical medium-speed, single-acting many- cylindered type, usually 4-stroke.

GROUP 2. MEDIUM GAS ENGINES Usually of horizontal single-acting, I- or 2-cylindered type, work- ing on the 4-stroke cycle.

GROUP 3. HEAVY OIL ENGINES

(a) Of large Diesel design, both 4-stroke and 2-stroke.

(b) Of " semi-Diesel " type, both 4-stroke and 2-stroke:

(c) Of the low-compression hot-bulb, or Akroyd type, and normal heavy oil engines with vaporizers.

GROUP 4. " LIGHT OIL " ENGINES

Small quick-revolution usually multi-cylindered engines of the 4-stroke Daimler, or 2-stroke Day type ; all single-acting, and usually vertical.

GROUP 5. SPECIAL TYPES

(a) The Humphrey Pump.

(b) The Holzwarth Turbine.

(c) The Still Engine.

Group i (a). Large Gas Engines. The period 191021 saw a considerable increase in the number of large engines of the slow- running horizontal type. Considerable difficulties were encoun- tered at first with large gas engines as the necessity of providing very complete cylinder cooling arrangements was not clearly realized, and cracked and seized pistons, failures of valves, and ruptured cylinders were not infrequent. These difficulties had by 1921 been completely overcome, and these large engines work with the utmost regularity and freedom from trouble.

The type under discussion frequently referred to as the " Nurnberg " or " M.A.N." engine, on account of the impor- tant part taken in its development by the Maschinenfabrili Augsburg-Niirnberg A.G. is illustrated in section in fig. i and i external appearance in fig. 2.

The engine shown is a 4-stroke, or " Otto " cycle, tandem, double- acting, single-crank, " blowing " engine of M.A.N. type constructed by the Lilleshall Co., Ltd., of Shropshire, England. Rated at 1,200 B.H.P. and running at 90 revolutions per minute on blast furnace gas, it is capable of compressing 26,000 cub. ft. of free air per minute to a pressure of 8 Ib. per sq. in. above atmosphere.

Within each of the working cylinders A and B is a piston F 35 in. in diameter and having a stroke of 43} in. ; the pistons are mounted upon a common piston rod which terminates towards the right in