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mentioned, one of which is movable, the other fixed to the body. The rings are pressed together by the cap of the fuze, which is screwed down tightly enough to prevent the movable ring from being shifted otherwise than by a spanner called a " fuze key," so as to ensure regular adjustment of the ring and to prevent it from slipping after being set. Many factors govern the rate of burning of the time rings. Primarily there is the pressure prevailing at the burning surface; this again depends on the pressure produced by the burning com- position at the escape outlets, and this again depends largely on the speed of revolution and of translation, the position of the outlets with regard to the body of the fuze, the shape of the fuze, the height to which the shell is fired, the barometric pressure, and the nature of the gun from which the shell is fired.

Time and percussion fuzes are used normally with shrapnel, but rarely with H.E. shell.

Lighting Pellet

Stirrup Spring

Percussion

Pellet

Stirrup Spring

Double ended Needle

Creep Spring

.."-Magazine

FIG. 10.

In the Time and Percussion Fuze No. 80 the upper time ring is fixed to the body by pegs, but the lower ring can be rotated for set- ting (fig. 10). Immediately below the cap is the mechanism for igniting the composition. This consists of a lighting pellet fitted with a detonator and supported by a stirrup spring which keeps it away from the needle below it.

On shock of discharge the pellet sets back, straightening out the arms of the stirrup spring, and the detonator comes in contact with, and is fired by, the needle. The flash from the detonator passes through a slanting channel and ignites the composition in the upper ring. The flame then travels in the direction in which the shell is rotating until, after an interval of time determined by the setting, it reaches a passage communicating with the lower time ring. Here there is a compressed powder pellet to ensure more certain ignition. The composition in this second ring, being thus ignited, burns in the reverse direction until, at the time determined by the setting, it reaches the compressed powder pellets in a passage leading to the magazine, the flash from which, passing down a tube in the shell, fires the opening charge.

The percussion pellet is held away from the lower point of the needle by a ferrule supported by a stirrup spring, a creep-spring preventing any forward movement during flight. On graze or impact the percussion pellet flies forward, and the detonator, impinging on the point of the needle, fires, and its flash passing through the pellet fires the magazine.

Gas Escape

Lighting Pellet

Time Rings;.'

Pivot of Needle block-

Percussion Pellet--

Centrifueral-"' Bolts

Split -ring

Spring "Plunger

h-rl Needle block

-- -Magazine

FlG. II.

In general arrangement Time and Percussion Fuze No. 85* greatly resembles No. 80. There are, however, some important differences (fig. n).

The lighting pellet is supported by a ring sprung into a groove in the pellet and resting on the rim of a recess in the stem of the body. On the shock of discharge the pellet escapes from the ring and sets back on to the needle which fires it, the flash igniting the time ring as in Fuze 80. Another interesting characteristic of this fuze is that the

1 This fuze, worked out by J. D. Gushing, was the American army fuze before the World War, and was used in large quantities for British field guns to supplement the available stocks of British fuzes.

gases, instead of escaping directly through holes in the rings, pass into a series of channels before finally escaping through a circumferential groove in the cap. With this modification the rate of burning of the time ring is less affected by variations in barometric pressure and the other disturbing factors above mentioned. The main difference between these two fuzes is, however, in the percussion arrangefnent. This consists of a pivoted needle pellet or block and a detonator supported in the upper part of the recess. The block, which is pivoted, is kept in position by centrifugal bolts. These in due course free the block, which swings round on its pivot, bringing the needle opposite the detonator. Spring -plungers prevent the pellet from creeping forward in flight. On graze or impact it flies forward, overcoming the spring plungers, and fires the detonator. The flash from this passes down a passage in the body (dotted lines) and fires the magazine.

In T. and P. No. 83, a variant designed specially for medium guns and howitzer, a special safety device is provided in the shape of a ball, which is trapped between the ferrule and the detonator pellet and the striker until the ferrule sets back, when the ball acts as a temporary safety between pellet and striker. When sufficient rotation has been set up, the ball flies up into a side channel, and the pellet is now only held back by a creep-spring which on impact is overcome.

Time fuzes without percussion elements are now of no special interest except in connexion with anti-aircraft fire. It is essential to render all shell fired at aircraft that might fail to burst in the air as harmless as possible on impact with the ground, owing to the danger to friendly troops and the populations of towns and villages, and buildings. Another use of time fuzes without percussion mechanism is with trench-mortar shell, which do not pitch nose first. In these and a few other special cases the ordinary stirrup spring is too stiff to arm, and a weaker spring is fitted with, as its corollary, a safety- pin for securitv in transport. All such time fuzes are simply T. and P. Fuzes of the various service patterns converted by the removal of the P. mechanism.

In general the time fuzes designed before and used during the World War were constructed to run for 22 seconds' time of night. In fuzes for certain longer-ranging ordnance, however (e.g. No. 83), 30 and 45 seconds' run was allowed for, and the latest representative of this class, No. 89, which has three time rings instead of two, runs for 60 seconds. Amongst all these No. 80 was by far the most gener- ally used, about 70,000,000 of this type having been manufactured during the war, as well as 18,000,000 of the American No. 85 and 8,000,000 of Nos. 83, 88 and 89. As an instance of how the industry of the country adapted itself to war needs, it is worth mention that the average pre-war output of No. 80 was 55,000 a year, and the output of one firm alone in 1918 250,000 a week.

Last, and for the future most important in the time-fuze category, comes the mechanical or clockwork fuze. This almost eliminates the variations of burst due to atmospheric conditions and to the quality and freshness of the composition used. It had long been sought for by inventors, but until 1916 no type had been produced which gave satisfactory results in practice owing to the great difficulties in designing a mechanism that would run under the conditions of shock and of rotation that a fuze must cope with.

In 1916, however, the Germans brought into use a clockwork fuze, (" Dopp. Z. 16") which proved very successful and has been copied, with little variation, by the British authorities, its British designa- tion being Fuze, Time, No. 200. The German fuze has a percussion arrangement in addition to the clockwork; this is omitted in the British model, which is a time fuze only.

Calth Gipi in ttinf

FIG. 12.

The description which follows must be understood as a description of principle and arrangement only, a technical demonstration being impossible without the actual fuze. It must be premised further that the clockwork element is very small in size, being in diameter about equal to an ordinary wrist watch and in thickness to two such watches.

Fuze, Time, Mechanical, No. 200. The clock train is driven, as in a watch, by a coiled spring in a barrel, but the escapement is original and peculiar. A straight steel spring takes the place of the hair-spring,