The present invention relates to a centrifugal clutch of disc type in accordance with the introduction to the main claim. In particular, but not exclusively, it relates to a clutch for internal combustion engine vehicles such as aircraft, watercraft, motorcycles or automobiles, but can also find application in the machine tools field, or be coupled to electric or any other type of motor. Centrifugal clutches find application in the most varied fields of engineering but are generally interposed between an engine and a user in order to be able to decouple these components when necessary. In many applications the torque transmitted by the engine to the user increases with increasing r.p.m. For example, below a certain r.p.m. the torque transmitted to the user must be zero or very low, and increase more or less proportionally with increasing r.p.m. This is particularly useful for example in starting the engine when the inertia of the user (or the resistant torque imposed by it) could be so high as not to allow the engine to start. Various types of centrifugal clutches exist; a first type presents an inner disc rotationally rigid with a driving shaft rotating within a bell housing rigid with a user shaft. Arch-shaped friction elements are hinged at one end to the inner disc, and present a return spring which maintains them in a gathered position detached from the bell. With the inner disc rotating, when the centrifugal force acting on the barycentre of the friction elements exceeds the return force of the spring, the friction elements rotate about their hinge point and project outwards form the inner disc, to rest against the rim of the bell and hence drag it into rotation. By suitably dimensioning the spring, the torque transmitted by the clutch can therefore be zero below a certain r.p.m. and increase with it. In a second type of clutch, instead of the hinged friction elements there are two or more friction elements slidable within a guide rigid with the inner disc under the opposing action of a spring. Its operation is similar to the preceding. These types of clutch present a small friction surface area compared with the size of the clutch itself. They also wear very quickly and, given the small friction area, the transmittable torque is modest. Again, the difference in peripheral speed between friction elements and bell creates considerable overheating, causing problems of rapid wear. To obviate these problems, centrifugal clutches with two or more discs have been designed. In their most elementary form, they present an inner disc torsionally rigid with a user shaft and a bell housing torsionally rigid with a driving shaft. In its interior, the bell presents, facing the inner disc, a friction disc torsionally rigid with the bell and axially movable towards the inner disc. Between the friction disc and the bell there is a perimetral wedge- shaped passage where a plurality of freely movable rollers are disposed. With the bell rotating, the centrifugal force urges the rollers into the wedge-shaped passage, the rollers by becoming wedged then urge the friction disc against the inner disc, which begins to rotate. This arrangement is very costly, given the careful machining which the bell has to undergo. In addition the movement of the rollers inside the clutch causes considerable noise and rapid roller wear. The bell also undergoes considerable mechanical stress due both to the movement of the rollers and to their considerable mass distributed over the rim of the bell. An object of the invention is therefore to provide a friction clutch of disc type which represents an improvement on the known art. This and further objects are attained by a centrifugal clutch constructed in accordance with the teachings of the accompanying claims. Further characteristics and advantages of the invention will be apparent from the description of a preferred but non-exclusive embodiment of the centrifugal clutch, illustrated by way of non-limiting example in the accompanying drawings, in which: Figure 1 is a section through a multiple disc clutch unit according to the present invention; Figure 2 is a detailed view of a driven and of a driving friction disc; Figure 3 is a front view of a disc pusher ; Figure 4 is the section 4-4 of Figure 3; Figure 5 is a perspective view of the disc pusher of Figure 3; Figure 6 is a rear view of the disc pusher of Figure 3; Figure 7 is a perspective rear view of the disc puscher of Figure 3; Figure 8 is similar to Figure 4, but shows a different embodiment of the disc pusher; and Figure 9 shows a further different embodiment of the disc pusher. Said figures show a centrifugal multi-disc clutch indicated overall by 1. Torsionally rigid with a driving shaft 2 there is a hub 3 keyed onto the shaft and resting on a step 5 via a spacer 4. The hub 3 presents a slotted profile 6 on which a series of nine driving friction discs 7, 8 are mounted, one of which can be seen in Figure 2. These (7, 8) are of conventional type and present an inner slotted profile 11 and lightening and ventilation holes 11 A. A bell 12 rotates about the shaft 2 via a bearing 13, and presents, torsionally rigid therewith, a first gearwheel 15A engaging a second gearwheel 15B torsionally rigid with a driven shaft 16. The bell 12 presents a slotted rim 13 in which a series of nine driven friction discs 14 are engaged to move axially, each lying between two driving friction discs 7, 8 and engaged in the slotted rim 13 via teeth 14a. A disc 20 is fixed, by screw means 19 engaged in holes 21 , to fixing points 30 of a support 18 which is keyed onto the driving shaft 2 in an axially fixed and torsionally rigid manner. The disc 20 is substantially a plate of hardened C72 spring steel, and presents through radial slots 22 defining segments 23. Centrifugal masses 24 are fixed by screw means 25 to adjoining pairs of said segments 23, which are provided with lightening and balancing holes 29. The centrifugal masses 24 are in the shape of an arch of toroidal section and present a protuberance 26 able to enter into contact with the driving friction disc 8 facing it and torsionally rigid therewith. The masses 24 also present a centre of gravity G in a plane γ (perpendicular to the axis of the driving shaft 2) spaced by a distance D from a fixing plane α in which the fixing points 30 of the disc 20 are located. The operation of the device of the invention is apparent from that described and illustrated. Specifically, when the driving shaft 2 is rotated, a centrifugal force is generated acting at the centre of gravity G of the centrifugal masses 24, this force causing the planes γ and α to approach each other to shorten the distance D between them, to flex the segments 23, and also at least partially the disc 20, in the direction of the force F. « Essentially the centrifugal masses 24, fixed to the segments 23 at the fixing points 30, overcome the elastic rigidity of the segments 23 (which tends to maintain the centrifugal masses 24 spaced from the driving friction disc 8), to axially press against the driving friction disc 8, and consequently against all the other driven discs 14 and driving discs 7, 8, hence activating the clutch. The greater the angular velocity of the driving shaft 2, the greater the thrust exerted on the driving friction disc 8 and hence on the pack of discs 7, 8, 14, hence the greater the torque transmitted by the clutch 1 to the driven shaft 26. Essentially, the torque transmitted by the clutch 1 is constant at a given r.p.m., as the pressing force F exerted by the centrifugal masses 24 is determined by the position of their centre of gravity G, by their mass and by the rigidity of the disc 20/segment 23 combination. Such a clutch 1 is particularly advantageous given that, by suitably designing the centrifugal masses 24 and disc 20 (and hence controlling the thickness and elastic rigidity of the segments 23), the times and manner of intervention of the clutch at a given driven shaft r.p.m. can be controlled. In alternative embodiments the masses 24 present centres of gravity in different planes. Hence, with increasing r.p.m. of the driving shaft 2, those centrifugal masses 24 the planes of which are closest to the fixing plane α act first. Likewise the centrifugal masses can be constructed of different materials, such as plastic, aluminium or iron. With increasing r.p.m. of the driving shaft 2, the iron centrifugal masses 24 will be the first to act on the disc pack, followed by the aluminium masses and then by the plastic masses. The length of the segments 23 can also be different, as shown in Figure 9, for example by varying the depth of the slats 22. In this manner they obtain different rigidity, to hence act at different r.p.m. values. A different segment rigidity can also be obtained by superposing a number of segments 23a, 23b, 23c, as shown in Figure 8.