fig1 shows one of the possible constructions of the first embodiment of the invention , i . e . the embodiment wherein the general form of the backing strip 11 is convex towards the surface to be cleaned . in the center portion of the wiper blade , said backing strip 11 is positioned beneath the horizontal line 12 when the wiper blade is free , i . e . not in contact with the surface to be cleaned . the wiper blade of fig1 essentially comprises a main bridge 1 , a secondary yokes 2 , two backing strip 11 and a resilient wiping element or squeegee 4 . the secondary yokes 2 are provided with interior claws 5 and with exterior claws 6 which grip the backing strip 11 , said backing strip 11 being positioned in longitudinal grooves ( not shown ) provided in the resilient squeegee 4 . the backing strip is separated by a slot 3 , but it is secured , one to the other , at their two extremities 10 as well as at the sections r1 and r2 where there is an angular change of direction of the backing strip . at these same sections r1 and r2 , the interior claws 5 grip said backing strip 11 and consequently link them , as well as the squeegee 4 , to the superstructure of the wiper blade . the webs or bridges 9 , which secure one side to the other of the backing strip 11 at the sections r1 and r2 , diminish the flexibility of the strip 11 at said two sections r1 and r2 . it is , however , quite evident that this diminished flexibility may be obtained by other means , such as for example by a larger width or thickness of the backing strips 11 or by an appropriate transverse deformation of said strip 11 . furthermore , the flexibility of the backing strip 11 may regularly increase from the sections r1 and r2 towards the respective extremities 10 of the wiper blade . the pressure f of the wiper arm ( not shown ) is transmitted to the backing strip 11 by the following parts : the pivot pin 7 , the main bridge 1 , the pivots 8 , the secondary yokes 2 and the claws 5 and 6 . when the wiper blade is placed on a flat or a curved surface , the secondary yokes 2 rotate because of the reacting forces r1 and r2 and the backing strip 11 , and consequently the wiping element or squeegee 4 , adapt themselves to the form of the surface to be cleaned . because of the angular change of direction of the backing strip 11 at the sections r1 and r2 and because of the fact that at these same sections said backing strip 11 has a lower flexibility , the transfer of the pressure f of the wiper arm is more important in the central portion of the wiper blade as this would be in a wiper blade provided with a conventional backing strip . it is , however , evident that a uniformly distributed pressure all along the wiping element or squeegee 4 can only be obtained by judiciously choosing not only the form and the flexibility of the backing strips 11 , but also the dimensions of the main bridge 1 and of the secondary yokes 2 . the wiper blade shown in fig1 is only one of the possibilities for using the principle of the invention in which the general form of the backing strip 11 is convex towards the surface to be cleaned . without going into details , one may cite the following examples : a wiper blade comprising more than two secondary yokes or also comprising intermediate bridges and / or intermediate yokes . in this case there will be as many angular changes of direction as there are interior claws ; a wiper blade comprising one single and unique bridge which transmits the pressure of the wiper blade directly to the two extremities of the wiper blade and wherein the secondary yokes are , for example , replaced by leaf springs comprising one or two limbs . fig2 shows one of the possible constructions of the second embodiment of the invention , i . e . wherein the general form of the backing strip 31 is concave towards the surface to be cleaned . in the center portion of this wiper blade , said backing strip is thus positioned above the horizontal line 32 when the wiper blade is free , i . e . when it is not in contact with the surface to be cleaned . the backing strip 31 is exactly similar to the backing strip 11 of fig1 and all what has been said on the backing strip 11 is also true for the backing strip 31 , the only difference being that in fig1 the backing strip is mounted onto the superstructure of the wiper blade in such a way that they are convex towards the surface to be cleaned , whilst in fig2 it is mounted on the superstructure of the wiper blade so as to be convex towards said superstructure of the wiper blade . the superstructure of the wiper blade of fig2 comprises a bridge 21 in two parts , rotatable around the pivot pin 27 , and a central yoke 22 in one piece , also rotatable around the pivot pin 27 . in this embodiment , the pressure p of the wiper arm ( not shown ) is transmitted to the backing strip 31 through the central yoke 22 and the claws 25 . the backing strip 31 is separated by slots 23 extending from the two extremities 30 to the portions 29 of lower flexibility . a slot 23 also extends between the two portions 29 . under the pressure of the forces p1 and p2 thus transmitted to the backing strips 31 , said strips , and consequently the resilient squeegee 24 , adapt themselves to the surface to be cleaned , said surface being plane or curved . it is to be noted that contrary to some known wiper blades , the wiper blade of fig2 is not provided with a coil spring surrounding the pivot pin 27 , which spring in these types of wiper blades urges downwards the claws 26 on the remote ends of the two parts of the bridge 21 . however , the invention may also be adapted to wiper blades which are provided with such a coil spring . in the embodiment shown in fig2 the conformation of the backing strip 31 to the configuration of the surface to be cleaned is due to the action of the forces p1 and p2 and thanks to the special form as well as , above all , to the portions of lower flexibility 29 when transmit an important part of the forces p1 and p2 towards the two extremities 30 of the wiper blade . however , just as in the embodiment of fig1 the actual distribution of the pressure all along the squeegee 24 depends on the form and the dimensions not only of the central yoke 22 , but also on the form and the variable flexibility of the backing strip 31 . just as in the first embodiment , the wiper blade shown in fig2 is only one of the possibilities to use the principle of the invention wherein the general form of the backing strip 31 is concave towards the surface to be cleaned . the following examples may be cited : a wiper blade comprising not only a single central yoke , but an intermediate yoke and a secondary yoke , the intermediate yoke being linked on one of its extremities to the backing strip and on the other of its extremities to the secondary yoke . in this embodiment there will be three angular changes of direction of the backing strip and consequently also three portions of lower flexibility ; a wiper blade comprising a superstructure which is reduced to a connecting device , for the wiper arm , rigidly secured to the backing strip . in this embodiment there will be only one change of the direction of the backing strip and only one portion of lower flexibility . the change of direction and the portion of lower flexibility will be positioned at the section corresponding to said connecting device . fig3 to 5 show the third embodiment of the invention wherein the general form of the backing strip 51 substantially corresponds to the form of an inverted and flattened w with a bottom angle 49 and two top angles 48 ( fig3 ). the backing strip 51 is linked together at their two extremities 50 and at their center portion 49 , i . e . at the section where there is an angular change of direction ( fig4 ). just as in the two preceding embodiments , the backing strip 51 is separated by slots 43 and the center portion or link 49 between the backing strip corresponds to a portion having a lower flexibility which may also be obtained by other means such as disclosed with respect to the embodiment corresponding to fig1 . lastly , the width of the backing strip 51 increases from their two extremities 50 towards their center portion 49 , the effect of this being a still lower flexibility near the center of the backing strip . fig5 shows the complete wiper blade according to the third embodiment of the invention . the pressure b of the wiper arm ( not shown ) is transmitted to the backing strip 51 , and consequently to the resilient squeegee 44 , by the pivot pin 47 , the sole bridge 41 and the claws 45 of said bridge . as already said above , the two claws 45 of the sole bridge 41 grip the backing strip at the two upper angles 48 . when the wiper blade of this embodiment is placed on a flat surface , the first contacts of the squeegee 44 with the surface to be cleaned will be at the sections s1 , s2 and s3 . under the pressure b of the wiper arm , which is transmitted to the backing strip 51 through the claws 45 , said backing strip 51 flatten and the resilient squeegee 44 is uniformly pressed against said plane surface . when the same wiper blade is placed on a curved surface , the first contact will be at the section s2 and because of the reacting force at s2 , the two portions of the backing strip 51 , on both sides of section s2 , will slightly rotate around the two upper angles 48 corresponding to the claws 45 of the sole and unique bridge 41 . consequently the two extremities 50 of said backing strip 51 will move downwards with respect to the bridge 41 and they will be pressed against the curved surface to be cleaned . thus , a wiper blade has been provided which is very economical and of a very small height and which independently of the advantages resulting therefrom , also provides a uniformly distributed pressure all along a plane or a curved surface to be cleaned . it will be understood that the invention is not limited to the exact constructions shown and described , but that various changes and / or modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims .