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BACKGROUND OF THE INVENTION 
     This invention relates generally to ladder scaffolds, and in particular to a ladder scaffold with a safety mechanism for a worker on the scaffold. 
     Ladders are well known for use in building construction, painting and repair. For the repairs, maintenance or painting of exterior walls workmen usually stand on the rungs of a ladder propped against the wall. While convenient and easy to use, ladders limit the access of a workman to the work surface. If a ladder is leaning against a wall, a worker standing on the ladder has the ladder as a barrier between himself and the wall. He can reach the wall for only a short distance through or on either side of the ladder. 
     To overcome the limitation of working from ladders, scaffolds have been developed in the prior art which attach to a pair of ladders. Typically, a ladder scaffold is a flat plank attached to two ladders leaning against a wall. The plank is generally attached to an upper rung of each ladder by means of a ladder jack. See FIG.  1 . The scaffold provides an easy way for a worker to substantially expand his coverage of a work surface. 
     The drawback with using a scaffold, however, is safety. With a ladder, the worker is actually laying against the rungs of a ladder and usually is holding on to one of the ladder rails or rungs with his nonworking hand. While on a scaffold, a worker has nothing to hold onto and relies completely on balance. One mistake or a slight backward misstep from the scaffold plank at a usual working level above the ground can be catastrophic for the worker. This is an especially serious problem for workers who are using high pressure hoses to wash down or sand blast a wall surface, the pressure from the hose tending to push the worker backward and making the worker&#39;s position on the scaffold quite unstable. 
     The prior art has made various modifications to ladder scaffolds to improve safety. These modifications, however, generally limit the worker&#39;s access to a work surface or limit his mobility on the scaffold. 
     SUMMARY OF THE INVENTION 
     The present invention&#39;s general purpose is to overcome the difficulties with prior art safety modifications to ladder scaffolds. The present invention provides a worker back support which is laterally and slidably attached to the scaffold. The back support has a belt which is releasably attached about the waist of the worker. The worker can thereby move laterally along the scaffold with his hands free and no obstructions between him and the wall work surface. The belted back support provides the worker with backward support and a safe attachment to the scaffold. In one embodiment of the invention, a high pressure hose used for washing or sand blasting a wall surface may be attached to and braced by the back support, thereby reducing the working pressure on a worker. 
    
    
     These together with other objects of the invention, along with various features of novelty which characterize the invention, are pointed out with particularity in the claims annexed hereto and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and the specific objects attained by its uses, reference should be had to the accompanying drawings and descriptive matter in which there is illustrated a preferred embodiment of the invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a prior art ladder scaffold attached to a pair of ladders by means of typical ladder jacks. 
     FIG. 2 is rear elevational view of the invention attached to two ladders by means of ladder jacks. 
     FIG. 3 is a bottom elevational view of a plank used in the present invention. 
     FIG. 4 is a bottom perspective view of a plank assembly used in the present invention. 
     FIG. 5 is a close up view of the plank assembly of FIG. 2 with a back support. 
     FIG. 6 is a view of the bottom portion of the back support of FIG.  5 . 
     FIG. 7 is a view along the line  7 — 7  of FIG.  5 . 
     FIG. 8 is a view along the line  8 — 8  of FIG.  5 . 
     FIG. 9 is a top view of the back support of FIG.  2 . 
     FIG. 10 is a close up view of the plank assembly of FIG. 2 with a back support modified with a clamping means. 
     FIG. 11A is a view of the bottom portion of the back support modified with a braking means engaged. 
     FIG. 11B is a view of the bottom portion of the back support modified with a braking means disengaged. 
     FIG. 12 is a bottom perspective view of a plank assembly used in another embodiment of the invention. 
     FIG. 13 is a cross-sectional view of the plank shown in FIG.  12 . 
     FIG. 14 is a front side view of the plank shown in FIG.  12 . 
     FIG. 15 is a view along the line  15 — 15  of FIG.  13 . 
     FIG. 16 is a cross-sectional view of the invention embodiment of FIG. 12 with lower engagement portion. 
     FIG. 17 is a cross-sectional view of the invention embodiment of FIG. 12 with the back support. 
     FIG. 18 is a top view of the second invention embodiment. 
     FIG. 19 is a side view of the second invention embodiment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings in detail wherein like elements are indicated by like numerals, there is shown in FIG. 1 a prior art scaffold  10  attached to a pair of ladders  15  by means of typical ladder jacks  1 . The scaffold  10  is comprised of scaffold platform comprised of a flat plank  11  having opposing first  12  and second  13  lateral end portions attached to individual ladder jacks  14  for supporting the platform in an elevated horizontal position intermediate two spaced apart ladders  15 . The ladder jacks  14  are each removably attached to two adjacent rungs  16  of each ladder  15 . 
     Referring to FIGS. 2-9 there is shown one embodiment of the present invention. The present invention is comprised of a plank assembly  20  with a back support  40  attached thereto, said plank assembly  20  being attached to individual ladder jacks  14  for supporting the plank assembly  20  in an elevated horizontal position intermediate two spaced apart ladders  15  leaning against a generally flat wall surface  2 . The ladder jacks  14  are each removably attached to two adjacent rungs  16  of each ladder  15 . 
     A basic ingredient of the plank assembly  20  is a standard rectangular plank  21 . See FIG.  3 . The plank  21  has an upper surface  22  and a lower surface  23 , said surfaces laying in planes positioned horizontally with respect to the ground  1 . The plank  21  has a first long edge  24  and a second long edge  25 , said first long edge  24  being nearest to the wall surface  2 . The plank  21  also has a first lateral end  26  opposite a second lateral end  27 , said lateral ends defining the longitudinal axis of the plank  21 , said plank longitudinal axis lying horizontally parallel to said wall surface  2 . An elongated, rectangular box  30  is fixedly attached to the plank lower surface  23 , along and adjacent to the plank second long edge  25 . See FIG.  4 . 
     Referring more particularly to FIGS. 4,  5 ,  7  and  8 , where FIG. 7 is a sectional view along the line  7 — 7  of FIG.  5  and FIG. 8 is a sectional view along the line  8 — 8  of FIG. 5, there is shown an elongated, rectangular box  30  having a top side  31  attached to the plank lower surface  23 , an opposite bottom side  32 , a first open side  33 , an opposite second side  34 , said second side  34  having an elongated, horizontal aperture  35  formed therein, a first end  36  and an opposite second end  37 , said ends defining the longitudinal axis of the box  30 , said box longitudinal axis being parallel to the plank longitudinal axis. The box sides  31 ,  32 ,  33 ,  34 , and ends  36 ,  37  define a box interior  38 . The length of the box  30  is less than the length of the plank  21 . The box  30  is longitudinally centered along the plank  21  longitudinal axis. 
     Referring more particularly to FIGS. 5-8, the invention is further comprised of a back support  40 . The back support  40  is comprised of a lower engagement portion  41  and an upper portion  50 . The lower engagement portion  41  is comprised of a set of wheels  42  rotatably attached to an elongated, horizontal cross bar  43 . The cross bar  43  has a horizontal, longitudinal axis parallel to the radial axis of the wheels  42 . A vertical element  44  is attached to the cross bar  43  by means of a horizontal offset piece  45 . The horizontal offset piece  45  has a horizontal, longitudinal axis perpendicular to the cross bar longitudinal axis. The cross bar  43  and wheels  42  are enclosed within the box interior  38 . The offset piece  45  protrudes through the box second side aperture  35  and joins the vertical element bottom  46 . The vertical element  44  contains a plurality of spring pin buttons  47  in a longitudinal row along its surface  48 . In an alternate embodiment of the invention, a reinforcing cross bar  49 , external to the box, may interconnect with the horizontal cross bar  43  to provide reinforcement and additional stability to the back support  40 , said reinforcing cross bar  49  being a parallel, mirror image of the horizontal cross bar  43 . 
     The back support upper portion  50  is comprised of an elongated sleeve element  51  having a bottom  52 , an upper end  53 , a surface  54  and a plurality of apertures  55  in a longitudinal row along its surface  54 , said sleeve element  51  adapted to fit over said lower engagement portion vertical element  44 , sleeve element bottom  52  first, said upper portion apertures  55  adapted to engage one or more lower engagement portion vertical element spring pin buttons  47 . 
     A curved back rest  60  is attached to the upper portion sleeve element upper end  53 . The back rest  60  has a top  61 , a bottom  62 , two lateral side ends  63 ,  64 , a first surface side  65  and a second surface side  66 , said first surface side  65  facing the wall surface  2 . A horizontal strap assembly  70  is also provided, said strap assembly  70  adapted to laterally engage the waist of a worker. The horizontal strap assembly  70  is comprised of two attachment elements  71  affixed to the back rest lateral sides  63 ,  64 , and an adjustable horizontal strap  72  with two ends  73 ,  74  each of which is removably attached to an attachment element  71 . The horizontal strap  72  may be separated into two sections  75 ,  80 . The first section  75  has a removable attachment end  73  and an opposite end  76  attached centrally to a conventional buckle  77 , said buckle  77  fitted over the first strap section  75  forming an adjustable loop  78 . Sliding the buckle  77  along the first strap section  75  makes the loop  78  smaller or larger thereby lengthening or shortening the effective length of the first strap section  75 . The loop  78  is adapted to engage a fastener  79 . The second strap section  80  has an attachment end  74  and an opposite end  81  terminating in a fastener  82 , said fastener  82  adapted to engage said first section fastener  79 . The fasteners  79 ,  82  may be hook and pile fasteners, sold under the Velcro trademark, or other conventional fastening means. 
     Referring more particularly to FIG. 10, the upper portion sleeve element surface  54  may also have a supplemental strap or clamping means  56  attached thereto, said clamping means  56  adapted to engage hose assemblies such as those of a high pressure washer or sand blaster. 
     Referring more particularly to FIGS. 11A and 11B, the back support, lower engagement portion  41  may have a simple brake assembly  90  in order to lock the wheels  42  laterally in place along the box bottom  32 . The brake assembly  90  is comprised of an elongated arm  91  pivotally connected by a pivot pin  93  to the horizontal cross bar  43 . The arm  91  is connected to either the lower engagement portion vertical element  44  or upper portion sleeve element  51  by means of a tension spring  92  holding the arm against the vertical element  44  or sleeve element  51 . When the arm  91  is at rest against the vertical element  44  or sleeve element  51 , two braking elements  94 ,  95  pivotally attached to the arm  91  at the pivot pin  93  brakingly engage the wheels  42 . When the arm  91  is pushed away from the vertical element  44  or sleeve element  51 , the braking elements  94 ,  95  are disengaged from the wheels  42  and the wheels  42  are free to rotate. 
     Referring to FIGS. 12-19, there is shown another embodiment of the invention. This embodiment of the invention is comprised of a metallic plank assembly  120  with a back support  140  attached thereto, said plank assembly  120  being attached to individual ladder jacks  14  for supporting the plank assembly  120  in an elevated horizontal position intermediate two spaced apart ladders  15  leaning against a generally flat wall surface  2 . The ladder jacks  14  are each removably attached to two adjacent rungs  16  of each ladder  15 . 
     Because this embodiment of the invention is metallic, preferably aluminum for lightness, rectangular plank  21  of the first embodiment must be structurally strengthened to handle vertical loads. The resulting plank  121  has an upper surface  122  and a lower surface  123 , said surfaces laying in planes positioned horizontally with respect to the ground  1 . The plank  121  has a first long edge  124  and a second long edge  125 , said first long edge  124  being nearest to the wall surface  2 . The plank  121  also has a first lateral end  126  opposite a second lateral end  127 , said lateral ends defining the longitudinal axis of the plank  121 , said plank longitudinal axis lying horizontally parallel to said wall surface  2 . The first long edge  124  has a downwardly extending first flange  128  terminates in an inward second flange  129  extending toward the second long edge  125  and having a plane parallel to the plank lower surface  123 . The second flange  129  extends approximately half way beneath said plank  121 . The second long edge  125  has a downwardly extending first flange  130  terminating in an outward second flange  131  extending away from the plank at an approximate 90° angle with the first flange  130 , said second flange  131  having a plane parallel with the plane of the plank lower surface  123 . The second flange  131  terminates in a third flange  132  extending upwardly away from the second flange at an approximate 90° angle, said third flange  132  having a plane parallel with the plane of the first flange  130 . The three flanges  130 ,  131 ,  132  form a rectangular channel  135  with the first and third flanges  130 ,  132  forming vertical sides, the second flange  131  forming a horizontal bottom, and the channel  135  having an open top. The second long edge first flange  130  has an elongated, horizontal, central aperture  133  formed therein, said aperture  133  having a longitudinal axis parallel to the longitudinal axis of the plank  121 . The second long edge third flange  132  also has an elongated, horizontal, central aperture  134  formed therein, said aperture  134  having a longitudinal axis parallel to the longitudinal axis of the plank  121 , said apertures  133 ,  134  being parallel to each other. The flange arrangement extending from each long edge  124 ,  125  provides longitudinal, horizontal channel reinforcement of the plank thereby strengthening the metallic plank&#39;s ability to bear vertical loads. 
     Referring more particularly to FIGS. 16-19, the invention is further comprised of a back support  140 . The back support  140  is comprised of a block-like lower engagement portion  141  and an upper portion  150 . The lower engagement portion  141  is comprised of a rectangular box  142 , having a top surface  146 , bottom surface  143 , two interconnecting, opposing parallel sides  144 , and two opposing, parallel ends  145 , said ends  145  defining the longitudinal axis of the said box  142 , said box longitudinal axis being parallel to the longitudinal axis of the plank  121 . The box  142  is adapted to fit into the channel  135 , said box sides  144  being positioned adjacent the first and third flanges  130 ,  132 , said box bottom surface  143  being adjacent the second flange  131 . The lower engagement portion  141  is further comprised of two pins  135  extending through each of the box sides  144 , said side pins  147  protruding from the sides  144  and engaging the second long edge first and third flange apertures  133 ,  134 . 
     The back support upper portion  150  is comprised of a vertical post extending upwardly from the lower engagement portion box top surface  146 . A curved waist grip  152  is attached to a sleeve  153  slidably affixed to said post  151 , said sleeve  153  adapted to being locked into position by a locking pin  154  engaging one of a plurality of pin apertures  155  formed in said post  151 . 
     Referring more particularly to FIGS. 18 and 19, the upper portion  150  may have a simple brake assembly  190  comprised of an elongated arm  191  pivotally attached to the post  151 . The elongated arm  191  has a bottom terminating in a 90° spike  192  adapted for engagement with one of a plurality of holes  193  formed in the second long edge third flange  132  beneath the elongated aperture  134 . 
     It is understood that the above-described embodiment is merely illustrative of the application. Other embodiments may be readily devised by those skilled in the art which will embody the principles of the invention and fall within the spirit and scope thereof.

Summary:
A worker back support which is laterally and slidably attached to a scaffold plank. The back support has a grip which is releasably attached about the waist of the worker. The back support provides the worker with backward support and a safe attachment to the scaffold. A high pressure hose used for washing or sand blasting a wall surface may be attached to and braced by the back support, thereby reducing the working pressure on a worker.