You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
     In order to load and unload manifest items from a boat it is customary to provide a gangplank for passengers and small cargo items. Changing water levels give rise to the need to provide adjustable loading and unloading ramps. Ramps are not only necessary for ocean going boats but are also needed for loading and unloading manifest items of barges. Changes in river water levels present problems similar to changing tides. Although a flat ramp may be satisfactory for loading and unloading operations if the slope of the ramp is not steep, the changing tides and river levels give rise to a need for a safer conveyance surface with steps being generally preferred. In providing a pier mounted access ramp to a barge there is a need to maintain the same orientation of the barge boarding platform regardless of the slewed condition of the ramp. In other words it is desirable for the exit from the barge boarding platform to face the same direction relative to the barge regardless of the slewed position of the ramp. 
     BRIEF SUMMARY OF THE INVENTION 
     The boat access stairway of this invention provides a versatile conveyance for transferring the manifest items to and from a boat such as a barge. A landing is provided for attachment to a pier which includes a first part fixed to the pier and a second part pivoted to the first part on a vertical axis. A ramp is pivotally connected to the second part of the landing on a horizontal pivot axis. These two pivot connections permit raising and lowering of the ramp and slewing of the ramp. The boat access stairway includes power apparatus for slewing the ramp and for raising and lowering the ramp. Additionally, the boat access stairway includes a platform at the free end of the ramp which can be set down on a boat. The platform has a first segment pivotally connected to the outer end of the ramp on a horizontal axis and a second segment pivotally connected to the first segment on a vertical axis. The second segment includes a floor. Two laterally spaced upright spars are pivotally supported on the stationary part of the landing on vertical axes and two laterally spaced upright spars are pivotally supported on the second segment of the platform. Upper and lower links are pivotally connected at their corresponding opposite ends to the spars on the landing and the platform and when the ramp is slewed the platform retains the same general orientation relative to the boat. The access stairway may be slewed to compensate for the distance between the pier and the boat, but in doing so the exit from the platform onto the boat maintains it direction orientation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       One embodiment of the invention is shown in the drawings, in which: 
         FIG. 1A  is a top view of the boat access stairway with certain parts removed for illustration purposes; 
         FIG. 1B  is a top view of the boat access stairway similar to  FIG. 1  but with the stairway slewed; 
         FIG. 2  is a side view of the boat access stairway adjusted for access to a boat deck substantially lower than the pier; 
         FIG. 3  is a side view of the boat access stairway with parts removed for illustration purposes and showing its boat access position when the boat deck is about the same elevation as the pier; 
         FIG. 4  shows the boat access stairway in a raised position; 
         FIG. 5  is a section taken on the line V—V in  FIG. 3 ; 
         FIG. 6  is a section taken on the line VI—VI in  FIG. 5 ; 
         FIG. 7  is a section taken on the line VII—VII in  FIG. 1A , 
         FIG. 8  is a section taken on the line VIII—VIII in  FIG. 2 , and 
         FIG. 9  is a side view of the boat access stairway with parts removed for illustration purposes. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The conveyance illustrated in the drawings is a boat access stairway  11  designed for attachment to a pier  14 .  FIG. 1A  is a top view of the boat access stairway  11  having a landing  12  which has a first part  13  rigidly secured to the pier  14 . The landing  12  also includes a second part  16  pivotally connected to the first part on a vertical pivot axis  17 . The stationary first part  13  and the pivotable second part  16  of the landing  12  have floors  18 ,  19  on the same level with confronting complimentary semi-circular edges. 
     Referring also to  FIGS. 5 and 6 , the first part  13  of the landing  12  includes a sleeve bearing  21  supported by struts  22 ,  23  and the second part  16  of the landing includes a vertical pivot tube  24  pivotally supported in the sleeve bearing  21  for pivotal movement about the vertical axis  17 . Pivotal movement of the second part  16  of landing relative to its first part  13  is effected by extension and contraction of a linear fluid power actuator  26  having of a cylinder  27  pivotally connected to a cylindrical support member  28  on the first part  13  and a piston having a rod  29  pivotally connected to a lever arm  31  nonrotatably fastened to the pivot tube  24 . 
     As shown in  FIGS. 1A and 1B  the boat access stairway  11  includes an elongated ramp  36  having one of its longitudinal ends pivotally connected to the second part  16  of the landing  12  on a horizontal pivot axis  37  which is transverse to the direction of elongation of the ramp  36 . The other longitudinal end of the ramp  36  is pivotally connected to a first segment  41  of a platform  42  on a second horizontal pivot axis  43  which is parallel to the axis  37 . The ramp  36  is a stairway with self leveling steps  46  pivotally supported on and between a pair of parallel stringers  47 ,  48 . Corresponding ends of the stringers  47 ,  48  are pivotally connected on the axis  37  to the second part  16  of the landing  12  and on the axis  43  to the first segment  41  of the platform  42 . 
     As shown in  FIGS. 3 ,  8  and  9 , the second part  16  of the landing  12  includes a pair of laterally spaced vertically extending support plates  51 ,  52  to which the stair stringers  47 ,  48  are pivotally connected, respectively. A pair of laterally spaced vertical stand posts  53 ,  54  are rigidly secured at their lower ends to the second part  16  of the landing  12 . As shown in  FIGS. 2 ,  3 ,  4 ,  7  and  9  a pair of laterally spaced vertically extending rigid posts  56 , 57  are rigidly secured to the first segment  41  of the platform  42 . A pair of laterally spaced parallel rails  58 ,  59  have corresponding opposite ends pivotally connected to the upper ends of stand posts  53 ,  54  and the rigid posts  56 ,  57  on parallel horizontal axes  61 ,  62 . Axes  37 ,  43 ,  61 ,  62  define corners of a parallelogram. Intermediate vertical posts  63 ,  64  are rigidly connected at their lower ends to a longitudinally spaced pair of steps  46  and are pivotally connected at their upper ends to the rail  58 . Intermediate vertical posts  66 ,  67  are rigidly connected at their lower ends to the same longitudinally spaced pair of steps  46  and are pivotally connected at their upper ends to the rail  59 . As shown in  FIG. 9 , stair leveling links  71 ,  72 ,  73  serve to pivot the steps  46  to maintain their treads horizontal when the ramp  36  is pivoted up or down relative to the pier  14 . 
     The platform  42  includes a second segment  76  pivotally connected to the first segment  41  on a vertical pivot axis  81 . As shown in  FIG. 7  the second segment  76  includes a floor  77  and a pivot sleeve  78  pivotally supporting a tube component  79  of the first segment  41  inserted in and in cylindrical surface bearing relation with the interior of the sleeve  78 . Thus the first and second segments  41 ,  76  of the platform  42  are connected for relative pivotal movement about the vertical pivot axis  81 . The second segment  76  includes a pair of laterally spaced upstanding structures  86 ,  87  and a safety railing  89  around the floor  77  except for the arc cut out portion of the floor mating with a semi circular part of the floor  41  and except for a gateway  88 . An inward opening gate  91  is provided at the gateway  88 . 
     Referring to  FIG. 8 , the landing  12  includes a pair of laterally spaced vertical walls  92 ,  93  to which the upper and lower ends of a pair of laterally spaced parallel vertical spars  94 ,  96  are pivotally connected on parallel vertical axes  101 ,  102 . In a similar manner a pair of laterally spaced parallel vertical spars  103 ,  104  are pivotally connected at their upper and lower ends to the upstanding structures  86 ,  87  of the second segment  76  of the platform  42  for pivotal movement about a pair of vertical axes  106 ,  107 . As shown in  FIG. 1A  the vertical axes  101 ,  102  and  17  lie in a common vertical plane  111  and the axes  106 ,  107 ,  81  lie in a common vertical plane  112 . The pivot axes  101 ,  102 ,  106 ,  107  define the corners of a parallelogram. Axes  101  and  102  are equal distances from axis  17  and axes  106 ,  107  are equal distances from axis  81 . This configuration permits slewing of the stairway  36  as illustrated in  FIG. 1B . 
     A parallel lift linkage arrangement is provided for raising and lowering the gateway  42  and for maintaining the orientation of the platform  42  when the stairway  36  is slewed. The lift linkage includes a pair of laterally spaced parallel upper links  116 ,  117  having first corresponding ends pivotally connected to the upper ends of the first pair of upright spars  94 ,  96  on parallel horizontal axes  118 ,  119 , respectively, and having second corresponding ends pivotally connected to the upper ends of the second upright spars  103 ,  104  on parallel horizontal axes  121 ,  122 , respectively. The lift linkage also includes a pair of laterally spaced parallel lower links  126 ,  127  having first corresponding ends pivotally connected to the lower ends of the first upright spars  94 ,  96  on horizontal axes  128 ,  129 , respectively, and second corresponding ends pivotally connected on parallel horizontal axes  131 ,  132  to the lower ends of the second pair of upright spars  103 ,  104 , respectively. The axes  118 ,  121 ,  128 ,  131  define the corners of a parallelogram and the axes  119 ,  122 ,  129 ,  132  define the corners of a parallelogram. The links  116 ,  117 ,  126 ,  127  include braces  151 ,  152 ,  153 ,  154  along their laterally outer sides. 
     The stairway  36  and platform  42  are raised and lowered by a pair of linear actuators in the form of fluid rams  136 ,  137  interconnected between the first pair of upright spars  94 ,  94  and the lower links  126 ,  127 . 
     The herein described and illustrated boat access stairway can be secured to a pier to afford access to a boat or barge  144  adjacent the pier.  FIG. 2  shows the stairway  36  sloping downward at a 40 degree angle  145  and  FIG. 4  shows the stairway sloping upward at an angle  146  of 30 degrees. The desired elevation of the platform  42  will depend on the water level and the elevation of the barge deck on which the platform  42  is set down. The pivoting of the upright spars  94 ,  96 ,  103 ,  104  on vertical axes and the pivot connections between parts and segments of the landing and platform, respectively, permits the stairway to be slewed in either direction. As shown in  FIG. 1B  the stairway  36  is slewed by an angle  147  of 20 degrees. It is desirable that the gateway  88  of the platform  42  face the same direction regardless of the slewed position of the stairway. This orientation maintenance is provided by the parallel raising links  116 ,  117 ,  126  and  127  interconnected between the landing  12  and the platform  42 .

Summary:
A boat access stairway having a landing attachable to a pier, a ramp which can be raised, lowered and slewed and a platform at the free end of the ramp whose orientation relative to a boat positioned parallel to the pier remains the same regardless of the slewed condition of the ramp.