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 
       [0001]    Sinks that temporarily hold fluid such as water, commonly have a drain through which water rapidly leaves. The drain has holes that pass small solid objects such as hair without the hair blocking the drain. It is desirable to construct the drain so the holes are oriented and shaped to minimize blockage from debris. 
       SUMMARY OF THE INVENTION 
       [0002]    In accordance with one embodiment of the invention, a grid drain is provided that can be constructed at low cost and that minimizes blocking. By increasing air flow through the drain, the drain can be used as a cleanout to service the drain. The drain includes a vertical tube and a dome cover that is removably mounted on the top of the tube, the cover having multiple holes. The cover holes are arranged in two circles, including a middle circle where the cover is flat (horizontal), and an outer circle where the cover is curved to form a dome. 
         [0003]    The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  is an isometric view of a drain of the invention, with the cover attached to the tube. 
           [0005]      FIG. 2  is an exploded enlarged view of a top portion of the drain tube and of the cover. 
           [0006]      FIG. 3  is a sectional view taken on line  3  of  FIG. 5 . 
           [0007]      FIG. 4  is an isometric view of the cover of  FIG. 3 . 
           [0008]      FIG. 5  is a plan view of the cover of  FIG. 3 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0009]      FIG. 1  shows a drain  10  of the invention, which includes a tube  12  that extends along a vertical axis  14  and a cover  20  that is mounted on the top of the tube.  FIG. 2  shows that the tube  12  has a flange  22  at its top, that forms a support surface  24 . The cover has a periphery  30  with three downward projections  31  that lie on the support surface. A vandal resistant screw  32  ( FIG. 4 ) passes through a hole  34  in the cover and screws into a threaded hole  36  at the top of the tube, to hold the cover in place. The screw has a pair of holes that allow it to be screwed only by a special tool. The cover is preferably a cast metal part. 
         [0010]    The cover  20  ( FIG. 2 ) is symmetrical about the vertical axis  14 . The cover has two rings of holes, including an inner ring  40  and an outer ring  42 . As shown in  FIG. 5 , the outer ring  42  has sixteen outer holes  44  that are uniformly (by 22.5°) spaced about a circle  46  centered on the axis. The inner ring  40  has eight inner holes  50  that are uniformly (by 45°) spaced about a circle  52 . As shown in  FIG. 5 , each inner hole lies on an imaginary radial (to axis  14 ) line  54  and each outer hole lies on an imaginary radial line  60 , with lines  60  angled apart by 22.5° and lines  54  angled apart by 45° and lying halfway between pair of lines  60 . 
         [0011]      FIG. 3  shows that the cover has a top part, or top plate part  70  with a flat (horizontal) middle portion  72  and with a curved outer part, or periphery  74 . The periphery  74  extends in a quarter circle as seen in the sectional view of  FIG. 3 , with the upper surface  80  and the lower surface  82  parallel and each curved by one-quarter circle. This results in the cover forming a dome, with only the periphery of the cover being curved. In a cover that applicant has constructed, the over had a diameter of 2.6 inch, and a cover thickness of 0.1 inch. The bottom and top surfaces of the periphery were each curved about a point  84  lying 0.7 inch below the top plate part  70 . The point  84  is spaced from the axis  14  by an amount A that is more than half the radius of the cover and by the height B of the cover. The cover is formed with the three bottom projections  31  that each lies against a support surface. The above construction results in a cover with uniformly stiff wall holes and with the holes occupying a maximum amount of the area of the cover. 
         [0012]    The inner ring  40  of holes have hole axes  92  that are vertical. This allows efficient flow because when water reaches the inner holes  40 , the water is moving close to the vertical. The outer ring  42  of holes have hole axes  94  ( FIG. 4 ) that are angled about 45° from the vertical and from the horizontal (i.e. the outer hole axes  42  extend 15° to 75° to the vertical and to the horizontal), which is parallel to the direction of water flow through the cover. The top  100  ( FIG. 2 ) of the outer holes face upward while the bottom  102  of the outer holes face horizontally. Solid particles, especially hair, tend to easily flow through the cover as they flow with the movement of water. Applicant can construct the drain in a number of ways, but prefers to make it of cast metal. 
         [0013]    Thus, the invention provides a drain that facilitates the flow of water and of solid particles such as hair. The drain includes a cover of a generally dome shape, formed by a curved periphery and a flat (horizontal) middle. Holes in the cover include a ring of inner holes that lie in a flat (horizontal) wall of the cover, while a ring of outer holes lies in a curved periphery of the cover. The inner holes have vertical hole axes. The outer holes have axes that are angled about an eighth circle (about 45°) from the horizontal and from the vertical. The fact that the cover  20  is held by a screw and therefore can be easily removed from the tube  12 , allows a plumber to remove the cover and run a “snake” through the tube and pipes to clean them. 
         [0014]    Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art, and consequently, it is intended that the claims be interpreted to cover such modifications and equivalents.

Summary:
A grid drain for use on a sink to prevent large objects from entering the drain. The drain includes a tube ( 12 ) and a cover ( 20 ) that attaches to the top of the tube. The cover has a flat middle ( 72 ) and has a peripheral portion ( 74 ) that is curved to form a dome, with two circles of holes ( 46, 52 ).