1. Know all men by these presents: Provisional Application SN 60/575,123 filed 05/28/2004 This application claims priority from United States Provisional Application SN 60/575, 123, filed May 28, 2004 by the undersigned applicant. That the undersigned, who name, address and citizenship is set out below, has invented a certain new and useful improvement in s CUTTING ASSEMBLY FOR ROTARY TRIMMER, for which the following constitutes the application for United States patent by the undersigned, describing and claiming the said invention as follows: CUTTING ASSEMBLY FOR ROTARY TRIMMER 2. This invention relates to a cutting assembly for rotary trimmers and is more particularly directed to a rotary paper trimmer having such an assembly for office and similar use wherein the cutter is mounted on a cutting board and slidably secured for cutting on a fixed rail, and which is characterized by selective vertical movement toward an underlying paper or other target to be cut and by horizontal movement along the rail defining a plane upon which a cut is performed. BACKGROUND AMD SUMMARY OF THE INVENTION 3. Rotary cutters are well known in the art. 4. Conventional cutters may be movably secured on and move in a horizontal plane along a rail, spring loaded or fixed. Such rotary cutters may be found in Johnson Patent #0,791 ,793, Mori Patent #5,069,097, Boda Patent #5,322,001 and Daley Jr. Patent #6,098,515, amongst others. Sometimes these cutter have clamps which hold the paper or other target during cutting, for example see Cornell et al Pat. #5,996,459. 5. Some cutters tend to twist out of their intended cutting path, and make an irregular cut. Often, more or less pressure is required to effect a desired cut, but known conventional rotaiy cutters lack sufficient structure for the person manipulating the cutter to adjust the cutting pressure as required to make the desire cut, as varying pressure may be required depending upon the thickness, texture, or structure of the target material, which affects the cutting pressure which must be applied to effect an appropriate cut. ■ 6. Some prior art cutters lack proper paper compression for non-skewed cuts, and the cut on stacked sheets is misaligned. Other cutters lack convenient means for detecting the position of the cutting edge or for measuring and ascertaining the exact location of the cut. The present cutter assembly is intended to overcome these deficiencies in the prior art. SUMMARY OF THE PRESENT INVENTION 7. In the present invention, the cutting assembly is secured in a carriage which is slidably secured on a fixed rail overlying a cutting table and cutting assembly carriage is adapted to move horizontally along that rail in a fixed plane. Preferably the rail is mounted over a spring loaded ruler or guide, which may be biased above the cutting table, thus permitting a target paper or other sheet to be arranged between the ruler and the cutting table. 8. The carriage which houses the cutting blade is spring loaded on the rail and spring biased above the rail and cutting table and rail, so that no cutting action can be achieved unless the carriage and its cutter blade is manually depressed against the carriage spring toward the cutting table. The carriage is ergonomically formed, so that hand pressure applied to it will move the cutting blade downwardly and not to a size or otherwise skew the blade relative to the target to be cut. 9. The ruler also performs a measuring and clamping function as it is pressed against the paper or other target when the carriage is depressed to expose its blade for cutting action. The marked edge of the ruler or guide defines the position of the cutting action, and its measurement marking permit the cut to be achieved in a specific intended dimension and location. 10. Before and after the cut is made, the spring action of the carriage draws the cutter blade and its. carriage to a position normally elevated above and spaced apart from the table; and the ruler is also pushed up by its springs toward the rail. At this point, the cut paper or other target material can be removed from the table. This cutting action is such that the user can feel the pressure applied against the paper or other material target to make the cut, responsive to feel of the target material and its physical resistance. The carriage is ergonomically designed to accommodate the hand and palm of the user, so that there is positive feeling of the hand pressure required by the person making the cut. OBJECTS AND ADVANTAGES OF THE INVENTION 11. It is the object of the present invention to provide a Cutting Assembly for a Rotary Trimmer of the character described. 12. Another object is to provide a cutting assembly including a carriage and cutter blade which travels horizontally on a fixed rail above a cutting table so that the cutting assembly can be manipulated to make a controlled cut according to the nature and characteristices of the target to be cut. 13. Another object is to provide a novel cutting assembly having self lubricating means which is movably secured on a rail to permit smooth action of the cutter assembly. 14. Another object is to provide a ruler for a cutter assembly which is arranged between the cutter assembly and the underlying table and which performs both a measuring guide function and a clamping function for pressing the underlying target material in position for the cutting. 15. Another obj ect is to provide a fixed rail for a spring loaded carriage containing a rotary cutter which is rigid and not deflectable. 16. Another object is to provide a spring loaded carriage containing a rotary cutter which is ergonomically arranged a movable only in a horizontal direction and in a vertical direction to make a cut. 17. Another object is to provide a spring loaded carriage containing a rotary cutter which has tracking structure for moving the carriage only in selected directions and not otherwise. 18. Another object is to provide a cutting assembly for a trimmer which is easy and efficient in use but not substantially expensive or critical to manufacture. 19. These and other objects and advantages will become apparent as this description proceeds, taken in conjunction with the accompanying drawings. Brief Description of the Drawings 20. In the drawings: 21. Figure 1 is a perspective view of an exemplary paper trimmer embodying the present invention. 22. Figure 2 is a perspective view of the novel carriage assembly housing the cutter blade arranged on the rail of the of the exemplary paper trimmer shown in Figure 1. 23. Figure 3 is a detail view of the novel carriage assembly with its cover removed. 24. Figure 4 is a detail sectional view of part of the novel carriage assembly, substantially at a right angle to the view shown in Figure 3. 25. Figure 5 is a detail sectional view of part of the end of the rail. 26. Figure 6 is a detail sectional view of the rail substantially at a right angle to the view shown in Figure 5. Description of a Preferred Embodiment 27. With reference to the accompanying drawings and particularly to Figure 1 , a typical trimmer 10 for paper or other similar material, comprises a table or bed 11 , which may have a series of cross lines 12 to indicate dimensions or the like, over which is secured a rail 13, usually spaced apart at opposed ends of the table 11. This rail 13 has secured to it a cutter carriage assembly 15 which is normally elevated above and movable along the rail from and supported by the end supports 16 of the rail. The rail 13 and its end supports 16 are rigidly fixed on the table 11 and are not intended to move or twist. 28. The cutter carriage assembly 15 is ergonomically designed with a graspable cover 17, and within the carriage assembly and underlying the cover, as shown in Figure 2, there is arranged a rotary blade 18 journaled for rotation on an axle 19. Preferably, the rail 13 has an arched upper face 20 and straight side walls 21 , and the lower surface 22 of the carriage assembly 15 is correspondingly contoured, so that the fit of the carriage assembly when depressed on the rail is fairly exact, thus supporting the carriage assembly without tilting or twisting when a cut is made, as hereinafter explained. The rotary blade 18 is guided along a ruler or edge guide 23 which defines the location of the cut as the cutter carriage assembly 15 is depressed and moved along the rail 13. The rail end supports 16 are secured to the bed 11 preferably by screws 24 or similar fasteners. 29. The side of the carriage assembly 15 opposed to the rotaiy blade 18 is shown in Figure 3, wherein a nylon or similar lubricating block 25 is shown within the cutter carriage assembly and shaped for bearing on the rail 13. Upstanding from this block is a boss 26 which is apertured in a vertical direction to receive a post 27 adapted for reciprocal movement in the boss . Surrounding this boss 26 there is an expansion spring 28 arranged over the boss and having one end bearing on the nylon block 25 and its other end bearing on the underside of the carriage cover 17. This expansion spring 28 is depressed when a cut is made, the carriage assembly 15 is moved downwardly to expose the rotary blade 18 and make the cut along the defined edge of the guide 22. Reciprocation of the blade 18 in the carriage assembly 15 is held in alignment by means of a walled recess 30 formed in the bottom side 29 of the carriage assembly cover 17 as shown in Figure 4, and additional positive alignment means for moving the blade 18 only in the intended direction and path is provided by means of a plurality of shafts 31 on the block 25 which engage in aligned recesses 32. 30. As shown in Figure 6, at the ends of the rail 13, the edge guide 23 may be held elevated by means of guide springs 33 on end shaft 34. 31. While a preferred embodiment of the invention has been described in considerable detail, it should be understood that changes in the structure can be made without departing from the spirit or scope of the invention. The invention should be limited to the exact structure disclosed and described.