I Drill Barrel and Core Bit Clearer FIELD OF THE INVENTION [0001] The present invention relates to diamond drilling equipment, in particular an implement for clearing drill barrels and core bits of debris. BACKGROUND TO THE INVENTION [0002] Diamond drilling is used extensively in the mineral exploration field for extracting core samples. A core bit is attached to the end of a drill barrel and progressively drilled into the earth with drill rods being successively attached behind the drill barrel to make a rod string which can extend 1 kilometre or more into the earth. To collect the core as it is drilled an inner tube is sent down the rod string and is seated within the core bit just behind the cutting teeth. As the core bit progresses the inner tube fills with the core sample. When the inner tube is filled it is pulled back to the surface by a wire line, the core is extracted and the process repeated. [0003] When the inner tube is first pulled back to retrieve the core, the core sample is snapped off from the rock being drilled by means of a core lifter located in a core lifter case fitted to the bottom of the inner tube. This process is reasonably reliable, however on occasion a fragment of rock may get jammed in the drill barrel or core bit. The rock fragment prevents the inner tube from passing through the drill barrel and seating in the core bit, thus preventing collection of further samples. [0004] At present, to remove the rock fragment the drill barrel and core bit must be returned to the surface. This entails completely extracting the rod string from the drill hole. The drill barrel and core bit are then cleared and the rod string sent down the drill hole again. This exercise could readily take 4 or more hours to complete for a drill depth of 500 metres. This is clearly a time consuming, expensive and undesirable problem. [0005] The object of this invention is to provide a drill barrel and core bit clearer that alleviates the above problem, or at least provides the public with a useful alternative. SUMMARY OF THE INVENTION [0006] In a first aspect the invention comprises a drill barrel and core bit clearer comprising a cylindrical body of a first diameter, wherein a first end of the body comprises an internally threaded section and a second end of the body includes a protrusion of a second diameter being less than the first diameter. [0007] It should be noted that any one of the aspects mentioned above may include any of the features of any of the other aspects mentioned above and may include any of the features of any of the embodiments described below as appropriate. BRIEF DESCRIPTION OF THE DRAWINGS [0008] Preferred features, embodiments and variations of the invention may be discerned from the following Detailed Description which provides sufficient information for those skilled in the art to perform the invention. The Detailed Description is not to be regarded as limiting the scope of the preceding Summary of the Invention in any way. The Detailed Description will make reference to a number of drawings as follows. [0009] Figure 1 is a perspective view of a preferred embodiment of the invention. [0010] Figure 2 is a side view of Figure 1. [0011] Figure 3 is a rear perspective view of Figure 1. [0012] Figure 4 is not part of the invention, but shows a core bit that the invention seats within and clears. [0013] Figure 5 shows the invention seated within the core bit of Figure 4. DETAILED DESCRIPTION OF THE INVENTION [0014] The following detailed description of the invention refers to the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings and the following description to refer to the same and like parts. Dimensions of certain parts shown in the drawings may have been modified and/or exaggerated for the purposes of clarity or illustration. [0015] The present invention provides a drill barrel and core bit clearer as depicted in Figure 1. The clearer is fitted in place of the core lifter case at the end of an inner tube and when the inner tube is sent down a drill hole the clearer will impact and dislodge rock fragments that are jammed in a drill barrel and/or core bit. This allows a -Y jammed drill barrel and core bit to be cleared in situ thus saving considerable time and effort. The clearing process may take only 20 minutes to complete at a depth of 500 metres. The invention can be used when a drill barrel, a core bit or both are blocked. [0016] Details of the clearer can be seen in Figure 1-3 which show the clearer in front perspective, side and rear perspective views. The clearer 10 comprises a cylindrical body 20 and rounded cone 40. The body 20 is shaped and sized as per a conventional core lifter case so that it may take the place of a core lifter case at the end of an inner tube and seat within a core bit. The cone 40 passes through the centre of a drill barrel and bit dislodging any rock fragments jammed within. [0017] The body 20 comprises a solid cylinder 22 with a threaded box end 24, chamfered top 26 and flow holes 28. [0018] The threaded box end 24 has threads as per conventional core lifter cases, allowing the clearer to be threadedly fitted to the end of a conventional inner tube. [0019] The chamfered top 26 is also as per a conventional core lifter case and allows the clearer to seat within a corresponding chamfer of a core bit. Normally an inner tube is pushed down a rod string using pressurised water. When the inner tube reaches the bottom of the rod string the chamfered end of a core lifter case will seat in the corresponding chamfer of the core bit and can travel no further. A seal is made between the core lifter case and the core bit resulting in a 'blow back'. This is how the drill crew know that the inner tube is in place. As the clearer has the same features as a core lifter case it will behave in a similar manner, however when it seats in the core bit its job of clearing the jam in the drill barrel and core bit will be complete. The drill crew will notice the resulting blow back and know that the inner tube with clearer can be removed and normal drilling recommenced. [0020] The flow holes 28 provide a fluid connection between the inside and outside of the inner tube as it descends down the rod string and help increase the speed of descent. The exact size and distribution of the holes is not critical. The holes sit in a channel 27 within the chamfered top 26, thereby increasing the blowback affect when the clearer seats within the core bit. The channel 27 collects debris as it descends down the drill string and through the blockage so that the debris does not stay within the drill string and cause further jams. The bottom of the channel 27 is curved to prevent debris getting stuck. When the clearer is seated within a core bit, the water -r flow through the flow holes 28 will clear out any debris accumulated in the channel 27. [0021] The cone 40 sits atop the body 20 and is designed to pass through the end of a core bit. The base 42 of the cone is stepped in from the chamfered top 26 of the body so that it clears the teeth of a core bit. The tip 44 is rounded to optimally impact and dislodge rock fragments. The sides 46 are at an angle of approximately 15 degrees to the centreline of the cone. [0022] The operation of the clearer can be appreciated with the aid of Figure 4 which shows a core bit 100 and Figure 5 which shows a core bit with a clearer 10 seated within. The core bit 100 as shown is well known in the art and comprises a cylindrical body102 topped by a ring of cutting teeth 106 separated by gaps 108. The centre 104 of the core bit is hollow to allow a core sample to pass through. When the clearer is inserted into the core bit 100, any rock fragments that may be caught in the centre 104 will be dislodged. In Figure 5 it can be seen that the core bit clearer partially passes through the core bit 10. [0023] Preferably the clearer is made from steel which is heat treated after forming. Other materials and finishes as is known in the art can be readily envisaged. [0024] Many embodiments of the invention can be envisaged. A simple cylinder with a thread at one end and a protrusion at the other could perform the same basic function, albeit not as well as the preferred embodiment. [0025] The reader will now appreciate the advantages of the present invention which provides a core bit clearer that can clear a jammed drill barrel and/or core bit in situ thus saving considerable time and effort. The clearer is also easy to use at it mimics the operation of existing equipment [0026] Further advantages and improvements may very well be made to the present invention without deviating from its scope. Although the invention has been shown and described in what is conceived to be the most practical and preferred embodiment, it is recognized that departures may be made therefrom within the scope and spirit of the invention, which is not to be limited to the details disclosed herein but is to be accorded the full scope of the claims so as to embrace any and all equivalent devices and apparatus. Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in this field. [0027] In the present specification and claims (if any), the word "comprising" and its derivatives including "comprises" and "comprise" include each of the stated integers but does not exclude the inclusion of one or more further integers.