This invention relates to accessories for spirit levels. Spirit levels are widely used in the building trade and similar areas to check whether a surface is horizontal or vertical. A wide variety of spirit levels is available in commerce and many of these are provided with two or more captive bubble units set fixed in an elongate member and located so that the bubble is central between the customary two lines on the exterior of the bubble container when the elongate member is vertical or horizontal.

If the elongate member of a spirit level is laid against a surface which is not precisely vertical or horizontal, the user can see the angle from which the surface varies from horizontal or vertical by moving one end of the level away from the surface while keeping the other in contact with it; the degree of movement necessary to render the bubble central gives a visual indication of the deviation from vertical or horizontal of the surface in question.

When the level is being used in order to ensure that a movable building component is installed accurately vertically or horizontally, the component itself is moved until the desired position is achieved. This is the standard and conventional use of a spirit level.

However, it is well known to use spirit levels to determine fall on a floor or like surface, or for example of a drainage pipe. One end of the level lies against the surface or pipe and the other is lifted until the level itself is horizontal. By measuring, for example using a tape measure, the distance by which the end of the level has been raised, it is possible to work out a gradient relative to the horizontal of the surface in question. If the gradient is 1 in 10, then if the length of the elongate member of the spirit level is 100 cm, the height to which one end needs to be raised to render the level horizontal is 10 cm. Working this way is prone to inaccuracy due to the difficulties of taking a measurement, e.g. with a tape measure, while holding the level in place, and even if two persons attempt to do this, the results are often imprecise. US-A-4543731 describes a spirit level having, rotatably fixed to one end, an endpiece, arranged to rotate about the central axis of the elongate member of the spirit level and having a noncircular periphery. One part of the outer contour of the rotatable member is straight and it can be brought into alignment with one working face of the elongate member of the spirit level, so enabling the level to be used in normal fashion. Rotating the rotatable endpiece brings its exterior periphery to a range of positions where it projects from the lower edge of the elongate member, and by turning it with one end of the elongate member on a surface and the other on a lower portion of that surface, it is possible to measure the gradient of the surface between those two points. The range of gradients which can be conveniently measured by a device of this sort is relatively small because measuring larger gradients would require an inconveniently large rotatable member on the end of the elongate member. We have now devised an accessory for a spirit level which can convert a conventional spirit level into a gradient measuring device and which is easy to fit to a conventional spirit level and which enables the spirit level to be used to measure existing gradients accurately or to set a building component such as a pipe to a given inclination.

According to the present invention there is provided an accessory for a spirit level which comprises a pair of units each of which comprises a releasable clamping portion adapted to fit onto the elongate member of a spirit level, and at least one of which has a planar surface on the clamping portion configured to be perpendicular to the plane of the working face of the level when fitted thereto, a movable member mounted for sliding movement against the planar face, one end of the movable member having a foot, means for locking the movable member relative to the clamping member or to free one relative to the other, so enabling sliding movement to occur, and wherein one of the planar face of the clamping portion and the movable member bears at least one index mark and the other a scale.

Preferably the arrangement is such that the sliding movement of the movable member is constrained to be linear. This may be adjusted by providing a slot in the movable member and two spaced apart posts fixed relative to the clamping member and extending into or through the slot to constrain the movable member to move exclusively in the direction of the slot.

Alternatively, the movable member may slide in a track on the clamping portion or an edge of the movable member may abut a rib or rail on the clamping portion. It is preferred to provide a simple screw clamp member to hold the movable member in position, and this may conveniently take form of a manually rotatable traditional wingnut mounted on a threaded post which passes through the slot. Such an accessory may be quickly and easily fitted to an existing level by clamping the clamping portion on to the elongate member of the level, preferably two identical units being fitted at a distance apart along the length of the level. Once the units have been clamped on to the level, the feet of the two movable members may be placed on a surface and then, by adjusting the position of one of the movable members on to the clamping portion, the level may be brought to lie horizontally. At this point the relative positions of index mark and scale can provide a direct reading of inclination, for example expressed as a "1 in X" gradient or expressed as a geometrical angular measurement.

More than one scale may be provided on one or both units of the accessory in order to enable it to be fitted to the level with different spacings between the two units. For accurate measurement of relatively small gradients the two units may be widely spaced along the length of the elongate portion of the level. For measuring steeper gradients, closer spacing is needed. A particular advantage of a level carrying an accessory in accordance with the present invention is that it may be used on uneven or stepped surfaces, or even to determine, for example, whether a floor level one side of a door frame having a protruding seal or stop bar or the like running across the floor in the door frame is the same as the floor level on the other side, This can be achieved simply by extending the slidable portions to the same extent and then placing the level in the manner of a bridge across the obstruction with the foot of one unit one side of it and the foot of the other unit on the other side. The foot on the slidable member will normally be flat. However if the accessory is to be used on a level which is to be used to determine the fall of a pipe, for example a sewer pipe or drainage pipe, the foot may have an appropriate shape to fit the contour of the top of the pipe. The invention is illustrated by way of example with reference to the accompanying drawings in which:

Figure 1 is a general diagrammatic view of a builders level to which an accessory according to the present invention has been fitted.

Figure 2 shows in section along the lines A-A in Figure 1 , on a larger scale one unit of the accessory according to the present invention fitted to the body of the level. Figure 3 shows a view of one of the units showing the scale and index marks; and Figure 4 is an exploded view showing the components of one unit of the accessory.

Referring to the drawings, the accessory according to the invention is designed to be easily and quickly fitted to a standard builder's level 1. As shown in Figure 1 , this has a central captive bubble unit 2 to indicate when the level is horizontal and two captive bubble units 3, one at each end, which show when the level is vertical. As shown in Figure 1 , two units constituting the accessory of the present invention are denoted 4 and 5 and they are clamped on to the main body of the level at a distance apart which may be easily set by using a tape measure or, for example, by aligning with marks made on the body of the level itself. Although the units shown in Figure 1 are positioned about one quarter and three quarters along the length of the elongate body of the level itself, they can be placed at any other positions, and it is not necessary that they are placed equidistantly from the ends/centre of the level.

What is important is that the scale used, if it measures inclination,

corresponds to the distance apart of the two units.

As shown in the Figures, each unit consists of a short generally C-section member 10. One end of the C-section has a lip 11 so that the body of the level 1 can be located as shown in Figure 2, while the opposite part of the C- section 10 has an internally threaded collar 13 fixed on it through which a threaded screw 12 passes. The base of the screw bears a knurled or otherwise configured knob 14 which is easy to turn, below which is a resilient disc 15. By turning knob 14, the disc 15 may be pressed against the top of the body of the level 1 as shown in Figure 2, to clamp C-shaped member 10 firmly on the body of the level . Extending from C-shaped member 10 is a threaded stud 23 and this passes through a slot 22 in a movable scale-bearing member 20. The scale-bearing member 20 may be clamped firmly against the face of member 10 by means of a wing nut 24 threadedly engaged on stud 23 and having a conventional clamping washer 25 between it and the scale-bearing face of member 20.

Fixed to the bottom of member 20 by means of four screws 31 (shown in Figure 4) is a foot 21 . As can be seen, foot 21 has two rearwardly extending slightly angled bars on to which are fitted rubbery plastics feet 30.

As can be seen most clearly in Figures 1 and 3, the edges of clamping member 10, which is slightly wider than scale member 20, bear index marks 30 near the bottom. The scale-bearing member 20 has two scales, one on each side, and, as can be seen most clearly in Figure 3, one is graduated in gradient measurements, the other has a linear distance scale.

In order to use the level, e.g. to measure the gradient of a slope indicated diagrammatically by the dashed line 51 in Figure 1 relative to the horizontal, indicated by the dashed line 50 in Figure 1 , the two units constituting the accessory are assembled on to the level 1 by clamping on to the body of the level and ensuring that they are the right distance apart for the scale marking in question. Once that is done, the level is placed on the slope 51 and, as can be seen, unit 5 will come to rest on level 51 first. By releasing wing nut 24 on unit 4, the scale-bearing member 20 may then be allowed to drop down while the level is held generally level (easily seen by looking at the bubble in bubble unit 2) and then, once the foot 21 of unit 4 is on the sloping surface 51 , the precise position of level 1 relative to the foot of unit 4 may be adjusted by looking at bubble unit 2 until the bubble is located in the centre between the conventional centre markings in the bubble unit. At this point, wing nut 24 may be tightened and the degree of inclination then read off from the scale on unit 4. As just described, the accessory of the invention fitted to a level is used to determine a gradient of a fixed slope. It is equally possible to use the level with the accessory the other way round, i.e. to set the feet at different distances from the base of the level corresponding to a desired inclination, for example of a waste pipe, and then to place the level on the pipe and move the pipe until the desired inclination is reached, shown by the bubble in the central bubble unit 2 being centred.

Although, as illustrated in the drawings and as described above, the inclination is measured or set relative to a horizontal, the same procedure can be used to measure or set deviations from the vertical by observing one of the bubble units 3 when adjusting one of the feet of units 4 or 5.

As can be seen, the scale-bearing member 20, together with its foot 21 , can be easily detached from the clamping member 10 simply by unscrewing wing nut 24 and removing the member 20. This enables the exchange of one member 20 for another, for example one graduated to measure inclination when the two units 4 and 5 are, for example, 80 cm apart on a one metre length level, and the other when they are 1.5 metres apart on a longer level, such as might be used for ensuring fall when laying a patio or similar deck structure.

The accessory in accordance with the invention may be quickly and easily fitted to a level when desired. However, when the level is to be used in customary fashion purely as a spirit level, they can be kept separate for deployment when next needed.