Background of the invention The recommended safe manner in which to use a conventional ladder is to have the feet of the ladder resting on a horizontal surface, so that the ladder extends vertically.

When the surface is uneven however, the ladder has to be used in an unsafe, non-vertical configuration unless special measures are taken.

It is not uncommon for the user to place additional supports such as bricks or blocks beneath one of the legs to enable the ladder to be used in a vertical configuration. Often however, there is a risk that such additional supports will move when the ladder is in use, because they are not secured to the ground. The ladder is therefore liable to topple sideways, and may cause injury to the user or others as a consequence.

We are aware of United States patent US 5678656 (Lanzafame) which describes extension legs of adjustable length for ladders to provide a stabilizing and levelling effect. The extension legs include an outer housing within which can slide an inner member which carries a ground engaging foot.

A spring-loaded ratchet device is provided between the outer housing and the inner member, by operation of which the length of the extension leg can be adjusted. The outer housing is permanently connected to one leg of the ladder.

A product marketed under the name"Levelok" (Trade Mark) is of similar construction.

We are also aware of United States patents US 16611057 (Edward R Neil) and US 5669462 (Paul J Jennings) which both describe extension devices for a ladder, to enable the ladder to stand on an uneven surface, comprising a channel- shaped member having an open top end to slidably receive one leg of the ladder, wherein the channel-shaped member is bolted directly to the leg of the ladder.

Although the above devices may work satisfactorily, the permanent connection of the outer housing to one leg of the ladder means that two such devices need to be provide, one for each ladder leg. Furthermore, the arrangement for adjusting the length of the extension leg is complicated and therefore costly to manufacture.

There is therefore a need to provide a device enabling a ladder to be used on an uneven surface, which device is relatively simple and less costly to produce and which can be selectively connected to one or the other leg of a ladder according to the user's choice.

SUMMARY OF THE INVENTION According to this invention, there is provided an extension device for a ladder, to enable the ladder to stand on an uneven surface, comprising a channel-shaped member having an open top end to slidably receive one leg of the ladder, a support block dimensioned to fit within the channel, means for securing the support block in a selected one of a plurality of positions along the length of the channel, and a non-slip foot provided at the bottom end of the channel- shaped member.

The channel-shaped member may be of U-shaped cross-section, but more preferably it is of C-shaped cross section, to provide greater strength.

The channel shaped member may be formed of a material selected from rigid plastics materials, aluminium, alloys thereof and, less preferably, steel.

The non-slip foot may be formed of a material selected from rubber and elastomeric synthetic materials.

In a first embodiment of the invention, the means for securing the support block in a selected one of a plurality of positions along the length of the channel may comprise at least one peg-shaped member extending from the support block and a plurality of holes positioned along the length of the channel, in a selected one of which the peg-shaped member is located. Ideally, one or two such peg-shaped members are provided, the provision of one peg-shaped member being ideal for quick adjustment of the device while the provision of two peg-shaped members provides a more secure connection with the channel-shaped member. The or each peg-shaped member may be a bolt, carried by the support block, and the peg-shaped member is preferably rigidly secured to the support block.

In this embodiment, the plurality of holes may be arranged in at least two rows, running generally parallel to the axis of the channel. In a preferred embodiment, the holes in adjacent rows are staggered with respect to each other, so as to enable the support block to be secured at different angles within the channel, thereby to provide a smaller adjustment distance then would be the case with just one row of holes.

In this embodiment, the ladder leg supporting surface of the support block is preferably curved to spread the load.

In a second embodiment of the invention, the means for securing the support block in a selected one of a plurality

of positions along the length of the channel comprises a support bracket for supporting the support block and a plurality of slots positioned along the length of the channel, in a selected one of which the support bracket is capable of being located.

In this embodiment, the support block may be supported on the support bracket by the location of a peg carried on the support block in a hole provided in the support bracket.

In either embodiment, a releasable clamp device may optionally be provided adjacent the top end of the channel- shaped member to clamp the leg of the ladder within the channel, to provide an even more secure connection between the leg of the ladder and the extension device.

The invention will now be further described, purely by way of example, with reference to the accompanying drawings, in which : Figure 1 shows the combination of a ladder and a first embodiment of an extension device according to the invention, being used on uneven ground; Figure 2 is a view of the extension device shown in Figure 1, taken in the direction"II"in Figure 1 ; Figure 3 is a view of the extension device shown in Figure 1, taken in the direction"III"in Figure 1 ; Figure 4 is a simplified top elevational view of the extension device shown in Figure 1, taken in the direction "IV"in Figure 2; Figure 5 is a perspective view from the rear of the support block used with the extension device shown in Figure 1;

Figure 6 is a perspective view from the rear of an alternative support block; Figure 7 shows the channel-shaped member of a second embodiment of the invention; Figure 8 is a top elevational view of the channel-shaped member of Figure 7; Figure 9 is a 3-dimensional view of the channel-shaped member of Figures 7 and 8; Figure 10 shows a support bracket for use in the second embodiment; Figure 11 is a 3-dimensional view of a support block for use in the second embodiment; Figure 12 is a top elevational view of the support block of Figure 11; Figure 13 is an end elevational view of the support block of Figure 11; Figure 14 is a 3-dimensional view of a non-slip foot for use in the second embodiment; Figure 15 is a side elevational view of the non-slip foot of Figure 14; Figure 16 is a top elevational view of the non-slip foot of Figure 14; and Figure 17 is an end elevational view of the non-slip foot of Figure 14.

As shown in Figures 1 to 6, an extension device 13 for a ladder 10, partly shown in Figure 1, is provided to enable the ladder to stand on an uneven surface 12. The extension device includes a channel-shaped member 14, having a length of, for example, 575 mm. Member 14 is of C-shaped-cross section with dimensions of, for example, 75 mm by 31 mm and having a mouth opening slightly wider than the thickness of the ladder rungs 15. The member 14 is formed of galvanised steel, having a thickness of, for example, 1.5 mm. It has an open top end 16 to slidably receive that leg 18 of the ladder 10, which would not otherwise be in contact with the ground if the ladder were to be positioned upright.

A non-slip ground engaging rubber foot 32 is provided at the bottom end of the elongate member 14.

A support block 20 of generally elliptical configuration fits within the channel 22 of member 14. As shown in Figure 5, two bolts 24 are rigidly secured to the back of the support block 20 and extend therefrom. A number of holes 26 are arranged in the web of the member 14 in two rows, running generally parallel to the axis of the channel 22.

The holes 26 in adjacent rows are staggered with respect to each other.

A releasable clamp device 30 is provided adjacent the top end 16 of the channel-shaped member 14.

In use, the support block 20 is located in a suitable position along the length of the member 14 by location of the bolts 24 is selected holes 26 and by tightening the wing nuts 34, thereby to secure the support block 20 to the member 14 (for the sake of clarity only one such wing nut is shown in Figure 5). The appropriate holes 26 to be used are chosen according to the unevenness of the surface 12 and the intended orientation of the ladder. The leg 18 of the

ladder 10 is inserted in the channel and is supported by the curved surface 28 of the support block 20. The leg 18 is clamped within the channel 22 by the clamp device 30. The foot 32 rests on the ground. The lower end of the other ladder leg 19 rests directly on the ground. The ladder can now be used safely.

If the ladder is moved to ground which is uneven in another direction, the extension device 13 can be quickly removed from the ladder leg 18 by loosening the clamp 30, and placed as desired on the other ladder leg 19.

In the alternative embodiment shown in Figure 6, the support block 22a carries an off-centre single bolt 24a, for securing the block to the member 14 with the aid of a wing nut 34a. This embodiment has the advantage of quicker adjustment. The block 20a is smaller than the block 20 shown in Figure 5, to enable it to be rotated within the channel 22 of the member 14 so that either the surface 28a or the surface 28b may be used to support the ladder leg 18.

As shown in the Figures 7 to 17, an extension device includes a channel-shaped member 42 of C-shaped-cross section having a channel 44 and a mouth opening slightly wider than the thickness of the ladder rungs. The member 42 has an open top end 46 to slidably receive that leg of the ladder, which would not otherwise be in contact with the ground if the ladder were to be positioned upright.

A non-slip ground engaging plastics material foot 48 is provided at the bottom end of the elongate member 42 and is bolted thereto by bolts (not shown) passing through bolts holes 70 and 72 in the channel-shaped member 42 and the foot 48 respectively.

An L-shaped support bracket 50 is provided to fit into a

selected one of a number of slots 52 in the web of the channel-shaped member 42. The horizontal arm of the L- shaped support bracket 50 has a locating hole 54.

A support block 56 is provided having a shape generally complementary to the shape of the channel 44. The top face 58 of the support block slopes towards a groove 60 along which is located a peg-shaped protrusion 62. The bottom face 64 of the support block 50 is similarly sloped towards a groove 66 along which is located a further peg-shaped protrusion 68. The depth of the groove 60 is less than the depth of the groove 66 by an amount which is about half of the distance between the slots 52. By this arrangement, the support block can be used in the orientation as shown, or upside down to give two alternate extension positions for each of the slots 52.

In use, the support block 56 is located in a suitable position along the length of the member 42 by location of the protrusion 62 in the locating hole 54 of the. support bracket 50, and the fitting of the support bracket 50 in a selected one of the slots 52 in the channel-shaped member 42. The appropriate slot 52 to be used is chosen according to the unevenness of the surface on which the ladder is to stand and the intended orientation of the ladder. The leg of the ladder is inserted in the channel 44 and is supported by the upper face 58 of the support block 56. The sloping top face 58 of the support block 56 serves to help maintain the channel-shaped member 42 in line with the ladder leg.

The foot 48 rests on the ground. The lower end of the other ladder leg rests directly on the ground. The ladder can now be used safely.

If the ladder is moved to ground which is uneven in another direction, the extension device can be quickly removed from the ladder leg and placed as desired on the other ladder

leg.

In an example of the second embodiment, the channel-shaped member 42 has a length of 600 mm and cross-sectional dimensions of 80 mm by 30 mm. The mouth opening of the channel is 40 mm. The channel-shaped member 42 is formed of galvanised steel, having a thickness of 2.0 mm. The L- shaped support bracket 50 is formed of 7 mm thick aluminium, and has a horizontal arm of 45 mm length, a vertical arm of 57 mm length and a width of 20 mm. 15 slots 52 are provided, each having a dimension of 20 mm by 7 mm, spaced apart by about 20 mm.

The dimensions of the support block 56 are 35 mm width by 24 mm height by 40 mm depth. The depth of the groove 60 is 3 mm, and the depth of the groove 66 is 13 mm.