a helmet 112 comprises an outer shell 116 , typically of a hard plastics material , within which is provided a double compressible layer 124 , 128 and an optional soft internal comfort liner 120 . as best illustrated in fig3 , the inner compressible layer 124 comprises an array of integral conical protuberances 130 which fit closely within corresponding conical recesses 132 of the outer compressible layer 128 . the materials of the layers 124 , 128 are of different compressibility , which gives an advantageous compression characteristic as compared with a conventional unitary liner of single compressibility . particular details of the prior art construction can be obtained by reference to the description of wo 2010 / 001230a , and will not be further described here . the invention will be described with reference to a helmet of the kind illustrated in fig1 - 3 , it being understood that the compressible liner of the invention may be used in apparatus other than helmets , as previously mentioned . fig4 illustrates a first embodiment of the invention . a compressible liner 1110 comprises an inner layer 1124 , an outer layer 1128 and an intermediate layer 1160 . the inner layer 1124 has many protuberances 1130 which project into matching recesses 1161 of the intermediate layer 1160 , and the intermediate layer has many protuberances 1162 which project into matching recesses 1132 of the outer layer 1128 . the protuberances 1130 , 1162 and corresponding recesses 1161 , 1132 are integrally formed from a respective base region 1134 , 1163 of relatively uniform thickness , and may have variability in size , shape and spacing , though , as illustrated in this embodiment , the protuberances of the inner and intermediate layers are uniform . in this example the outer layer has a continuous surface layer 1122 of relatively uniform thickness . the inner layer also includes inwardly facing projections or ribs 1164 to engage a comfort liner , but the inner surface may also be smooth . each of the three layers 1124 , 1128 , 1160 typically comprises a shock absorbing expanded polystyrene material ( or other suitable thick absorbing material as previously described ). the layers may be respectively homogeneous . adjacent layers are of different compressibility so as to permit greater variation in the compression and crushing gradients across the thickness of the liner 1110 . as will be appreciated the invention permits three different densities of material in three different layers ( i . e . a factorial three possibility ) which provides many more potential combinations than the prior art , but maintaining a comparatively low manufacturing cost . an alternative embodiment is illustrated in fig5 , to show a degree of variation which is possible with the invention . in fig5 , the inner and outer layers 1224 , 1228 have the same compressibility , whereas the intermediate layer 1260 is different . furthermore the underside of the inner layer 1224 is planar , and at the outer side , the peaks of the protuberances 1262 of the intermediate layer 1260 are permitted to appear through the outer layer 1228 , thus permitting a substantial sharing of an orthogonal impact load . in both embodiments of fig4 and 5 , it will be understood that a straight liner is shown for ease of illustration , but that in practice a three - dimensional form may be required as in the case of the helmet liner illustrated in fig1 - 3 . fig6 and 7 illustrate two examples of different shapes of protuberance 1301 , 1401 which allow the material of the protuberance to have a changing effect as the degree of compression increases . it will be understood that a corresponding recess is provided in the adjacent layer . in fig6 , a broad protuberance 1301 has a first portion 1302 comprising a regular conical tip 1303 with an included angle in the range 80 - 120 °. a second portion 1304 comprises a regular circular supporting pillar 1305 which constitutes the main body of the protuberance , and has a slight outward taper in the range 5 - 15 ° towards the base . the first portion 1302 has an axial height which is about 25 % of the total height of the protuberance . in this embodiment the base region 1306 is of substantially constant thickness across the layer . the protuberance 1301 exhibits a resistance to compression which increases quickly over the tapering point 1303 . the main body 1305 of the protuberance is of substantially constant section , and exhibits substantially increased stiffness . the shaft taper of the main body ensures a snug fit in the corresponding recess . in fig7 , a slim protuberance 1401 also has a first portion 1402 comprising a regular conical tip 1403 with an included angle in the range 30 - 60 °. a second portion 1404 comprises a tapering shallow frustoconical base 1405 having an included angle in the range 120 - 160 °. the first portion 1402 has a height which is in the range 75 - 125 % of that of the second portion 1404 . as illustrated the height of the first portion 1402 is greater than that of the second portion 1404 . in this embodiment the base region 1406 , as before is a substantially constant thickness across the layer . the protuberance 1401 exhibits a resistance to compression at the tapering point 1403 which is slight . the main body 1403 of the protuberance permits only further compression before the entire base thickness 1404 is engaged to resist compression . it will be appreciated that the protuberance 1401 squashes down more readily than the protuberance 1301 . fig8 illustrates a double compressible liner , of the kind shown in fig4 , incorporating the triple layered construction of the present invention whereby a common inner layer 1524 is surmounted by respective intermediate layers 1560 and outer layers 1528 on either side . in the embodiment of fig8 it will be understood that the inner layer 1524 may be constituted by a single component such as a one - piece moulding , or may comprise two inner layers of single compressible liners placed back to back and secured together , if required , by any suitable means . this embodiment may also be characterised on a common outer layer ( placed innermost ) surmounted by respective intermediate and inner layers . fig9 - 15 illustrate the variety of configurations which are possible with the interlocking structure of the present invention , by reference to a curved liner ( for example for a helmet ). fig9 illustrates three layers with relatively small inner cones 1601 aligned with somewhat larger outer cones 1602 , the outer cones being somewhat inward of a smooth outer surface 1603 , and the inner surface 1604 being also smooth . fig1 corresponds to fig9 , but in this case the outer cones 1602 just reach the outer surface 1603 . fig1 corresponds to fig9 and 10 , but in this case the outer cones 1602 appear in truncated form on the outer surface 1603 . fig1 illustrates a reversed cone arrangement , corresponding to fig1 , with the inner and outer cones 1605 , 1606 facing inwardly . a reversed arrangement corresponding to fig9 and 11 is also possible . fig1 corresponds to fig9 , and illustrates a somewhat narrower intermediate layer 1607 having outer cones 1608 of reduced wall thickness ; the inner cones 1609 are of somewhat greater height than those illustrated in fig9 . fig1 illustrates one element 1701 of an inner or intermediate layer , having cones 1702 in a regular pattern . the edges 1703 of the element 1701 have a male or female locking form or key 1704 , 1705 whereby adjacent elements can be retained together against transverse forces , in the manner of a jigsaw puzzle . it will be appreciated that the arrangement of fig1 permits adjacent elements to be of different material , different size and / or different compressibility . the element of fig1 is rectangular , but this aspect of the invention is not limited to edge shape — curved and non - regular shapes are possible , and may be necessary for a helmet liner . the outer layer ( not shown ) is one piece . fig1 illustrates how adjacent elements 1801 , 1802 of an intermediate layer have a junction 1803 which does not correspond with junctions 1804 , 1805 between adjacent elements 1806 , 1807 , 1808 of an inner layer . such an arrangement provides a more stable and strong construction . the outer layer 1809 is one piece . in the variations disclosed in fig9 - 13 , the cones have substantially the same apex angle , it will however be understood that the inner and outer cones may have a different apex angle , and / or be different between adjacent keyed elements . the invention comprises layers whose comparative densities ( or portions thereof ) may be characterized as follows (‘ a ’ being the outer layer ; ‘ b ’ being the intermediate layer , and ‘ c ’ being the inner layer ): densities of the respective layers ( or portions thereof ) are in the following ranges : in an embodiment of the invention , the materials of the respective layers are foam expanded polystyrene and / or a viscoelastic foam material . the material may be isotropic ( having a material property that is identical in all directions ) or anisotropic ( having a material property that preferentially shears in one direction ) to give a shearing in the direction substantially parallel to the layer direction . thicknesses of the respective layers in a helmet gives an overall thickness in the range 15 - 45 mm , but is typically in the range 20 - 30 mm . the three layers may each have a uniform thickness , which may not be equal between layers , or may have a varying thickness . a comparative impact test using a variety of anvil shapes and ambient conditions has been carried out , with the following characteristics and results . a ‘ standard ’ single layer liner had a thickness of 30 mm and consisted of expanded polystyrene foam with a density of about 60 kg / m 3 . a triple layer liner according to the invention had an average thickness of 30 mm ( 25 mm to 35 mm ) and consisted of expanded polystyrene foam having an outer layer density of 60 kg / m 3 . the middle layer had bigger cones than the inner layer . the density of the cones of the middle layer at the front , back and sides was 55 kg / m 3 , whereas on the top the density was 40 kg / m 3 . the density of the cones of the inner layer at the front , back and sides was 45 kg / m 3 , whereas on the top the density was 40 kg / m 3 ( the same as the corresponding cones of the middle layer ). the helmet angle is the rotational position of the impact , with respect to the anvil ; front being 0 °, rear being 180 ° and so on . the test helmet in which the comparative liners were tested at a standard impact , and included a dummy head of appropriate size and mass ( about 5 kg in total ). impacts were in each case translational . for impacts where the helmet was dropped onto a flat steel anvil , the drop height was 1 . 92 m and for impacts onto hemispherical anvil , the drop height was 1 . 43 m . it may be seen by comparison that the triple layer liner according to the invention provided a substantial percentage improvement ( i . e . increased compression ) over a single layer liner of the same thickness . the comparative g - forces measured during the tests exemplified in table 1 are as follows : although the invention has been herein shown and described in what is conceived to be the most practical and preferred embodiments , it is recognized that departures can be made within the scope of the invention , which are not to be limited to the details described herein but are to be accorded the full scope of the appended claims so as to embrace any and all equivalent assemblies , devices and apparatus .