referring to fig1 , the exemplary gas spring 10 comprises a metal tube 22 , with a base 24 and guide bush 26 welded thereto to define a container 12 . a ram 28 extends through an aperture in the guide bush 26 . a stainless steel bellows 30 is 15 welded at one end to bush 26 and at the other end to ram 28 , thereby to define a closed or sealed space 14 together with the ram 28 , tube 22 , base 24 and bush 26 , i . e . outside the bellows 30 and inside the container 12 . the base 24 is provided with a filling aperture 32 and valve 34 whereby the closed space may be filled with an inert gas 33 under pressure , such as nitrogen . the gas is indicated by dots in the figure . in use , the pressure of the gas acts upwardly as shown in the figure on the ram 28 . the ram 28 has a circumferential stop 36 for limiting the maximum extension of the ram , upwardly in the orientation shown in fig1 . the base 24 is provided with a threaded bore 38 which receives a reduced threaded end 40 of a guide retainer 42 . the retainer has means for driving it into the thread 38 , such as a slot 44 . the retainer 42 guides and retains a mechanical compression spring 46 in the form of a stack of belleville washers 48 made of a heat - resistant metal . as is known , belleville washers are domed washers which can be stacked with the individual domes either upward or downward . by selecting the number of washers and their orientations it is possible to control the initial length and spring constant of the spring formed by the stack of washers . as can be seen in fig1 , the ram 28 has a blind internal bore 49 , and the spring 46 is disposed partially within the blind bore 49 so as to be interposed between the ram 28 and the container 12 . the spring 46 engages a step 50 on the underside of the ram 28 , in the interior of the bore 49 , to urge it in the same direction as the force of the gas pressure , i . e . upwardly in the figure . fig2 shows an alternative design in which the retainer 42 is eliminated . a spigot 54 is formed on the base 24 and serves to guide and locate some of the belleville washers 48 . others are located by the internal bore 49 and others are indeed located by both the spigot 54 and the bore 49 . fig3 shows another design in accordance with the invention in which the retainer 42 or spigot 54 and washers 48 are replaced by a compression spring 46 seated in a circular groove 56 and engaging the underside of the ram 28 , i . e . the blind end of the bore 49 . in this example the spring is fabricated in the form of a bellows 58 , but alternatively a coil spring could be used . in this case also , the mechanical compression spring 46 urges the ram 28 in the same direction as does the gas pressure . the other parts are similar to , or perform similar functions to , the equivalent parts in the fig2 embodiment . it is to be noted that in each of the above embodiments the mechanical compression spring 46 is disposed in a sealed space within the container , this being the same sealed space as contains the gas . we have found that metal springs in such a configuration can continue to operate reliably without suffering any adverse effects due to the environment . fig4 shows another design in accordance with the invention in which the base 24 has a raised nose 60 extending partially into the blind bore 49 and the mechanical compression spring 46 is seated on the nose 60 and thereby is disposed wholly inside the blind bore . this arrangement provides for the use of a shorter spring , which may be useful to give the required performance in certain applications , whilst also providing a very stable configuration against relative lateral movement of the various components . fig5 shows a stage in the assembly of the gas spring of fig1 . the guide bush 26 , ram 28 and bellows 30 form a separate part from the rest of the device and are prefabricated . the spring 46 is assembled on the retainer 42 which is driven into the thread 38 . in this embodiment the spring 46 is a stack of belleville washers only indicated schematically and they are in their relaxed condition . a clamp is used to press the guide bush 26 towards the base 24 and during this process the step 50 comes into engagement with the spring 46 and compresses it . this applies a preload on the spring 46 , resulting in the condition shown in fig1 . the bush 26 is then welded to the tube 22 and the sealed container can then be charged with gas . it will be appreciated that the force extending the ram results from both the gas pressure and the force of the mechanical compression spring . by providing a blind bore in the ram and locating the mechanical spring in the bore , a mechanical spring of appropriate length can be employed whilst still providing a compact overall design . by selecting the properties of the mechanical compression spring and by selecting the initial gas pressure , it is possible to tailor the initial force at room temperature , the force at higher temperatures , and the slope of the force / temperature curve . by using a lower initial gas pressure , the increased gas pressure inside the new gas spring as the temperature increases is less than in a gas spring where the total force is made up of gas alone . depending on the ratio between gas force and spring force the force / temperature curve can be manipulated to provide the desired shape . having a secondary spring inside the unit also addresses the problem of immediate and complete failure . once the gas has been expelled , the force from the spring remains , providing a limited amount of load to the plates . in an example of the kind shown in fig1 , the force at ambient temperature on the new gas spring was split with 275 kgf provided by the mechanical spring ( an alloy designed to withstand temperatures up to 700 ° c .) and the remaining 225 kgf provided by the gas . the forces of the new gas spring were compared against a known gas spring without a mechanical spring . both springs were taken up to temperature and compressed to their working length . the results in the table below compare the new gas spring with the known gas spring . the results are plotted in fig6 , and show that the gradient of the new spring is more gradual . in use in one example mechanism there are eight springs and the operating temperature is around 400 ° c . at this temperature one new gas spring will apply a force which is 509 kgf less than the known spring , reducing the total force by 4072 kgf . the performance and life of the gas spring are greatly improved as the external pressure exerted on the outer bellows is reduced .