since the over centre mechanism means that the lever 7 only has to be driven marginally past the centre position for it to snap shut the extent of travel of the plunger 10 is limited to the distance between the brass bush 15 and the annular shoulder 13 which prevents the plunger 10 from being ejected from the casing which might itself present a hazard once the actuating device has been activated it is not necessary to replace the entire device rather the original bush 15 can simply be removed and a new tube 12 with a bush pre welded therein positioned on the end of the casing part 11 so that the plunger 10 can be depressed back into the casing and screwed to the new bush the present invention therefore provides a relatively simply and thus reliable mechanical mechanism which operates to shut the valve 2 in response to a rise in temperature in the locality of the valve a second actuating device according to the invention is illustrated in fig4 the actuating device 30 is attached to a ball valve 31 of the type fitted to liquid petroleum gas lpg tanks and is arranged to automatically move the ball valve 31 to a closed configuration when the temperature in the vicinity of the ball valve 31 rises above a predetermined level the actuating device 30 operates in the same manner as the first actuating device described in relation to fig1 to 3 a cylindrical copper tube 32 contains a plunger not visible in fig4 which is biased by a helical spring towards an outer end of the tube 32 the plunger is welded to the tube 32 using a relatively low melting point metal alloy or solder the weld restrains the plunger from moving in accordance.