that the coefficient of thermal expansion of the actuator ring be different than that of the shroud support 23 since it is this shroud support or an extension thereof the t shaped hanger bracket 37 to which the actuator is attached at its base or fixed end 57 the other end 54 of the actuator ring is of course free to grow relative to the shroud support 23 in response to temperature changes to thereby rotate the ring gear 52 in operation the cooling air which is bled off from the compressor so that its temperature is dependent on the speed of the engine is introduced by the conduit 19 into the plenum 21 where it directly contacts the shroud support 23 to change the temperature and therefore the size thereof a portion of the air is directed through the holes 61 for the cooling of the low pressure nozzles 16 since the shroud support 23 the t shaped hanger bracket 37 and the actuator ring 53 are all so closely associated they are at substantially the same temperature further since the coefficient of thermal expansion is different for the shroud support 23 and the actuator ring 53 the actuator ring 53 will grow or shrink with respect to the shroud support 23 as the temperatures thereof are changed this relative thermal growth or shrinkage will in turn rotate the ring gear 52 and the gears 51 to cause a radial movement of the shroud segment 12 in a pattern which facilitates the.