To hold a torpedo at a fixed depth a very delicate hydrostatic valve is employed which operates the diving rudders. The valve is set for a certain depth or weight of water. If the torpedo goes below this depth, the weight of the water bearing on this valve presses it down and thereby tilts the diving rudders until the torpedo comes back to the required depth. If it rises above the depth for which it is set, the valve feels the reduction of water pressure and tilts the rudders in the opposite direction to correct the deviation from the predetermined line of travel. Once the valve has been set the torpedo takes care of itself automatically. It may be discharged from any depth or be dropped from torpedo tubes on the deck of a ship and after a few undulations it will find its depth and hold it as long as it keeps running. Torpedoes have actually been discharged from aeroplanes and it is possible that in the next great war swiftly flying aeroplanes may actually bring death and destruction to powerful dreadnoughts. In fact winged destroyers and battle cruisers of the air may render obsolete all our modern fighting machines. As we have already observed, it was war that first aroused the inventive instincts in man. Fortunately this instinct was not confined to destructive devices and engines, and the mechanical conquest of the earth, although it continues to derive much benefit from the stimulus of war, may well afford to break off relations with so grim and horrible a partner and proceed to develop under the stimulus of its own successes. CHAPTER XXIV SUMMARY OF MECHANICAL PROGRESS THE REAL beginning of the present age of machinery dates from Watt’s invention of the double-acting steam engine which was patented in 1782. Prior to that date the steam engine was used merely for pumping water, but now in its new form it was adapted for use in driving industrial machinery. But, as we have shown in another chapter, it was not until John Wilkinson invented his boring machine in 1774 that it was possible to bore the cylinders that Watt used in his engine. In the field of machine tools there were several notable inventions. Henry Maudslay invented the slide rest for lathes and later in 1797 the screwcutting lathe. Joseph Bramah invented the hydraulic press and Maudslay furnished the cup leather packing that made it a success. Woodworking machinery, and particularly mortising machines for making pulley blocks, were invented by Samuel Bentham and patented in 1791 and 1793.