In his address to the miners of England he also says, "that the frequent disorders and cumbersomeness of water engines then in use encouraged him to invent engines to work by this new force; that though they were obliged to encounter the oddest and almost insuperable difficulties, yet he spared neither time, pains, nor money, till he had conquered them." In Bradley's _Improvements of Planting and Gardening_, 1718, the author thus speaks of an engine erected by Savery:— "Supposing the situation of a house or garden to be a considerable height above any pond, river, or spring, and that it has at present no other conveniency of water than what is brought continually by men or horses to it. In this case, the wonderful invention of the late Mr. Savery, F.R.S., for raising water by fire, will not only supply the defect, by flinging up as much water as may be desired, but may be maintained with very little trouble and very small expense. "It is now about six years since Mr. Savery set up one of them for that curious gentleman Mr. Balle, at Cambden House, Kensington, near London, which has succeeded so well that there has not been any want of water since it has been built; and, with the improvements since made to it, I am apt to believe will be less subject to be out of order than any engine whatever." It is remarkable that, notwithstanding the high pressure steam necessary for the operation of Savery's engine, he does not appear to have adopted the obvious expedient of a safety valve. The safety valve had been previously known, having been invented about the year 1681, by Papin, for his digester, which was a close boiler, contrived by him for stewing meat and digesting bones, by submitting them to a higher temperature than that of water boiling in an open vessel. The safety valve which has ever since been used for steam boilers of every kind is a valve which opens outwards, and is fitted to an aperture in the boiler, so as to be steam tight. It is pressed down by a weight, the amount of which is regulated by the maximum pressure to which it is intended the steam [Pg058] shall be limited. Thus, if the magnitude of the valve be a square inch, and the pressure of the steam be limited to 10 lbs. per square inch above the pressure of the atmosphere, then the valve would be loaded with a weight of 10 lbs.; but as it was found necessary to vary from time to time the limiting pressure of the steam, or the load of the safety valve, these valves were usually constructed so as to be held down by the pressure of a lever having a sliding weight upon it. By moving the weight on the arm of the lever, the pressure on the valve could be increased or diminished at the discretion of the engineer. This contrivance was first applied to Savery's engines, by Desaguliers, about the year 1717, before which year Savery died.