Progress of Invention. 77 



is easily moveable on the rod, and the other is connected with the 

 rod : or one end is fixed to a support, and the ; upper end of the 

 pendulum is attached to the other — the arrangement being such that 

 when the spiral rod expands or contracts, it raises or depresses the 

 bob on the pendulum rod, or raises or depresses the pendulum itself 

 through the fork ■ the effect being a practical shortening or lengthen- 

 ing of the pendulum exactly to the extent that the change of tempera- 

 ture has lengthened or shortened the pendulum rod; the acting length 

 of the latter, therefore, never alters. In applying this principle to 

 the watch or chronometer, the spiral spring and the compensator 

 are made of the very same material and dimensions : and it is so 

 arranged that, when change of temperature alters the length of the 

 spiral spring, the same change alters that of the compensator which 

 acts on the "regulator;" and thus the leDgth of the acting part of 

 the spiral spring is kept without change. 



Improvement in the Safety Valve. — Without an effective and 

 reliable safety-valve, the use of a steam-boiler must be constantly 

 attended with the most imminent danger, Whatever care may be 

 bestowed by the manufacturer on the construction of the safety-valve, 

 it may be rendered nugatory by the ignorance or temerity of the 

 person in' charge of the engine : since h e may overload the valve, and 

 thus create a pressure within the boiler which it was not constructed 

 to endure, and which it may not be capable of bearing. The evil may 

 indeed be prevented by the use of two safety-valves, one of which is 

 beyond the power of the engine man. But ingenuity has devised a 

 still more simple remedy ; one that not only prevents the production 

 of steam at too high a pressure, but which actually causes every 

 attempt to produce it to be accompanied by a reduction of pressure, 

 and thus removes all temptation to tamper with the valve. The 

 new form of safety-valve differs little from the ordinary kind, and is 

 extremely simple. In the ordinary kind the fulcrum of the lever is 

 absolutely immovable : in the new kind, it is fixed; in ordinary circum- 

 stances, being kept down by a spiral spring. But attempting to 

 overload the valve brings the lever down on a stud, which is at the 

 side of the valve most remote from the fulcrum, and which comes 

 into action as a new fulcrum by supporting the lever, changing the 

 latter from the third to the first order. The former fulcrum yields to 

 the additional weight, the spiral spring being compressed, and 

 is raised up, the safety-valve being at the same time opened, or 

 allowed to open ; and thus the steam is permitted to escape, though 

 at a pressure too small to raise the valve when weighted as it should 

 be. In its normal state the fulcrum of the lever is at one end, the 

 weight at the other, and the power — that is the tendency of the 

 safety-valve to rise — between the fulcrum and weight. When the 

 valve is overloaded, the weight — that is the resistance of the spiral 

 spring — is at one end, the power — that is the weight with which the 

 lever is loaded — is at the other, and the fulcrum — that is the stud on 

 which the lever has been brought down by the overloading — is 

 between the power and weight : the effect of the latter being aided 

 by the tendency of the steam to raise the valve. 



A New Glass for Optical Purposes. — The density imparted by 



