The application of the centrifugal principle was not a new invention, but had been applied by others to the regulation of water and wind-mills, and other things ; but Mr. Watt improved the mechanism by which it acted upon the machines, and adapted it to his en gines.
From the beginning, Mr. 'Watt applied a gage to show the height of the water in his little boiler, which consisted of a glass tube communicating at the lower end with the water in the boiler, and at the upper end with the steam contained in it. This gage was of great use in his experiments, but in practice other methods arc adopted. He has always used a barome ter to indicate the degree of exhaustion in his engines. Sometimes that instrument is, as usual, a glass tube 33 or 34 inches long, immersed at bottom in a cistern of mercury, and at top communicating by means of a small pipe and cock with the condenser. The oscil lations are in a great degree prevented by throttleing the passage for the steam by means of the cock.
But as glass tubes were liable to be broken by the workmen, barometers were made of iron tubes, in the form of inverted syphons, one leg being about half the length of the other. To the tipper end of the long leg a pipe and cock were joined, which commu nicated with the condenser ; .a proper quantity of mercury was poured into the short leg of the syphon, which naturally stood level in the two legs : A light float with a slender stem was placed in the short leg, and a scale divided into half inches applied to it, which (as by the exhaustion the mercury rose as much in the long leg as it fell in the short one) represented inches on the common barometer.
The steam-gage is a short glass tube with its lower end immersed in a cistern of mercury, which is placed within an iron box screwed to the boiler steam-pipe, or to some other part communicating freely with the steam, which, pressing on the surface of the mercury in the cistern, raises the mercury in the tube, (which is open to the air at the upper end) and its altitude serves to show the elastic power of the steam over that of the atmosphere.
These instruments are of great use where they arc kept in order, in showing the superintendent the state of the engine ; but slovenly engine-tenders are but too apt to put them out of order, or to suffer them to be so. It is the interest, however, of every owner of an
engine to see that they, as well as all other parts of the engine, are kept in order.
The barometer being adapted only to ascertain the degree of exhaustion in the condenser where its vari ations it small, the vibrations of the mercury ren dered t very difficult, if not impracticable, io ascertain the state or the exhaustion of the cylinder at the dif ferent periods of the stroke of the engine ; it became therefore necessary to contrive an instrument for that purpose, that should be less subject to vibration, and should show nearly the degree of exhaustion in the cylinder at all periods. The following instrument, called the Indicator, is found to answer the end suf ficiently. A cylinder about an inch diameter, and six inches long, exceedingly truly bored, has a solid piston accurately fitted to it, so as to slide easy by the help or some oil ; the stem of the piston is guided in the direction of' the axis of the cylinder, so that it may not be subject to jam or cause friction in any part of its motion. The bottom of the cylinder has a cock and small pipe joined to it, which, having a conical end, may be inserted in a hole drilled in the cylinder of the engine near one of the ends, so that by opening the small cock, a communication may be ef fected between the inside of the cylinder and the in dicator.
The cylinder of the indicator is fastened upon a wooden or metal frame, more than twice its own length ; one end of a spiral steel spring, like that of a spring steelyard, is attached to the upper end of the piston-rod of the indicator. The spring is made of such a strength, that when the cylinder of the indi cator is perfectly exhausted, the pressure of the at mosphere may force its piston down within an inch of its bottom. An index being fixed to the top of its piston-rod, the point where it stands, when quite ex hausted, is marked from an observation of a barome ter communicating with the same exhausted vessel, and the scale divided accordingly.