266 OF THE REGULATION AND [SECT. vm. 



so as to be capable of affording the required proof at any time. Mr. Watt has 

 most justly remarked, " 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." 1 



The instruments consist of a steam gauge, the condenser gauge, and the 

 indicator. 



558. STEAM GAUGE. The steam gauge, Plate vm. Fig. 1, 18. is a short 

 bent tube of iron, nearly half an inch in diameter, with one end fixed into the 

 boiler or the steam pipe, and open to it ; with a portion of mercury in the bent 

 part of the tube. The part joined to the boiler or steam pipe is freely open to 

 the steam, which, pressing on the surface of the mercury in the pipe, raises it in 

 the other leg of the tube, which is open to the air at the upper end, and the height 

 it is raised is measured on a scale, 20, by a slender stem from a light float on 

 the surface of the mercury ; which therefore shows the elastic power of the steam 

 over that of the atmosphere. The scale should be adjusted by allowing the air 

 free access to the mercury on both sides. 



The scale is commonly divided into inches and parts ; each inch corresponds 

 to 2 inches of mercury, and to a pressure of 0'775 Ibs. on a circular inch, or to 

 0-98 Ibs. on a square inch. If each of the divisions of the scale be made 1-3 

 inches, and these each divided into 10 equal parts, the pressure in Ibs. and tenths 

 on a circular inch will be shown by the gauge. Some divide the scale into half 

 inches, then each division represents an inch of mercury. 



Sometimes a cock, 19, is placed between the mercury and the steam, so as to 

 use it or not at pleasure. 



To render the divisions of the gauge larger, Mr. Watt made his gauge pipe of 

 glass, to terminate in a cistern of mercury enclosed in an iron box. The action 

 is then like a common barometer ; the steam having free access to the surface of 

 the mercury in the cistern. 



559. CONDENSER GAUGE. This is sometimes called the barometer gauge, 

 from its resemblance to a barometer. It is made of iron tube in the form of an 

 inverted syphon, Plate vm. Fig. 1,21. with one leg about half the length of the 

 other. To the upper end of the longer leg, 24, a pipe is joined which communi- 

 cates with the condenser, and has a stop cock, 22, to open or close it. When a 

 proper quantity of mercury is poured into the short leg of the syphon, and it is 

 open to the atmosphere at both ends, it naturally stands level in the two legs. 

 A light float with a slender stem is placed in the short leg, and a scale, 25, 

 attached, which is usually divided into half inches ; and as by the exhaustion in 



1 Robison's Median. Phil. vol. ii. p. 156. 



