38 Mr. S. Seaward*s Description of an improved Gauge 



lubec; it is quite plain, that when the mercury is forced by 

 the pressure of the gas or fluid into the chamber A, and rises 

 up to the bottom of the glass tube, the air in the tube C must 

 then be compressed 20 times, and will consequently indicate 

 a pressure of 20 atmospheres; and if the mercury is then 

 raised halfway up the tube C, the air will then be compressed 

 40 times, and will indicate a pressure of 40 atmospheres. 

 We have considered that the glass tube C should be eight 

 feet long ; but if, instead of eight feet, we make this tube only 

 four feet long, and fix to the top a small hollow ball D, of 

 equal capacity with four feet of the tube ; then if the mercury 

 rise four feet high in the tube, or up to the ball, the apparatus 

 will still indicate a pressure of 40 atmospheres^ as if the tube 

 eight feet long had been employed. 



By this means we have a compact instrument, which will 

 indicate in a satisfactory manner the various pressures within 

 the range of 20 and 40 atmospheres ; for the divisions on the 

 scale for the whole of this range will be as large as if a tube 

 of the common form 70 feet long had been employed. 



But it may be objected that this instrument is imperfect, 

 because it will not indicate any lower pressure than 20 atmo- 

 spheres, nor any higher than 40. To this it should be ob- 

 served, that in all practical applications of a gauge of this 

 sort, as the compression of gases, &c, it is only within certain 

 limits that it is desirable to ascertain the exact pressure : for 

 instance, in the case alluded to, it is of no consequence to 

 ascertain the pressure when below 20 atmospheres ; nor is it 

 ever required to be elevated above 32 atmospheres; therefore 

 a range, as here proposed, of from 20 to 40 atmospheres is 

 quite sufficient for practical purposes in working the appa- 

 ratus of a portable gas establishment, &c. Perhaps it is unneces- 

 sary to observe, that the various ranges may be altered at 

 pleasure, from the lowest pressure up to the greatest ; for by 

 properly arranging tl»e different sizes of the ball D and cham- 

 ber A, it may be made to indicate from 1 to 20 — from 20 to 

 40— from 40 to 60, and 60 to 100 atmospheres. And if it 

 should ever happen to be necessary to ascertain the exact 

 pressure of the fluids from 1 atmosphere to 100, which is a 

 tning perhaps never wanted, it can still be done with the 

 same degree of exactness throughout ; for, by using two or 

 more of these gauges graduated for different ranges, we thus 

 obtain a correct scale of reference, which it would be impos- 

 sible to do in the old method; say from 1 to 15 — 15 to 45 — 

 and 45 to 100. 



It is here proper to observe, that the weight of the column 

 ol mercury in the chamber A, and in the tube C, should be 



taken 



