Si 



Mr. Maddock's Improved Barometer. [Feb. 1, 



who signs Sijjma, in end, is lo be graduated to inches, &c, 



correspondent 



a recent Number of your Magazine, in 

 support of liis "new bottle-barometer," 

 as I felt assured, on reading his first 

 communication, that some of his 

 friends, more scientific than himself, 

 would have convinced him of his 

 errors: but that has not been the case, 

 and the instrument he prides himself 

 on inventing, which is such a complete 

 compound of thermometer and baro- 

 meter as to be entirely useless, is again 

 brought before the public, and notions 

 the most erroneous and unscientific are 

 again offered. Thatall fluids expanding 

 byheatform thermometers, not barome- 

 ters, — that the pressure of the atmos- 

 phere is as great in front of a fire as in 

 a cold room, excepting the effect pro- 

 duced by the draught of the chimney, 

 though the air in the former situation 

 be ever so rarefied; and, finally, that 

 cutting off the rim of a phial precludes 

 the possibility of a drop forming on the 

 rim, which it otherwise would do : that 

 these are truths, he is not aware of. 



Open to the same criticism, I beg 

 leave to propose a barometer that 

 approaches much nearer to perfection 

 than any 1 have yet seen. In the old 

 barometers, as the mercury descends 

 in the main tube, it rises in the shorter 

 one, in the bulb, or in the saucer, ac- 

 cording as their construction is. This 

 rise prevents the mercury in the main 

 tube falling so low as it otherwise 

 would do, and produces an error in 

 extent proportioned to the area of the 

 bulb, &c. For this reason, a wide 

 saucer is more accurate than a bulb ; 

 and the merely turned tube of the 

 dial-barometers are the most inaccu- 

 rate, as this effect is not taken into 

 calculation in forming the scale ; but 

 • the graduation, as in thermometers, is 

 extended from two or three observa- 

 tions, or, as I have frequently found, 

 made in a still less careful manner. 

 This defect I consider obviated in Ihe 

 barometer I offer to the public, which 

 is thus constructed : — A straight glass 

 tube, about the size usually used, and 

 thirty-three inches in length, closed at 

 one end, is to be filled with mercury, to 

 the total exclusion of air, and fixed to 

 a frame, with the open end downwards, 

 iuimerged half an inch below the sur- 

 face of mercury in a small tumbler, 

 previously half filled with that fluid. 

 A rod of wood or metal, about one- 

 eighth of an inch square, and of the 

 length of the tube, with a ball at one 



the scale commencing at about one- 

 ciglith of an inch ironi the ball. This 

 rod, when covered with a varnish im- 

 pervious to moisture, is lo be placed 

 ball downwards near to, and parallel 

 with, the tube, and confined to that 

 siluiilion by two loose-fitting staples. 

 A silk thread, fastened to the top of 

 the rod, is to pass over a pulley, and 

 have a sufficient weight at its end to 

 float the rod in the mercury, so that 

 the of the scale may be exactly on 

 the plane of Ihe surface of the mercury 

 in the tumbler. To consult this baro- 

 meter, observe what part of the rod is 

 exactly level with the surface of the 

 mercury in the tube, and the scale 

 will give the height of the colunni, 

 showing the exact pressure of the 

 atmosphere at the place and tempera- 

 ture where the observation is made. 



This instrument is more easily con- 

 structed and cleaned than others. To 

 prove it in order, observe that no air 

 has got into the tube, and that the of 

 the scale is exactly on a level with the 

 surface of the mercury in the tumbler. 

 It should be inclosed in glass, &c. and, 

 if required, it may have an index on 

 the common plan, in addition. 



Deptford; W. Maddock, 



Nov. 20, 1823. 



To the Editor of the Monthly Magazine. 



SIR, 



HAVING read, in your valuable 

 Miscellany for October, " Addi- 

 tional remarks on phial-barometers,'' 

 I am induced to submit to your notice 

 what appears to me an improvement 

 in the construction of them, as being 

 easily made, less likely to be affected 

 by a changeof temperature, and capa- 

 ble of graduation. It consists in taking 

 a well-cleaned Florence (oil) flask, 

 into which put a few drops of water, 

 and hold it over the flame of a candle, 

 to expel the air, which will be accu- 

 rately enough effected at the time the 

 water is converted into steam ; tiien 

 immerse the neck of the flask into a 

 transparent vessel of water, from which 

 the water will ascend into the inverted 

 flask as the steam condenses. A large- 

 mouthed phial will do to put the neck 

 of the flask into, if water is cautiously 

 supplied during the operation ; it will 

 not do to substitute a phial for the 

 flask, as, owing to the inequality of the 

 thickness of the glass, it would proba- 

 bly crack on cooling suddenly. A 

 scale, 



