VOL. LXVII.] PHILOSOPHICAL TRANSACTIONS. 2/1 



Rules to be observed in adjusting the boiling point. — The most accurate way 

 of adjusting the boiling point is, not to dip the thermometer into the water, but 

 to expose it only to the steam, in a vessed closed up in the manner represented 

 in fig. 14, where Asba is the vessel containing the boiling water, Dd the cover, 

 E a chimney made in the cover intended to carry of the steam, and Mm the ther- 

 mometer passed through a hole in the cover. Those who would make use of 

 this method must take care to attend to the following particulars. 



]st. The boiling point must be adjusted when the barometer is at ig.S inches; 

 unless the operator is willing to correct the observed point in the manner directed 

 below. 2dly. The ball of the thermometer must be placed at such a depth within 

 the pot, that the boiling point shall rise very little above the cover, for otherwise 

 part of the quicksilver in the tube will not be heated, and therefore the thermo- 

 meter will not rise to its proper height. The surface of the water in the pot also 

 should be at least an inch or two below the bottom of the ball; as otherwise the 

 water, when boiling fast, might be apt to touch the ball: but it does not signify 

 how much lower than that the surface of the water may be. 3dly. Care must 

 be taken to stop up the hole in the cover through which the tube is inserted, 

 and to make the cover fit pretty close, so that no air shall enter into the pot that 

 way, and that not much steam may escape. A piece of thin flat tin plate must 

 also be laid on the mouth of the chimney, so as to leave no more passage than 

 what is sufficient to carry off the steam. The size of this plate should be not 

 much more than sufficient to cover the chimney, that its weight may not be too 

 great; and the mouth of the chimney should be made flat, that the plate may 

 cover it more completely. It must be observed, that when the tin plate is laid 

 on the mouth of the chimney, it will commonly be lifted up by the force of the 

 steam, and will rattle till it has slipped aside sufficiently to let the steam escape 

 without lifting it up. In this case it is not necessary to put the plate back again, 

 unless by accident it has slipped aside more than usual. If the artist pleases, he 

 may tie each corner of this plate by a string to prongs fixed to the chimney, and 

 standing on a level with the plate, as thus it will necessarily be kept always in its 

 place;* but we would by no means recommend having it made with a hinge, as 

 that might be apt to make it stick, in which case the included vapour might be 

 so much compressed as to cause an error. We would also by no means advise 

 lining the tin plate with leather, or any other soft substance, for the sake of 

 making it shut closer, as that also might be apt to make it stick. The chimney 

 also ought not to be made less than half a square inch in area; for though a 



* Fig. 13 is a perspective view of the chimney and tin plate ; a bcd is the plate ; e the chimney; 

 Ff, Gg, Mm, and sn, the prongs fastened to the chimney, to which the 4 corners of the plate are to 

 be tied by the strings af, bg, cm, and dn ; the ends f, g, m, and n, of the prongs must be on a 

 level with the plate, and the strings should not be stretched tight. — Orig. 



