164 lutvr STEAM ClGfistER. 



part covered by the fame. Fig, 4. h i is the wooclen handle 

 turning round the fcrew g, Fig. 1 , 2. to conneft the metallic 

 planes. It is a kind of button or key. 

 Peculiar fafety My next objed \yas to render the inftrument fafe, and to 

 ^h^^^^(T^^'*^^ place the operator's mind independent of any apprehenfion of 

 he^egulatedr*^ ^^^^' accidents. For this purpofe, (befides the ftrength of the 

 veflel and its different parts, which ought to be as folid as con- 

 veniently can be made,) a metallic cylinder mn is fixed to the 

 plate a 6, to the inferior open extremity of which a metallic 

 fphere m b, perforated with fmall holes, adheres by mere fric- 

 tion, allowing the vapour, but not the folid or, other matters, 

 to pafs into the cylinder, which contains a folid pifton ko, made 

 of the fame metal * and exa6tly ground into it, fo as to fink 

 down by its own weight if lifted, without however permitting 

 the vapour to efcape. This vapour when heated to a certain 

 degree, will raife the pifton above one or more of the capillary 

 holes 5 4", on the fide of the cylinder and then efcape. The pifton 

 will remain ftationary or ofcillating at a height, where the 

 quantity of fteam efcaping is equal to the quantity produced 

 in the fame time by that fire, confequently at a low point if 

 the fire is weak, or, if the fire is intenfe, at the top, where a 

 larger hole prefents a compleat iflue to the fleam and prevents 

 the poflibility of accidents. 

 Conditions for To adapt the inftrument thus conftrucled to exa6l and cora- 

 accurate experi- parative philofophical experiments, three things feem to be 

 d?gefter. i. To ''^^"^''®^' ^' *^ afcertain the degree of heat of the interior 

 know the tern- fluid contents f by a thermometer; 2. to increafe that heat at 

 Trcommand p'^afure ; and 3, to fix it invariably, if it be defired, inde- 

 the heat, and 3. pendent of the poffible increafe of the fire, during the courfe 

 To render it of the experiment. 



itcady though • r r 11 • r 1 



the fire be not The firft is obtained by means of ai fmall iron cup p q, fol- 



{°: dered into the plate, and containing fome mercury. A. fine ther- 



The temper- ^\ / , , , • ^ , - , , 



ature meafured mometer pafled through a cork, bemg fixed m the cup, the 



te^ * ermome- ^ Great care (liould be taken to make both of the fame metal, for 

 in one inftance when I had the pifton made of the fame brafs with 

 the cylinder, but of another day's cafting, this infinitely fmall dif- 

 ference was ftillfuffieient to produce a different force of expanfion, 

 in confequence whereof the pifton, which moved freely at 212% 

 ceafed to move, and was quite fattened at 260'. 



f It is perhaps fuperfluous to obferve that a digeftor is only in- 

 tended for experiments with fluids or bodies furrounded with them, 

 and confequently never contains materials in a dry ftate. 



degree 



