248 DESCRIPTION of an 



yet nearly fill the caliber. It is to be allowed to move down- 

 wards till it be fully immerfed in the fpirit. After this has 

 once been effected, it will be found, that it is not difpofed to 

 part again from the fpirit ; but if the thermometer be held ver- 

 tically, with the bulb uppermoft, it will immediately defcend 

 to the extremity of the column but no farther : 1 here it reds ; 

 and if by a diminution of heat the fpirit contract, it is drawn 

 upwards at the extremity of the column, as this recedes towards 

 the bulb. Now, let the thermometer be Co difpofed, that its 

 ftem, inftead of being vertical fliall be horizontal, (and fuch is 

 the ordinary pofition of this thermometer), it may readily 

 be imagined, that, in this cafe, the conical piece fhall equally, 

 as before, follow any retraction of the column ; but mould this 

 lengthen again, in confequence of an increafe of temperature, 

 the conical piece does not advance with it, but remaining fixed 

 at the lowed point to which the column had defcended, it allows 

 the liquor to pafs freely beyond it, as that again expands. Hence 

 the point of the fcale at which the conical piece is found to reft, 

 denotes the lowed degree to which the liquor of the thermo- 

 meter has funk in the interval of the obfervations. To rectify 

 the inftrument for a frefh obfervation, nothing farther is requi- 

 lite than to elevate the bulb of the thermometer, in order that 

 the conical piece may fink, by its proper gravity, to the extre- 

 mity of the column. 



2. If it be required, that the thermometer fhould mark the 

 highed point to which the fluid has afcended in any given 

 time, then a mercurial thermometer is to be employed, fuch as 

 is reprefented by D E ; into the tube of which is introduced a 

 conical piece of ivory F, with its bafe turned towards the bulb. 

 When fuch a thermometer is placed in a vertical fituation, 

 the bit of ivory will fall down, and red upon the furface of 

 the mercury in the tube ; it will rife as the column is 

 lengthened, and defcend as this contracts. But if the Item be 



placed 



