[ + 2 4 ] 
wood, which ill that place fills a groove originally 
made throughout the whole length of the lead- 
board. 
50. To keep the dull; from getting through the 
opening of the tube, I (hut it up in a fmall ivory 
cafe. It cannot be lealed up, becaufe if air was left 
in, it would obflruft the riling of the mercury ; and 
if it was exhaufted, the mercury would be pufhed to 
the top by the preffure of the atmofphere upon the 
ivory pipe ; as 1 have experienced it. 
51. Hence however arifes a fmall inconveniency ; 
which is, that as the upper part of the column of 
mercury communicates with the air, if it rein fins 
long in the fame part of the tube, or moves but 
little in it, fome dirt may be left on the tides. This 
I ealily remedy, by means of a brafs wire, the extre- 
mity of which is dentated in the form of a file, in 
order to hold fome bits of cotton, which I put round 
it. The wire is eafily introduced into the tube, by 
means of the widening mentioned before (35). I 
put it in, when the mercury is below the part it has 
foiled, and eafily clean it by this means. It is on 
this account that the tubes to be employed are to be 
of about i. of a line internal diameter. 
52. The fcale of the hygrometer is marked upon 
a deal flip, which Aides along the groove I mentioned 
before (49). This, as well as all the other parts of 
the frame, mult be lined with paper, to mark the ne- 
ceflary fcales ; and this paper is afterwards varnilhed 
over. Thin plates of filvered brafs can be employed 
for the fame ufe. 
53. The mobility of the fcale of . the hygrometer 
ferves to corredt, in the obfervation itfelf, the effect 
1 of 
