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MN, and are furniflied with locking nuts //" on the outfide, to 

 prevent their loofening: when the inftrument is thus puilied up- 

 ward in the cafe, the needle-cap at L is raifed above the needle, 

 and cannot injure its point, but may be let down upon it. again 

 by withdrawing the fcrews DD. To fteady the weight and 

 mirror, any convenient liquor, as water, vinous fpirit, &c. is 

 poured into the cafe through the open orifice in its fide, to the 

 hei2;ht of the rina; F F^ and may be let out again by another 

 orifice, (occafionally flopped), near the bottom ; to prevent which 

 from running out at the fcrews DD^ plates of leather are inter- 

 pofed between the nuts ff and the bottom of the cafe : and if a 

 ring of leather were likewife fattened on the Shoulder g of the 

 weight PP^ or in the indenture h of the ring F F, and frefii 

 linfecd cil, efl'ential oil, or any other, which would not ruft the 

 needle, werelubfiiluted inRcad of water (if by experience oil fhould 

 be found admiflible) it would, by the prelTure of the {houlder of llie 

 weight PP., when fcrewed up againft the leather at h^ be always 

 kept in the bottom of the vefTel from running out, and nothing 

 would be wanting to fit the inftrument at all times for obferva- 

 tion but to take off the cover HH, to withdraw the fcrews P> D, 

 (keeping the nuts^yin their places, and holding the cafe uprigiit 

 that the oil may remain at the bottom), and then to fet down 

 the inflruiTient. But the fame oil could not be long ufed ; for 

 unlefs every part of the metal which it could touch were coated 

 with tin, it would grow foul and vifcid by diflblving the brafs, 

 as it would alfo of ilfelf in time; and in cold weather and 



climates 



