I 83 ] 



fuHicicnt to cover the weight, and the upper part is to have a 

 door in the fide to allow the infirument to be ftifpended and 

 its weight hung by a hook from the loop of the thread /, or its 

 vibrations to be ftopt ; for which purpofes the door fliould de- 

 fcend to near the furface of the water : it is to be fliut at other 

 times to prevent the wind from agitating the level. 



From this defcription it is evident, that when the infirument 

 is fufpended from the point A\ under the centre of the mirror, 

 and the weight appended to G, or the middle point of the hole 

 in the axis of the wire, thefe two points would be always in the 

 line of the diredion of gravity if the inftrument itfelf had no 

 weight, or if that line paffed through the centre of gravity of 

 the whole machine ; but if the glafs plate and metal frame could 

 be divided into two parts, of unequal weight, by a plane paffing 

 in any diredlion through ^ G, then the axis A G would be 

 thrown out of the diredion of gravity, and confequently the 

 mirror out of the level more or lefs as the weight appended at 

 C is lefs or more : this can fcarce happen if the point of fuf- 

 penfion N be in the centre of the plates ^/ and B, each of the 

 plates B, C and D be of uniform thic'knefs, the pillars <7, a, of 

 equal fize, equidiftant from A', diametrically oppofite and at right 

 angles to the plates B and C, and the flem E F, in the middle 

 of the plate Z), and perpendicular to it ; and thus the line J G 

 be the vertical axis of the whole and of each of the parts : this 



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