[ 8i ] 



In making this little inftrument care muft be taken to form 

 it fo, that when the frame B C, and its appendage D F, are fcrewed 

 together, then the common axis ^ G of the whole fhall pafs 

 through the centre of the plate ^ and be at right angles to its 

 plane ; for when this is the cafe, if it be contrived to fuftain the 



axis 



and if made of fteel, to be poliflied anew, fhould it ever contrail ruft ; to prevent 

 which it would be beft to keep always a drop of oil in it. The wire fliould be fo 

 thick at the place of the fcrews that its diameter may be only about -iV of an inch 

 lefs than the internal diameter of the pipe, and when the inftrument is adjufted the 

 wire fliould be as nearly as poffible in the middle of the pipe ; for if it were removed 

 too much to one fide, each pair of the fcrews, whofe ends are all equidiftant from 

 one another, would not prefs the wire in points of it diametrically oppofite ; in which 

 cafe the withdrawing of one fcrew alone might not permit the motion of the wire 

 to that fide, but it might be requifite to cafe two contiguous ones, and create a ne- 

 ceflity for altering all the fcrews, which would render the adjuftment very trouble- 

 fome. This will be avoided by making the wire as thick as it may conveniently be 

 juft at the place of the fcrews, and by having the pipe fixed at right angles to the 

 mirror. 



Another method of appending the weight to the ftem is reprefented enlarged 

 in fig. 6. The lower part of the ftem is formed as at S T, and a pin P made 

 of hardened fteel is fcrewed into it, in whofe end is a conical cavity with a 

 fine apex to receive the fine and hard point of a hook H, made of a thin plate 

 of fl;eel and fafhioned as in the figure : through a hole in the hook at h a ftring is 

 put to fuftain the weight, which it is plain will thus pull the ftem only by that point 

 in its axis which is the vertex of the conical cavity. Whether this or the above 

 method of fufpending the weight is the belt, I can't fay, though I have tried both ; 

 but would rather recommend the former as being more fimple, and confcquently 

 fitter for a portable inftrument, which fliould confift of as few parts and common 

 materials as poffible, left any fliould be loft, deranged, or damaged. I made trial of 

 a fmall ring (to hang the weight from) paffed through the end of the ftem which 

 was made thin, but it would not anfwer. 



Vox. VIII. L 



