j6o Defcrlpikn and Ufe of a Beam-compafs 



purchafed by Alexander Aubert, efq. ; and a third, of go inches, which !s now the property of 

 the Royal Society, is kept in their archives, and is faid to have been ufed by Mr. Bird, in 

 dividing his large mural quandrants *. Befides thefe, he made two ftandards of three feet, 

 by order of the Houfe of Commons, of which I fhall fpeak more hereafter. The mode of ufing 

 this inftrument is as follows. 



§. 8. Let the objecSt to be meafured be fuppofed to be about fix inches, and let it be 

 defired to compare it with the interval between the 20th and the 26th divifions in the 

 fcale c d : move by hand the microfcope A, with its Aiding plate, until the divifion of the 

 index at k coincide with the divifion of 20 inches on the rule oi f g; then move by hand 

 alfo the microfcope /, with its Hiding plate, and appendage I m n 0, until the index divifion 

 near m coincides with 26 on f g : the axes of the microfcopes, or centres of their crofs-wires,- 

 •will be at the approximate diftance of 6 inches. To correct this, examine if the wires of ^ 

 correfpond with a divifion on c d; if not, move the rukyj' backward or forward by the 

 {crew g till they do ; then will the microfcope b be adjufted. Now examine if the wires in i 

 cover exactly a divifion ; if they do fo, the true interval of 6 inches between the microfcopes 

 is obtained ; if not, move the. microfcope / a little by means of the fcrew / till they do, and. 

 both the microfcopes will be adjufled : then remove the rule c e d from its place, by taking out 

 the fcrews c e d, and place the objefl; to be meafured in its room, at the fame time taking' 

 care that it be exadlly in the focus of the objedt-glafs of the microfcope, in fuch a manner that 

 one extremity may. correfpond with the wires in the microfcope b ; that done, if the other 

 extremity coincide with the wires in /, the dimenfion of the obje£t is cxadtly 6 inches ; ifnot, 

 reftore the coincidence by turning the micrometer fcrew «, and tl»e divifions at will 

 give the difference in looothsand io,oooths of an inch + or — '6 inches- 

 §. 9. Defer ipt ion of the Hydrofiatic Balance. 



abed (PI. V. fig. 4.) is a box, which contains the whole apparatus when not in ufe ; and when 

 ufedj ferves as a foot to the hollow brafs pillar efg h, which is fixed into it by the four fcrews- 

 at the bottom e andy. This pillar contains another within it, and which is raifed up and 

 down about Tc irieh, by means of the fcrew x : a \s the beam, 27 inches long, and 3,9 inches 

 wide in its greatefl: diameter ; each arm of which is made hollow and conical, for ftrength and 

 litrhtnefs ; through the centre at m, pafles the axis of motion, the ends of which, when ufed, are 

 fufFercd to fall gently upon two cryftal planes, which are fet horizontally, by means of the 

 Ipirit levels k I, and the fcrews underneath the box, at c and b. The ends of this axis are of 

 hardened fteel, of a wedge-like fhape, and reduced to a fine edge, viz. to an angle of about 40"*, 

 fo as to move upon the planes with very little fridtion, and at the fame time fo hard, as (with 

 due care in ufing) to be in no danger of being blunted : to prevent which, the inner pillar has 

 a motion upwards, as has been faid,, by the fcrew x, and by means of a femi-circular arm at its 

 upper extremity, lifts the beam of its bearings, when it is not ufed, or is greatly loaded. This., 

 axis is placed carefully at right-angles to the beam ; and by means of two fmall brafs fprings 



f A faiiher account of thefe fcales is given in the appendix. 



that 



