Delicate Hydrostatic Balance. 119 



of a balance, which may be made for a few shillings, and 

 which will answer all the purposes of philosophical investiga- 

 tion, as well as the finest hydrostatic balance in existence. 



Let a slender beam of wood be procured, about eighteen 

 inches or two feet long, and tapering a little from the middle 

 to each end. Let a fulcrum of tempered steel, resembling 

 the blade of a pen-knife, be made to pass through the middle 

 of the beam a little above the centre of gravity. Similar 

 steel blades are also made to pass through the ends of the 

 beam for suspending the scales. The fulcrum rests on two 

 small portions of thermometer tube, fixed horizontally on the 

 upright support EF, Plate I, Fig. 8. The support has a 

 slit passing along the middle, to allow the needle EF to play 

 freely between the sides. A small scale made of card, and 

 divided into any number of equal parts, is placed at F, for 

 the purpose of ascertaining the point at which the needle re- 

 mains stationary. This balance possesses extreme delicacy. 

 It may even be made more sensible than that belonging to 

 the Royal Society of London. 



I have said nothing of the perfect equality of the two ends, 

 as this condition is not at all necessary to the accuracy of the 

 balance, according to the method of double weighing. 



To ascertain the weight of any body W, place it in one of 

 the scales, and bring the needle to any point, by means of 

 small shot placed in the other scale. Observe the point op- 

 posite to which the needle rests, or the middle between its ex- 

 treme points of oscillation ; remove the weight W, and put 

 into the scale as many known weights as will bring the needle 

 to the same division as before : these weights will evidently be 

 equal to the weight of the body W, whether the arms of the 

 balance be equal or not. 



For this simple and accurate method, we are indebted to 

 the sagacity of Borda. It is generally employed by the con- 

 tinental philosophers, and, though somewhat more tedious, is 

 obviously more accurate than the common" method. This 

 method is so simple and obvious, that we are surprised it was 

 not discovered as soon as the balance itself was known ; yet, 

 as the celebrated Biot justly remarks, philosophers knew the 

 motions of the heavenly bodies, and had actually ascertained 



