B 



B A L A NCK (Playfair. ) 



The balance, when well constructed, must have the following proper- 

 ties. (1.) It should rest in a horizontal position, when loaded with equal 

 weights. (2.) It should have great sensibility, i. e. the addition of a 

 small weight in either scale should disturb the equilibrium, and make 

 the beam incline sensibly from the horizontal position. (3.) It should 

 have great stability, i. e. when disturbed, it should quickly return to a 

 state of rest. 



That the first requisite may be obtained, the beam mu^t have equal 

 arms ; and the centre of suspension must be higher than the centre of 

 gravity. Were these centres to coincide, the sensibility would be the 

 greatest possible, but the other two requisites of level and stability would 

 be entirely lost. 



The 2d requisite is the sensibility of the balance. If a be the length of 

 the arm of the balance, and b the distance between the centre of suspen- 

 sion and the centre of gravity, P the load in either scale, and W the weight 



of the beam, the sensibility of the balance is as j-rr^ ^TT it is .". 



( f -f- W) 



greater, the greater the length of the arm, the less the distance between 

 the two centres, and the less the weight with which the balance is load- 

 ed. 



Lastly, the stability is proportional to (2 P -f- W) b. The diminution 

 of b ^., while it increases the sensibility, lessens the stability of the ba- 

 lance. The lengthening of a will, however, increase the former of these 

 quantities, without diminishing the latter. 



Hence the merit of balances depends upon the quantities a t b, and W. 



BALLOON. See JEronautics. 



BALLS iron and leaden, weight of. See Shot 



BAROMETER. 



1. Barometer, scale of. 



The usual scale of the Barometer is 31 very dry, or hard frost j 30. 5. 

 1 fair or frost; 30 fair or frost ; 29. 5. changeable ; 29 rain or snow ; 

 '. : '-. 5. much rain or snow ; 28 stormy. ( Young's Nat. Phil) 



, Barometer^ measurement of heights by. 



- rofessor Robison's formula in feet, without logarithms. 

 let / = mean temperature of air at the two stations ; d =. difference 

 33 



