LEV 



Example. Suppose a spring to be on one side of a hill, and a house on 

 ;,n opposite hill, with a valley between them ; and that the spring seen 

 from the house appears by a levelling instrument to be on a level with the 

 foundation of the house, which suppose is at a mile distance from it ; 

 then (by Table) the spring is eight inches above the true level of the 

 house ; and this difference would be barely sufficient for the water to be 

 brought in pipes from the spring to the house, the pipes being laid all the 

 way in the ground. 



In the above Table, the effects of refraction have not been considered, 

 which, however, should not be neglected, if the distances are consider- 

 able. In that case, the correct formula is 



which expression includes the effects both of curvature and refraction. 

 See Refraction terrestrial. 



LEVER. 



Levers may be divided into three kinds. In levers of the first kind, 

 the fulcrum is between the power and the Aveight, as in the balance, 

 steelyard, scissors, poker, &c. In levers of the second kind, the weight 

 is between the fulcrum and the power, as in oars, doors, cutting knives 

 fixed at one end, &c. In levers of the third kind, the power acts be- 

 tween the fulcrum and the weight, as in tongs, sheers for sheep, mus- 

 cles of animals, &c. 



1. Two weights or forces, acting perpendicularly upon a straight lever, 

 will balance each other, when they are reciprocally proportional to their 

 distances from the fulcrum. 



Cor. 1. When the power and weight act on the same side of the ful- 

 crum, and keep each other in equilibrio, the weight sustained by the 

 fulcrum is equal to the difference between the power and the weight. 



Cor. 2. If the same body be weighed at the two ends of a false balance 

 (one arm of which is longer than the other), its true weight is a mean 

 proportional between the apparent weights, 



Cor. 3. If a weight be placed upon a lever supported upon two props, 

 the pressures upon the props are Inversely proportional to their distances 

 from the weight. 



2. If two forces, acting upon the arms of ant/ lever, keep it at rest, they 

 are to each other inversely as the perpendiculars drawn from the centre 

 of motion to the directions in which the forces act} or inversely as th* 

 arms, multiplied into the bines of the angle*, which, the direction of the 

 forces make with them. 



