SPEEDY ELEVATOR. 



preflure of thefe piers on the cords «^, ZZ3L-I-3D-I-2CG 

 + B 13. The power on the windlafs muft be equal to this. 



In any feries the power muft lift a weight equal to the 

 firft piers, double the fecond, aid fo forth, till the laft pier 

 and its load multiplied by the number of moving pieces. 



The pulleys, cords, and ribs have fome weight, and 

 are to be counted as parts of their refped;ive piers. A 

 competent allowance is likewife required for the friftion of 

 the pulleys, which impedes the afcent, though the defcent 

 is advantageoufly retarded by it. 



The preflure on the pulleys of the pillars A A is double 

 the power. The ftrain in their tops is half of the weight 

 on the pulleys of b H. The difference of thefe forces, 

 z=4L + 4D + 3CCH-2BB, added to their own weight 

 is the preflure on the bafe. 



The ftrain in the tops of any piers in a feries is equal 

 to what the power would be, it the pair next above vv^as 

 the laft. The weight on the pulleys of the fame pair is 

 double the ftrain in the tops of the pair next below. The 

 flrain on their feet is equal to the ftrain in the tops of 

 the piers two ranks below. 



It is very neceffary to compute the ftrains and preffures 

 in order to fecure all the parts, and to fave needlefs bulk, 

 which would be a great difadvantage in the piers by the 

 additional expenfe of power. The preflure of vertical 

 pieces by their own weight muft be counted, though not 

 as equal to the fame quantity of external burden laid up- 

 on them : its operation is vifible in high maffive beams, 

 which bend without any load ; but in Ihort though flender 

 pieces it is not fufficient to break the internal cohefion of 

 the parts. The effed of external weight is according to 

 its quantity, and to the height and flimnefs of the piers ; but 

 not in uniform proportions. Divers kinds of wood have 

 alfo different degrees of weight, and of vertical firmnefs : 

 fome are both ftiff and light to an admirable degree : piers 



made 



