146 MACHINE FOR SAVING PERSONS 



arch. The lever having come to a horizontal pofitlon, 

 the arm by the continued rotation returns towards the 

 line a K, and makes the rope retrace the arch until 

 the (haft is down. If the reverfed angle of the arm is 

 then equal to a iiG, the rope comes to the point G from 

 whence it went, if lefs it goes beyond. In the firfl 

 cafe EG is the fide of a triangle which has aa for its bafe 

 and two angles equal to a EG ; in the fecond it is lefs 

 than this fide. As the elevation of EF is not above 

 50, its complement rfEG (40) is greater than the angle 

 of EF with the Ihaft, which with that fpan would hold 

 the arm too high from the ground. It confequently ex- 

 ceeds the reverfed angle w^hich (being oppofite) is equal 

 with this. Therefore the Ihort arm is lefs than the faid 

 fide, but the difference is not great. — The fituation of 

 the fhaft being mod critical in the higheft elevation, 

 the rope fliould then allow the lever very moderate rota- 

 tion, and thus leflen the kicking of the weight againft 

 the fulcrum. Its pofition will therefore be nearly per- 

 pendicular, when the defcent begins, that the arch, im- 

 mediately falling, or rifing very little, may keep down 

 the end of EG. This pofition requires a moderate ele- 

 vation of EF ; for if the angle DEG is too contraded, 

 EG will projevft far beyond the foot of the rope, in and 

 about its horizontal paflage, and thus produce a detri- 

 mental obliquity. It alfo limits the fall of EF, by re- 

 gulating the reverfed angle ; for the nearer the point G 

 returns to the fhaft, the further it goes from the rope, 

 and the more oblique is the outward draught. 



The obliquity of draught admitted in this machine, 

 appears eafy on the model : trials may prove a greater 

 elevation of the lever pradlicable. The proportion of the 

 fhort arm is fufficient while the whole weight to be 

 wound up does not exceed that of twelve perfons ; for 

 fo much, at leaft, four hands at the windlaifes are com- 

 petent. 



