SPEEDY ELEVATOR. 525 



Oblique prefTures cannot arife from the principles of 

 conftruclion ; but happen from inevitable imperfection of 

 materials and workmanihip in a linall degree, which is not 

 an objedt of ex ad calculation, but lliould have full allov>ranci; 

 for its efieds on the machine. The obliquity will be thu 

 greater as the folding is {hallow, and the fitting is loolc. 

 The efftd rcfults jomtly from the angle of declination, 

 the length of the pier, and its moment of weight. The 

 oblique bearings on the ends of the tongues, when the 

 machine is wound up will be dangerous, if thefe have not 

 a competent folidity. 



i^U the piers with their moments of weight bear on the 

 pillars ; and the preflure on their pulleys is the difference 

 between double the power and the ftrain of their heads, 

 which balance is very great. This prelfure remains there 

 when the machine is wound up, in every ftage of the ele- 

 vation, however great. "1 he common centre of gravity 

 of the pulleys thus prefled, the pillars themfelves, and the 

 bafe, is below the pulleys. Thus the machine has a great 

 ftability, and the bafe is accordingly not cxtenfive. 



This machine combines thefe advantages : ready approach 

 to heights othernife not a< ceflible without great trouble : 

 fpeedy afcent and defcent : convenient folding for keeping 

 under cover, and for eafy conveyance. It can be applied 

 to feveral ufeful purpofes : — Quick hoifting and lowering 

 of things on many occafions ; particularly faving of goods 

 from upper ftories in cafes of incend : High elevation and 

 fpeedy exchange of fignals : thefe being light may be raifed 

 three hundred feet, and above interjacent hills : hlevation 

 of a perfon for taking views, and quick defcent when re- 

 quired ; as on reconnoitring an enemy within (hot : a ma- 

 chine calculated for lifting him at leafl one hundred feet by 

 eight men can be hght enough for carrying on a waggon 

 by two horfes. 



