PROM HOUSES ON FIRE. 145 



fufficieivt firmnefs againft oblique preflures, arlfing from 

 the action of the machine, or from the cafual inchnation 

 of the bafe. The part from the brim to the pulleys in- 

 clufively is fortified by the iron-band BB, to fecure the 

 fhaft when drawn up. The height of that piece, its own 

 weight, the prefTure on its head, and the obliquity of 

 the fame at the time, together with probable external ac- 

 cidents, will determine the width ; it need perhaps not 

 exceed a fifth of the part below. 



The trunk and fliaft require ftiff and hard materials. 

 The lever being made of tough wood, may be further 

 ftrengthened by iron plates. The folidity and fhape of 

 the fulcrum and arms mufl: be carefully proportioned to 

 their refpedive weights. 



The fhaft, when down, reaches beyond the trunk fo 

 much as to clear the lever of the piers. This added to the 

 diflance of the brim from the pulleys, is the difference 

 between the fhaft and their height .i a. The altitude a E 

 is therefore « a doubled and that furplus. If the fliaft 

 bore a greater proportion in efl^eding this altitude, it 

 muft be longer ; confequently it would, when flanding, 

 hold the arm further from the ground, and render that 

 piece, with itfelf, lefs portable ; and, when up, increafe 

 the weight on the fulcrum, and the prefTure againft the 

 ibcket. 



The arm EF rlfes from a E on an angle about fifty 

 degrees with the horizon, thus making a good projeCiion, 

 aiid a confiderable increafe of altitude. Its proportion 

 to the fliaft, and angle with it when dov;n, are fuch as to 

 fet the bafket on the ground. 



Thefe angles, the fhaft, and the fpace a a (its eleva- 

 tion), accord in adjufting the proportion of EG, and the 

 pofition of the rope. When the lever defcends, the 

 arm EG opens, and throws out the rope, keeping it 

 ftretched all the while. This makes it defcribe a circular 



arch. 



