CanJlruB'ton and PhUofcphical Jpplicaiion of the Screw. 1 59 



■weight by fliding it along this thread, as along an inclined plane of the fame length and 

 height. The female fcrew, or nut containing an hollow fcrew, is in faft a weight adapted 

 to this fliding procefs, and does accordingly correfpond with the weight on the fimilar in- 

 clined plane, provided the power be applied at the very thread of the fcrew ; but if the ad- 

 vantage of a lever be taken to move the nut, the rcfiftance will be lefs in proportion as the 

 radius of the fcrew is lefs than the length of the lever. 



Hence it is feen that the fcrew affords a convenient and powerful application of the in- 

 clined plane to mechanical purpofes. To compute its effeft, whether a lever be ufed or not, 

 is equally fimple -, for the power, wherever it be applied, is direfled in a plane at right 

 angles to the axis of motion ; and the agent itfelf defcribes the thread of a fcrew conflantly 

 rifing through the fame quantity in each turn. The proportion of the power to the 

 ■weight, will therefore be as the diflance between two contiguous threads to the circum- 

 ference of the cylinder in which the power moves. Or in the form of a rule. Multiply 

 the length of the lever by 44, and divide the produdl by 7 v-the quotient will bear the fame 

 proportion to the diftance between thread and thread as the weight does to the power 

 ■when in equilibrio. 



In mixed hydraulic engines, fuch as water-mills, pumps, and the like, the ufual addition 

 allowed to overcome friftions, and give the velocity of working, is three-eighths of the 

 power in equilibrio. The fcrew requires more than this, but the quantity mutt vary ac- 

 cording to the obliquity of the thread. 



The obliquity is greater the coarfer the thread, and the fmaller the diameter of the ge- 

 nerating cylinder. For flow and ftrong prefTures a fine thread is to be preferred ; but for a 

 quick ftroke and fpeedy return, as in the printlng-prefs and coining-engine, the thread muA 

 rife with confiderable fpeed. 



The man of fcience is moft particularly interefled in the confideration of this mechanical 

 organ, from the extreme accuracy with which it may be applied in the meafurement of 

 lines or fpaces. Thus, for example, if the threads of a fcrew be one-fiftieth of an inch 

 apart, every turn in a fixed nut will caufe its extremity to advance or recede that quantity : 

 and if a graduated circular plate of one foot in diameter be fixed to the other extremity, 

 the parts of each turn may be afccrtained. One quarter of a degree, or the 1440th part, 

 on fuch a circle, will be a very conveniently vifible divifion, and will correfpond with the 

 1440th part of one-fiftieth of an inch, which is lefs than the fcvcnty-lhoufandth part of 

 an inch. 



A qucftion of no fmall importance immediately prcfcnts itfelf on this occafion. Does tJie 

 practice jufl.ify this theoretical conclufion ? or, in other words, Do not the irregularities 

 of workmanfhip fliew themfelves to be much greater than the quantities here attempted to 

 be meafured .'' The folution, which will fliew the true value of the meafuring fcrew, muft be 

 deduced from experiment. 



The moft obvious method of forming a fcrew confiRs in tracing helical lines upon the 

 furface of a cylinder, and filing or cutting away the part between the intended tllreads. 

 In this way fame large fcrews are a£lually made ; but it is immediately perceived on reflec- 

 tion, lliat the original tracing Ls fubjc£l to all the inaccuracies of divifion by hand; to which 

 muft alfo be added the ftill greater errors of filing or cutting. By this procefs alone it 

 wuuUl be impradlicable to make a fcrew even of moderate corrcclncfs. 



If 



