oXj'^ 



.MONTHLY JOURNAL OF AGRICULTURE. 



be redaced to a convenient bulk for tnuisporta- 

 tion. the screw is used to compress them, and 

 they are thus reduced into hard, dense masses. 

 In printins. formerly, the paper was urged by 

 r. severe and sadden pressure upon the types 

 by means of a screw. 



As the mechanical power of the scre^v de- 

 pends upon the relative masnitude of the cir- 

 cumference throuffh which the power revolves, 

 and the distance between the threads, it is evi- 

 dent that to increase the efficacy of the machine, 

 we must either increase the lensth of the lever 

 by which the power acts, or diminish the mas- 

 nitude of the thread. Although there is no limit 

 in theory to the increase of the mechanical effi- 

 cacy by these means, yet practical inconve- 

 nience arises which effectually prevents that in- 

 crease being carried beyond a certain extent 

 If the lever by which the power acts be increas- 

 ed, the same difficulty arises as vi-as already ex- 

 plained in the wheel and axle : the space 

 throaa-h ■which the povrer should act ■noaiii be 

 so un'.vieldy, that its application would become 

 impracticable. K on the other hand, the power 

 of the machine be increased by diminishiug the 

 -size of the tliread, the strength of the thread wUl 

 be so diminished, that a slight resistance will 

 tear it from the cylinder. The cases in w^hich it 

 is necessary to increase the power of the ma- 

 chine being those in which the greatest resi.st- 

 ances are to be overcome, the object will evi- 

 dently be defeated if the means chosen to in- 

 crea.se that power deprive the machine of the 

 strength which is necessary to sustain the force 

 to ^vhich it is to be .submitted. 



The.~e inconveniences are removed by a con- 

 trivance of Mr. Hxmter, which, while it gives to 

 the machine all the requisite strength and com- 

 pactness, allows it to have an almost imlimited 

 degree of mechanical efficacy. 



This contrivance consists in the use of two 

 screws, the threads of which may have any 

 strength and magnitude, but -which have a very 

 small difference of breadth. Wliile the work- 

 ing point is urged forward bj" that which has the 

 greater thread, it is drawn back by that which 

 has the less; so that, during each revolution of 

 the screw, instead of being advanced through a 

 space equal to the magnitude of either of the 

 thread-s, it moves through a space equal to their 

 difference. The mechanical power of sucJi a 

 machine wiU be the same a.^ that of a single 

 screw, having a thread who.se magnitude isequal 

 to the difference of the magnitades of the two 

 threads just mentioned. 



Thus, without inconvenientlv increasing the 



riz. 13. 



sweep of the pow 

 er, on the one hand, 

 or on the other, 

 diminishing the 

 thread until the ne- 

 cci-sary strent-th is 

 lost the machine 

 will acquire an ef- 

 ficacy limited by 

 nothing bat the 

 smallness of the dif- 

 ference between 

 the two threads. 



This principle w^as 

 first applied in the 

 manner represent- 

 ed in fig. 13. A is 

 the greater thread, 

 playing in the fixed 



cylinder, and playing in a concave screw, cut 

 within the greater cjlinder. Daring everv revo- 

 , lution of tlie screw, the cylinder A descends 

 I through a space equal to the distance between 

 I its threads. At the same time, the smaller cyl- 

 j inder B ascends through a space equal to the 

 ' distance berv\-een tlje threads cut upon it: the 

 I eliect is, that the board D descends through a 

 [ space equal to the difference between the threads 

 ; upon A and tlie threads upon B, and the ma^ 

 I chine has a power proportionate to the small- 

 , ness of this difference. 



' Thus, suppose the screw A has tv/ent>- threads 

 . in an inch, while the screw B has twenty-one : 

 during one revolution, the screw A will descend 

 I through a space equal to the twentieth part of 

 an inch. K, during this motion, the screw B 

 did not turn within A, the board D would be 

 advanced through the twentieth of an inch : bat 

 becau.se the hollow screw vi-iihin A turns upon 

 B, the screvs- B vrill, relatively to A, be raised 

 in one revolution tlirough a space equal to the 

 twenty-first part of an inch. Thus, while the 

 hoard D is depressed through the twentieth of 

 an iach by the screw A, it is raised through the 

 twenty-first of an inch hj the screw B. It is, 

 therefore, on the whole, depressed through a 

 space equal to tlie excess of the t^ventieth of an 

 inch above the twenty-fii-st of an inch — that is, 

 through the four hundred and twentieth of an 

 inch. 



The power of this machine will, therefore, be 

 expressed by the number of times the four hun- 

 dred and twentieth of an inch is contained in 

 the circtunference through which the power 

 moves. 



In the practical application of this piinciple at 

 present the airangement is .somewhat different, 

 The two threads are usually cut on different 

 parts of the .same cylinder. If nuts be supposed 

 to be placed upon these, -which are capable of 

 moving in the direction of the length, but not of 

 revolving, it is evident that by turning the screw 

 once round, each nut will be advanced throogh 

 a space equal to the breadth of the respective 

 threads. By this means the two nuts will either 

 approach each other, or mutually recede, accord- 

 ing to the direction in which tlie screw is turn- 

 ed, throuu'h a space equal to the difference of the 

 breadth of the threads, and they will exert a 

 force either in compres.=ing or extending any 

 sub.stance placed between them, proportionate 

 to the sniailness of that diilf-Tence, 



A toothed -^vheel is sometimes u.sed instead of 

 a nut, .so that the same quality by -which the re- 

 volution of the screw urges the nut forward is 

 applied to make the wheel revolve. The screw 

 is in this case called an endless screw, because 

 its action upon the wheel may be continned 

 without limit Thi.s application of the screw is 

 represented in fig. 1 4. P is the winch to which the 

 power is applied ; and its effect at the circamfer- 



Fig. 14. 



ence of the wheel 

 is e.stimated in 

 the same man- 

 ner as the effect 

 of tlic - screw 

 upon the nut — 

 This effect is to 

 be considered^ as 

 a power acting 

 upon the circnin- 

 fertnce of the 

 wheel ; and its 

 proportion to the 

 weight or resi.stance is to be calculated in the 



