MECHANICS. 



[ Lesson X. 



of an inch apart ; because ^, multiplied by 

 j^, are equal to -fo the difference between 

 the distances of the threads of John Hunter's 

 compound screw. 



127. T. Is there any method by which 

 we can combine velocity with power in 

 the screw? 



P. Yes ; by having a screw with two, 

 three, or more threads, instead of one. 

 Suppose that we had a screw with a very 

 broad thread say, for example, one inch 

 and that we wished to increase its velo- 

 city, we have only to form three threads 

 in each, each one a quarter of an inch 

 in width, with a space the one-eighth of 

 an inch between them, and the thing is 

 accomplished. This is a fact, and as the 

 thing is applied to printing-presses, any 

 curious person may see it for himself. 



128. T. Are not houses sometimes 

 moved by means of the screw ? 



P. Yes. It has been done in England, 

 but is chiefly practised in America. The 

 annexed diagram will 

 explain the method. 

 The building to be 

 moved must be a 

 detached one. Open- 

 ings are made in the 

 end walls, just above 

 the ground, large 

 enough to insert 

 beams, about fifteen 

 inches square, across 

 the building (1) ; the 

 end of each beam is 

 supported on blocks 

 of wood, fixed into 

 the ground, and clear 

 of the walls, and each 

 and tight by driving 

 wedges between the beams and the upright 

 block. When this has been done, the 

 foundation is cleared away, and a clear 

 space left. Then other beams (2) are 

 placed under the first, and resting like 

 them on blocks of wood, and by this means 

 the front and back walls are supported, 

 and now the whole foundation is exposed. 

 The screws (3) are placed under the ends 

 of the second beams ('2), whic v> are forced 

 upwards by their means, and the weight of 

 the whole building sustained by them. 

 The ground underneath being all removed, 

 set of grooved-ways, or beams (6), are 



beam 



placed where the end walls formerly stood, 

 and the cradles (5), which are brains with 

 a projection corresponding to the groove 

 in the ways, are placed on them, both being 

 previously well greased. Large beams (!) 

 are placed over the cradles, between them 

 and the beams (2) which support the ends 

 of the house, and wedges driven in to 

 render the whole tight and secure, which 

 is also effected by additional blocks (7). 

 The screws are now removed, and being 

 placed horizontally, are made to act together 

 against the cradles, and move them along 

 the ways, at the rate of about four feet a 

 day, to the place the house is to occupy. 

 Then, by inverting the operations, the 

 beams are removed, and the house firmly 

 fixed, without sustaining any injury, and 

 often without even moving as much as a 

 chair from the house. 



129. T. What is the principle of the 

 screw applied to ? 



P. Various purposes. Among others, 

 we may enumerate the cider-mill, oil-null, 

 common printing-press, napkin -press, 

 bookbinder's press (see Fig. 40), patent 

 cork-screw, vice, clamp, augers, gimlets, 

 and many similar machines, which owe 

 their efficacy to the principle of the sen \\. 

 Common screws combine the principle of 

 the wedge with that of the screw, for they 

 are smaller at the point than at the lever 

 end. There is another form which must 

 also be mentioned the endless screw. 

 This is a solid screw, revolving on fixed 

 axes, the thread of which is adapted to 

 teeth on the circumference of a wheel. 



GENERAL QUESTIONS ON LESSON X. 



1. Describe the difference between a 

 male and female screw. 



2. Describe the difference between a 

 spiral and a helical line. 



3. Define a screw. 



4. Describe the difference between a 

 screw with a triangular thread, and one 

 with a parallelogram. 



5. How is the inclined plane referable 

 to the screw ? 



6. What is the helical line, and what is 

 the term derived from ? 



7. How can you estimate the effect of a 

 screw ? 



8. Can the power of the screw be in- 

 creased ? 



