CHAMBERS'S INFORMATION FOR THE PEOPLE. 



Fig. 29. 



called a nut, as represented at M, fig. 29. The 

 threads and corresponding grooves are some- 

 times of a triangular form, as 

 in fig. 29 ; and sometimes flat. 

 To produce pressure with the 

 screw, either the screw or the 

 nut must be fixed. Whichever 

 is free, is then turned, and made 

 to press against the resistance. 



Practically, the screw is never 

 used as a simple machine, the 

 power being always applied by 

 means of a lever, passing either 

 through the head of the screw or through the nut. 

 The screw, therefore, acts with the combined 

 power of the lever and inclined plane; and in 

 investigating the effects, we must take into ac- 

 count both these simple mechanical powers, so 

 that the screw now becomes really a compound 

 machine. 



To arrive at the proportion of the power to the 

 weight in this compound machine, we may apply 

 the principle of virtual velocities. The power acts 

 at the end of the lever L (fig. 29), and in turning 

 the screw once round, it describes a circle, of which 

 the lever is the radius. The circumference of this 

 circle represents the velocity of the power. In the 

 meantime, the weight has moved over a space 

 equal to the distance between two threads, which 

 is its velocity. Therefore, in the screw, the power 

 multiplied by the circumference it describes, is 

 tqual to the weight multiplied by the distance 

 between two contiguous threads. 



Applications of the Screw. 



The most common purpose for which the screw 

 is applied in mechanical operations, is to produce 

 reat pressure accompanied with constancy of 

 action, or retention of the pressure ; and this 

 quality of constancy is always procurable from 

 the great friction which takes place in the pressure 

 of the threads on the nut, or on any substance, 

 such as wood, through which the screw penetrates. 

 The common standing-press 

 used by bookbinders for 

 pressing their books, affords 

 one of the best examples of 

 the application of. the screw 

 to produce great pressure 

 (fig. 30). The screw A has 

 a thick round lower ex- 

 tremity B, into holes in 

 which the lever is inserted. 

 This extremity B is attached 

 by a socket-joint to the 

 pressing-table C, so that 



Fig- 30. 



when the screw is turned in one direction, the 

 table sinks, and when turned in another, the table 

 rises. The books, D, lie upon a fixed sole S, 

 below the table H is a cross-beam above, in 

 which is the box or overlapping screw, to give the 

 necessary resistance. 



STRENGTH OF BODIES AND OF STRUCTURES. 



A knowledge of the simple principles of 

 mechanics above considered, together with that 

 of the general .properties of matter, enable us to 

 determine what forms of bodies and what posi- 

 tions are best calculated to resist forces that tend 



216 



to break, crush, or overthrow them. This consti- 

 tutes engineering. A few of the more important 

 general truths thus arrived at may be here noticed. 

 The strains to which the parts of machines and 

 edifices are subjected, are chiefly of four kinds : 

 I. Tension, as when a rope is pulled in the direc- 

 tion of its length ; 2. Compression or crushing ; 

 3. Transverse or Cross Strain ; and 4. Torsion 

 or twisting. 



Tension. 



When a rod of wood is suspended vertically, 

 and a weight attached to it tending to tear it 

 asunder, all its fibres act equally, and its strength 

 evidently depends on the strength of the indi- 

 vidual fibres and their number that is, the area 

 of the cross-section of the rod. If there are two 

 rods, then, of the same material, the one having 

 an area of one square inch in its cross-section, 

 the other an area of three square inches, the last 

 will sustain three times the weight that the other 

 will sustain, and this whatever be the shapes of 

 the sections. The same reasoning applies to a 

 rope or a rod of iron or other metal. 



The power of bodies to resist tension or tearing 

 force, is called their Tenacity, Direct Strength, 

 Absolute Strength, or Tensive Strength. It 

 depends upon the force with which their atoms 

 cohere ; and differs in different substances. The 

 following are the absolute or tensive strengths of 

 some of the more important materials, as deter- 

 mined by experiments : A rod of ash, i square 

 inch in section, sustains a weight of 17,000 Ibs. ; 

 a similar rod of English oak, from 8000 to 12,000 ; 

 of red pine, 12,000; cast-iron, 18,000; wrought- 

 iron, 29 tons or 65,000 Ibs. ; iron wire, 41 \ tons or 

 93,000 Ibs. ; cast-steel, 134,000 Ibs. ; copper, 21 

 tons or 47,000 Ibs. ; hempen rope, 6000 Ibs. From 

 these numbers the tensive strength of a rod or 

 beam of any given dimensions may be found, by 

 multiplying by the number of square inches in the 

 section of the rod or beam. Ex. What weight 

 will a cylindrical rod of wrought-iron, 3 inches 

 in diameter, sustain? Here area of section is 

 3 2 X 7854 = 7*0686 square inches ; and 29 X 7'o68 

 = 205 tons, nearly. 



Compression. 



Numerous experiments have been made on the 

 power of different materials to resist crushing 

 force ; but no law has been found by which we 

 can deduce with any accuracy the strength of 

 bodies of dimensions different from those experi- 

 mented upon. 



Transverse Strain. 



Let ab be a beam fixed at one end into a wall 

 ag, and loaded with a weight W at the other end. 

 If the beam is to break (supposing it of uniform 

 dimensions throughout), the rupture will evidently 

 be at the wall, because there W acts with the 

 greatest leverage. Before actual rupture, a portion 

 of the fibres in the upper part of the section from 

 a to c are drawn out or stretched, and a portion 

 in the lower part, from c to f, compressed ; and 

 there must therefore be a line of fibres somewhere, 

 as at c, which are neither stretched nor compressed. 

 This line is called the neutral axis of rupture; 

 and as W tends to bring the beam into the posi- 

 tion marked by the dotted lines, we may conceive 

 c to be the fulcrum of a bent lever with three 



