MKC 



MED 



length of the winch twelve inches, the 

 circle described by the handle of the 

 winch where the power acts, will be 76 

 inches nearly, or about 152 half inches ; 

 and consequently 152 times as great as 

 the distance between the spirals ; and 

 therefore a power at the handle, whose 

 intensity is equal to no more than a single 

 pound, will balance 152 pounds acting 

 against the screw ; and as much additional 

 force as is sufficient to overcome the 

 friction, will raise the 152 pounds; and 

 the velocity of the power will be to the 

 velocity of the weight as 152 to 1. Hence 

 it appears, that the longer the winch is, 

 and the nearer the spirals are to one ano- 

 ther, so much the greater is the force of 

 the screw. 



A machine for shewing the force or 

 power, of the screw may be contrived in 

 the following manner : let the wheel C 

 have a screw (fig. 25.) on its axis, work- 

 ing in the teeth of the wheel D, which 

 suppose to be 48 in number, tt is plain, 

 that for every time the wheel C and screw 

 are turned round by the winch A, the 

 wheel D will be moved one tooth by the 

 screw ; and therefore, in 48 revolutions 

 of the winch, the wheel D will be turned 

 once round. Then, if the circumference 

 of a circle, described by the handle of the 

 winch A, be equal to the circumference 

 of a groove round the wheel D, the velo- 

 city of the handle will be 48 times as great 

 as the velocity of any given point in the 

 groove. Consequently, if a line G goes 

 round the groove, and" has a weight of 48 

 pounds hung to it, a power equal to 1 

 pound at the handle will balance and sup- 

 port the weight. To prove this by expe- 

 riment, let the circumferences of the 

 grooves of the wheels C and D be equal 

 to one another ; and then if a weight H, 

 of one pound, be suspended by a line go- 

 ing round the groove of the wheel C, it 

 will balance a weight of 48 pounds hang- 

 ing by the line G ; and a small addition 

 to the weight II will cause it to descend, 

 and so raise up the other weight. 



If a line G, instead of going round the 

 groove of the wheel I), goes round its 

 axle I, the power of the machine will be 

 as much increased as the circumference of 

 the groove exceeds the circumference of 

 the axle : which supposing it to be sis 

 times, then one pound at H will balance 

 six times 48, or 288 pounds, hung to the 

 line on the axle : and hence the power or 

 advantage of this machine will be as 288 

 to 1. That is to say, a man, who by his 

 natural strength could lift an hundred 

 weight, will be able to raise 288 cwts. by 

 this engine. If a system of pullies were 



applied to the cord H, the power would 

 be increased to an amazing degree. When 

 a screw acts in a wheel in this manner, it 

 is called an endless screw. When it is not 

 employed in turning a wheel, it consists 

 of two parts : the first is called the male, 

 or outside screw, being cut in such a 

 manner as to have a prominent part going 

 round the cylinder in a spiral manner ; 

 which prominent part is called the thread 

 of the screw ; the other, part, which is 

 called the female, or inside screw, is a so- 

 lid body, containing a hollow cylinder, 

 whose concave surface is cut in the same 

 manner as the convex surface of the male 

 screw, so that the prominent parts of the 

 one may fit the concave parts of 'the 

 other. A very considerable degree of 

 friction always acts against the power in 

 a screw ; but this is fully compensated by 

 other advantages ; for on this account the 

 screw continues to sustain a weight, even 

 after the power is removed, or ceases to 

 act, and presses upon the body against 

 which it is driven. Hence the scre\v will 

 sustain very great weights, insomuch, that 

 several screws, properly applied, would 

 support a large building, whilst the foun- 

 dation was mending, or renewed. 



The screw is of extensive use in the 

 printing press, and in the press for coin- 

 ing money, and in a great variety of other 

 purposes. It has lately been employed 

 in the flour-mills in America, for pushing 

 the flour which comes from the mill- 

 stones to the end of a long trough, from 

 which it is conveyed to other parts of the 

 machinery, in order to undergo the re- 

 maining processes. In this case, the spi- 

 ral threads are very large in proportion to 

 the cylinder on which they are fixed. As 

 the lever used with the screw moves 

 through a large space, when compared 

 with the velocity of its other extremity, 

 or of any body which it puts in motion ; 

 the screw is of very great use in subdivid- 

 ing any space into a great number of 

 minute parts. Hence it is employed in 

 the engines for dividing 1 mathematical in- 

 struments, Sec. See OSCILLATION, 



SlOS, &C. 



MEDAL. This word has generally 

 been supposed to be derived from Metal- 

 lum, from which we have the English term 

 metal ; but it may admit of some doubt 

 whether the derivation is correct, as the 

 word appears to have too comprehensive 

 a sense to particularize a piece of gold, 

 silver, brass, or copper, impressed with 

 figures to convey to posterity some great 

 historical occurrence, or to perpetuate the 

 memory of a person who had rendered the 



