larger wire and taking the impression of the spiral thus formed. Thus the screw is exactly like the inclined road winding up a tower or round a hill. You can yourselves illustrate this if you take a piece of paper cut into the form of a right-angled triangle (Fig. 21) with the side A' E the

A

E

Fig. 21.

same length as a common round ruler, which you must also have for the experiment. If you now put the edge A E lengthwise to the ruler, and wrap the triangle round it, you will find that the slanting side or inclined plane of the triangle will make a spiral, or screw, going four times round the ruler.

The turns or spiral of the screw are called its threads.

The power of the screw is made use of by causing it to move through a nut (N, Fig. 19), the screw itself being in some cases fixed and immovable, while the nut is made to turn, while in others the nut is fixed and the screw is made to turn through it.

16. Fig. 19 represents a combination of the lever and the screw, which is the ordinary form of the use of the screw, for practically it is never used alone. The power is applied to the end of the lever at P, and the weight or pressure W is sustained by the screw, as in the common screw press. If the distance A B between any two threads of the screw be half an inch, and the circle described by turning round the end of the lever P be five feet, or 120 half-inches, then a force or pressure of one pound at P will sustain a weight of 120 lbs. at W.

Thus you see that the power exerted at the end of the lever, or, in other words, at the circumference of the circle made by the power, is to be multiplied by this circumference, and the result, multiplied by the distance between two threads of the screw, is equal to the weight or resistance that has to be overcome. Don't go further till you understand all this.

17. If the weight or resistance to be overcome be known, and the power required to overcome it be the point to be ascertained, the process is, in some measure, reversed. You have to divide by the circumference instead of multiplying by it. Thus, suppose the lever of a screw were 60 inches long, reckoning from the centre of the screw, and the distance between the threads one inch, what would be the power required to produce a pressure of 4,000 lbs. ?

Now the diameter of such a circle would be twice 60 inches = 120 inches, and the circumference, in round figures, would be, say, three times as much, or 360 inches.

The pressure-4,000 lbs. multiplied by the distance between the two threads-1 inch-would be 4,000 × 1 4,000. The power required would therefore be 4,000 divided by 360, which would give the result as 113 lbs. But this is only what it would be if there were no friction to be overcome in turning the screw, and hence as the friction is equal to the exertion of one-third more power, that amount must be added, and thus, in this case the real force required would be nearly 15 lbs.

The power of the screw may be increased either by increasing the length of the lever, and thus enlarging the circle round which the power moves, or by increasing the number of turns of the thread. The same

B

Fig. 22.

f a wrought-iron lever, with a solid cast-iron ball at each end; B the body of the press; ss four screws to keep the bed or bottom die d in its place; p the puller-off, which detaches the metal from the die after the stamping. B' the bottom plate of

the machine.

power which sustained a weight of 120 in a screw, in which the threads are half an inch apart, would sustain 240 lbs. if they were only a quarter of an inch separate, that is, if the lever were in both cases the same length. But, of course, the threads must not be too fine else they will

break when used.

18. The Screw is used very largely where a great pressure is needed for the moment, as in the Cutting-Press (Fig. 22) for punching holes in metal, moulding thin pieces of metal into various shapes, and stamping devices on medals and coins. It is also very much in use where great continuous pressure is needed, as in Bookbinders' Presses (Fig. 23). The continuousness of the pressure is caused by the great friction produced by the pressure

E

of the threads of the screw on the nut of iron or wood through which it is made to pass.

Sometimes the screw is employed in connection with a wheel which it turns by acting on its teeth, as in the Candle-cutting Machine (Fig. 24).

Candle-cutting Machine.-f the endless screw; gg' toothed wheels; d a fly-wheel; k a fast and loose pulley, over which a leather band is passed; 1 framework; c mass of prepared candle composition.

Each turn of the screw, you will notice, catches a fresh tooth of the wheel and pushes it forward, thus causing the wheel to revolve continuously. This is called an endless screw, because its action never stops.

RACES OF EUROPE.-Hyde Clarke, D.C.L.

ALL mankind belong to several great stocks or families of races, as the Indo-European, the Chinese, the Syro-Arabian, the Americo-Indian; and each of these is again further divided. Thus, under the Indo-European come the Germanic, the Slavonic, the Celtic, and other great families. The Germanic stock again includes or brings together the English, High Dutch, and Scandinavian. The Celtic stock includes the Welsh, Irish, Manx, and Breton.

The leading great stocks in Europe are the following:

INDO-EUROPEAN; as English, Welsh, Irish, French, Russians, Gipsies. UGRO-TARTARIAN or FINNISH; as Magyars, Fins, Laps, Basques. TURKISH; as Turks.

SYRO-ARABIAN; as Jews.

The Indo-European is the greatest of these, and the mightiest in the world, although more souls belong to the Chinese stock. The Indo-Europeans reach from Burmah through Western Asia, spreading over Europe, and, further, throughout North and South America; and the European branches are thus divided :

GERMANIC:

Anglo-Suevian.. English, Americo-English.

Low Dutch
High Dutch

Old Saxons, Burgundians, Goths,
Vandals, Jutes, Longbeards (or
Lombards), Danes.

Frisians of Friesland, Heligo-
land, and Jutland.

Netherlanders, Hollanders, and
Flemings.

.Low Dutch, or Germans.
High Dutch, or Germans.
German Swiss.

Scandinavians..Danes and Norwegians.

Swedes.
Icelanders.

Welsh of Wales.

Old Cornish.

Bretons of Brittany.

.Irish of Ireland.

Irish or Erse of the Scotch High

lands.

Manx of the Isle of Man.
.Italians.

.French Canadians.

French Swiss.

Savoyards.

The leading living tongues belonging to the Germanic stock are the English, Frisian, Netherlandish or Flemish (sometimes called Dutch), High Dutch (sometimes called German), Danish, Swedish, and Icelandic. These again may be thus classified :ANGLO-SUEVIAN.. English.

Frisian.

.Flemish or Netherlandish.
Low Dutch or Platt Dutch.
.High Dutch.

.Danish or Norwegian.
Swedish.

Icelandic.

Our islands seem to have been first settled by the great Euskardian or Iberian stock, belonging to the Ugro-Tartarian, which once spread over Western Europe, but of which all that is left are the Basques in Spain, unless the Fins of several kinds are likewise to be reckoned, and unless, in the west of Ireland, some Iberian blood still lingers under the name of Spanish, as the common belief there is. The Iberians were slaughtered or driven out of Britain by the Celts; but in the time of the Romans, the Silures, in South Wales, still had a Spanish look, as Tacitus tells us. The Celts were yielding before inroads of the Belgians in Britain and Ireland, when the Romans made themselves masters of the islands, and for a time stayed the downfall of the Celts. In the fifth century after Christ, however, the Roman might tottered, the Germani rushed on the Roman empire and swept all before them. In these islands the Celts threw off the Roman yoke, but did not long hold sway; for the Northern Germani, or English kin, came over the North Sea and began to settle on the eastern shore, slowly spreading themselves till all the islands fell under their yoke.