Machine. If our young friends be sufficiently interested in the machine we are describing, they will naturally wonder why the eight cylinders only produce seven sheets. The reason is this: in consequence of six of the cylinders having a reversing motion, by which they print first forwards and then backwards, it is necessary that the two end cylinders (the 1st and the 8th) should work slower, while the reversing of the other six takes place; thus it is explained why the end cylinders only print one sheet each, while the remaining six produce two sheets each, from every revolution of the table.

Now while in the Four Cylinder Machine only half the cylinders are working, in the Double Action Machine seven out of eight are continually occupied in printing, and another great advantage possessed by the latter machine is the great saving effected in the distance the forme of type has to travel. This, combined with the constant working of seven cylinders, causes the machine, as we have before stated, to produce from 10,000 to 12,000 newspapers per hour, while the Four Cylinder Machines print only 4,500 in the same space of time.

Before the invention of the Printing Machine, the newspaper offices, in order to supply the number of copies required for daily publication, were compelled to set up or compose the types twice, and on extraordinary occasions even three and four times over. In those days the newspapers were necessarily much smaller than at the present time, and were worked by hand presses, the types being inked with the sheepskin balls already described. The speed at which the men were compelled to work required such great exertion that the stoutest constitutions, after a few years' time, fell sacrifices to such laborious occupation. Hence, beyond the advantage of rapidly producing the daily papers, we may regard the invention of the Printing Machine as highly beneficial to humanity.

To attempt to describe the several portions of the Printing Machine would be useless, as it will be by far the best under

stood by the engraving which is given at page 71. Other plans have been adopted, and several flat surface machines, which communicate the impression by a platten like the ordinary press, and are admirably adapted for fine book-work, are now in use. Their motion is similar to that of the hand press, and the work produced by them almost equals that from the hand press in excellence.

The most recently constructed platten machine is that of Messrs. Napier & Son, in which the inking apparatus is brought to very great perfection. A considerable portion of the "Boy's Own Library" is printed by this machine.

Another highly important invention connected with the art of typography is the process of Stereotyping, by which all the letters, forming a page of type, are cast in one piece or plate of type-metal about the eighth of an inch thick.

This process was first practised by William Ged, of Edinburgh, in the year 1725, who, after much perseverance, formed an arrangement with the University of Cambridge for casting their Bibles and Prayer-Books, thereby saving the necessity of employing a large quantity of type; but the plan

received so much opposition from the workmen, in making errors and injuring the plates, that it was discontinued, and the stereotypes ultimately melted down at the Caslon Foundry.

The merits of the invention were, however, eventually recognised, and its adoption has become almost universal. We will now endeavour to explain the process. The moveable types of two or four pages, according to their size, are first slightly oiled over with a brush, and then burnt plaster of Paris (termed gypsum), mixed with water to the consistency of cream, is poured upon the pages, which are surrounded by narrow slips of wood, or metal, forming a wall just sufficiently high to retain enough of the plaster mixture to produce a matrix, or mould. This matrix is left until the greater portion of the moisture becomes evaporated. It is then lifted from the type and is put into an oven to dry, or rather bake out all the remaining moisture. It is next secured in a flat dipping-pan, surrounded by an iron frame which regulates the thickness of the stereotype plate, and the pan being covered is immersed in a cauldron of melted metal, where it remains sufficiently long for the metal thoroughly to insinuate itself into every part of the matrix. The dipping-pan being removed from the cauldron, sufficient time is allowed for the metal to set. It is finally cooled by being plunged into cold water, and the superfluous metal is sawn from the stereotype plate by a circular saw. The back of the plate thus produced is turned in a lathe to a certain guage, which regulates all the plates forming the pages of a book to one uniform thickness.

When stereotype plates are printed they are fixed upon wooden or metal stands of such thickness that when the plate is added the two combined become exactly the same height as the regular printing types. Stereotype plates are adapted for working both by hand presses and machines.

THE THERMOMETER.

HE origin of the Thermometer, like that of the mariner's compass, remains in obscurity. We only know that the idea of measuring the degree

of heat, which the atmosphere at different periods presents, was first conceived in Italy, that country which, during the latter portion of the middle ages, was distinguished by the attainments and discoveries of its scientific men.

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In the year 1626, there was a book published entitled, Commentaries on the Works of Avicenna," by a physician, named Santoria, who resided at Padua; and in this work he claims the honour of having invented the thermometer. Cornelius Drebble, of Alkmaar in Holland, makes the same claim, and after carefully examining the evidence, it appears, that although Santoria was the first to point out the use of this instrument, Drebble had also discovered and made its properties known before he heard anything of the invention of the Italian physician.

For some time after the invention of the thermometer, it was chiefly used for ascertaining the changes of temperature alone, and the instrument was of the simplest description. A glass tube was formed with a ball at one end; the other end was open, and inserted in a vessel partly filled with mercury or coloured spirit-generally the latter. Previous to this the air inside the instrument was heated by a lamp, so that when the

temperature of the atmosphere was increased, it caused the air within the ball and the tube to be rarified. As this expanded and occupied more space, it pressed down the spirit; and on the contrary, when the temperature was reduced, its pressure upon the surface of the spirit decreased, and the latter was forced higher up the tube, as the quantity of air within became contracted in bulk. A scale was then fixed alongside the tube, divided into certain degrees, so that the several changes could be measured as correctly as might be expected from the simplicity of the contrivance.

Such an invention was not long before it attracted notice; and after a few years the celebrated Robert Boyle, who had turned his attention to natural philosophy, and already made great improvements in the air-pump, devised an alteration in what might be called the form of the heat measurer. He left the tube open at both ends, and turned one of them upwards in a curve; this he sealed hermetically, by melting the glass to a vessel, on the top of which a hole was pierced, and the pressure of the atmosphere caused the spirit to rise and descend, in the upright portion of the tube, as the condition of the atmosphere was changed. Boyle, who was a son of the Earl of Cork, was a man distinguished in every way for noble qualities of mind and heart. After travelling through Europe, he settled in England; and during the great civil war which was waged between Charles I. and his Parliament, he had the good fortune to enjoy the favour of several eminent men on both sides; and having thus obtained protection both for person and property, he was enabled to follow his literary and scientific pursuits at leisure and in peace.

Boyle's chemical experiments date from the year 1646; and in all probability it was shortly after this period that he first turned his attention to the improvement of the thermometer. He settled at Oxford in the year 1654, and resided there till 1668, being during that time a member of the association which