Page images
PDF
EPUB

air pressing on the surface of the quicksilver in the basin without, and no air being in the tube at top, the quicksilver will continue in the tube, being raised by the air on the surface in the basin below.

6. The usual range of the barometer in this country, is from twenty-eight to thirty-one inches; when the air is pure and heavy, it raises the mercury to nearly thirty-one, and when light, and full of vapours, it falls to nearly twenty-eight.

7. In fine, dry weather, the air is rendered pure, free from all light vapours, and is consequently extremely heavy, so that it presses up the quicksilver.

8. In moist rainy weather the atmosphere being charged with vapours, clouds, and fogs, the air is then sensibly lighter, and presses upon the quicksilver with less force.

9. When high winds blow, the atmosphere is light, and the quicksilver generally is low, and it rises higher in cold weather than in warm.

10. During frost, the air is purest and heaviest and the barometer rises to its highest points.

11. This instrument is also serviceable in measuring the height of mountains.

12. In ascending mountains, quicksilver is found to sink about a tenth of an inch in ninety feet; so that if the quicksilver fall an inch we have ascended near nine hundred feet; but this is subject to variations, from change of temperature and other causes, which render various corrections necessary.

13. The general method, however, of determining altitudes by the barometer and thermometer is extremely useful and convenient ; and ingenious rules are given by Dr. Hutton, Dr. Gregory, and others to facilitate the computation.

[blocks in formation]

MANUFACTURE OF PAPER. 1. Linen or European paper is manufactured chiefly of linen rags; which after being sorted into different classes, according to their respective qualities, are first carried to a machine called the cutting table, where they are divided into minute pieces; and thence to an engine denominated the duster, which is covered with a wire-net, and put in motion by machinery; so that, by the rapidity of its motion, it separates the dust from the shreds, and forces it through the wire.

2. The rags are reduced to a pulp in mills, by the joint action of water and cylinders, provided with iron blades; after which, the stuff is conveyed to a repository, that supplies the vat, whence the pulp is drawn.

3. In order to cast this pulp into paper, the workman immerses in the vat a mould composed of wire cloth, and furnished with a frame to retain the stuff.

4. Thus he draws as much of the pulp as is necessary to form one sheet, on which he lays a felt, for the purpose of absorbing the moisture; and thus places alternately a sheet and a felt, till he has formed six quires of paper.

5. When the last sheet is covered with felt, the whole is pressed, after which, the sheets are suspended on cords to dry.

6. The next operation, sizing, is performed, by plunging a few sheets together, and turning them in a vessel full of size, into which a small portion of alum is thrown.

7. The paper is now carried to the drying-room, and after being gradually dried, it is conveyed to the finishing-room, where it is submitted to the action of the press; selected, examined, folded, formed into quires of twenty-four sheets, and finally into reams, consisting of twenty quires each.

8. This is termed writing paper; as it is adapted for this purpose by the process of sizing.

9. There are various kinds of paper, such as blotting, brown, and coarse papers, which will not bear the ink.

10. To the above may be added, the different sorts of paper intended for drawing, engraving, or printing; which, although prepared in the usual way, are nevertheless not sized so highly as those papers which are intended for the pen.

11. Among the various vegetable substances employed as

a substitute for linen rags in the manufacture of paper, barleystraw, perhaps, is the most profitable and abundant, but it will only serve for common purposes; the unpleasant tinge it communicates to the paper being extremely prejudicial to the sight.

12. Stained paper is made by applying, with soft brushes, any of the colours used for tinging other substances, after tempering them properly with size or gum-water.

13. If the paper is to be of a uniform colour, the latter must be fixed by several thin coatings, each being suffered to dry before another is applied; as the shade will otherwise appear unequal.

[blocks in formation]

ORDERS OF ARCHITECTURE. 1. To Greece we are indebted for the three principal orders of architecture, the Doric, the Ionic, and the Corinthian ; Rome added two others both formed out of the former, the Tuscan and the Composite.

2. Each of these has a particular expression ; so that a building or different parts of a building, may be rude, solid, neat, delicate, or gay, accordingly as the Tuscan, the Doric, the Ionic, the Corinthian, or the Composite are employed.

3. The columns of these several orders are easily distinguishable to common observers, by reason of the ornaments that are peculiar to their capitals; but the scientific difference consists in their proportions.

4. The Tuscan order is characterised by its simplicity and strength.

5. It is devoid of all ornament.

6. The Doric is enlivened with ornaments in the frieze and capital.

7. The Ionic is ornamented with a volute scroll, or spiral horn ; its ornaments are in a style of composition between the plainness of the Doric and the richness of the Corinthian.

1

8. The Corinthian order is known by its capital being adorned with two sorts of leaves ; between these rise little stalks of which the volutes that support the highest parts of the capital are formed.

9. The Composite is nearly the same as the Corinthian, with an addition of the Ionic volute.

10. In their private buildings the Roman architects followed the Greeks ; but in their public edifices they far surpassed them in grandeur.

11. During the dark ages which followed the destruction of the Roman empire, the classic architecture of Greece, and Rome was lost sight of, but was again revived by the Italians at the time of the restoration of letters.

12. The Gothic style was so called because it was first used by the Visigoths ; but at first it was vastly inferior to that

; which we now call Gothic, and which exhibits grandeur and splendour with the most accurate execution.

13. The Saxon and Norman styles were so called because they were respectively used by the Saxons before the conquest, and by the Normans after, in the building of churches.

14. The Saxon style was distinguished by the semicircular arch, which they seem to have taken partly from the Romans and partly from their ancestors on the continent.

15. The Norman was distinguished by the following particulars ; the walls were very thick, generally without buttresses, the arches, both within and without, semicircular, and supported by very plain and solid columns.

LESSON 154. - p. 263.

THE STEAM ENGINE. 1. General Principle. — The property by which steam is rendered useful, is its power of making more room for itself when confined in too narrow a space. Thus it will often move the lid of a tea-kettle, till it finds some place to escape

at; and if we contrive a moveable lid, the steam drives it up and down with great force.

2. First Stage. — Let us take the most simple method first. Suppose a box of brass, of a proper shape, called a cylinder, with a lid which moves up and down, but does not allow steam to escape; if steam be introduced into this box, it exerts its power and forces up the lid. This is the first

movement.

3. Second Stage. — The lid is now up, and before the first movement can be repeated, it must be brought down; it cannot descend while the steam which drove it up remains in the box; you must therefore cover the box with cold water, and so cool the steam, and change it to water, and immediately the air has power enough to drive the lid down. This is the second movement, and by the frequent repetition of these two movements, the lid moves up and down constantly.

Observe now that the moveable lid has a motion like that of the box in a pump, and by a little addition will give a pumping motion in any part which wants it.

4. From an inspection of a diagram, it will be seen, that as the lid moves up and down in the box, the arm of the lever, joined to it by a rod, will move a little up and down also, and consequently the opposite arm of the lever will move in like manner, and produce a pumping motion on the other side sufficient to draw water, as in a common pump. The whole machine has a sort of see-saw motion. Thus, steam may be applied to pumping: and this was one of its first uses.

5. It was at this stage that the steam engine first attracted the notice of Watt, who was gradually led to the considera. tion of the defects of the engine just described. He saw that these defects consisted in driving the lid or piston down by the force of the atmosphere; and this defect he succeeded in removing, by making a steam-box also above the lid, and so driving the lid down in the same way as it had been driven up.

« PreviousContinue »