Imágenes de página

LESSON 147.- Original. 405. From the following objects let the pupil select one or two with which he is most familiar, and give a Description drawn from his own observation :

1st. in structure of their habi1. Instances displaying the tations. Instinct of Animals, – 2nd. in their means of self

preservation and defence. 2. Transformation which several insects undergo.

[blocks in formation]

406. OBJECTS OF INVENTION. RULE 1. State the purpose for which the object is designed.

2. Its form or figure, with an analysis of its parts.

3. The materials of which it is made, and the manner or process of its construction.

4. The mode of working it.

5. A Diagram, accompanying the description, will frequently be of great assistance in conveying a more accurate conception of the various parts.

Memoriter Exercise. 407. 1. Read the following Description two or three times, noticing the sequence of the tences.


[ocr errors]

2. Reproduce the Example from recollection.

3. Institute a Comparison between your own and the original, when all deviations either in construction, punctuation, or sequence must be noticed.

408. MODEL.—The Common PUMP. 1. The Common or domestic Pump is employed in raising water, and depends, for its efficacy, on the principle of atmospheric pressure. 2. It consists of a long tube or barrel, called the pump-log, which reaches from a few feet above the ground to near the bottom of the well. At the lower part of the tube is a valve, opening upwards, call the pump-box. When the pump is not in action, this is always shut. 3. At some distance above the lower valve is placed a short movable cylinder, called the piston. The piston has an aperture through it, which is closed by a valve opening upwards like the lower valve.

4. The following is the mode of working the Pump.-Suppose the Piston pressed down towards the lower valve, then, on depressing the handle or lever at the top, a vacuum would be formed between the Piston and lower valve, did not the water in the well rise, in consequence of the pressure of the atmosphere on that around the pump-log in the well, and take the place of the air thus removed. Then, on raising the end of the lever or handle, the lower-valve closes, because the water is forced upon it, in consequence of the descent of the piston, and at the same time, the valve in the piston opens, and the water, which cannot descend, now passes above the valve. Next, on raising the piston, by again depressing the lever, this portion of water is lifted up to the piston, or a little above it, while another portion rushes through the lower valve, to fill its place. After a few strokes of the lever, the space from the piston to the spout is filled with the water, where, on continuing to work the lever, it is discharged in a constant stream.


LESSON 149.-Hints.

409. From the following Hints, which are given in regular succession, produce a Description, developed and expressed as nearly as possible in accordance with the previous rules :

410. TAE BAROMETER. I. Barometer philosophical instrument measuring weight atmosphere. 2. Barometer said invention Torricelli, observing column water about 33 feet equal in weight to one of air same base, concluded that column of mercury only 29] inches would be so too, such column mercury equal weight to 33 feet water.

3. Common barometer glass tabe about two-tenths inch diameter, its length at least thirty-one inches. 4. This tube filled mercury so not have air over it, the maker placing finger on end, immerses in basin of quicksilver, and then takes finger away. 5. Quicksilver in tube by own weight endeavours descend into basin : but external air pressing on surface of quicksilver in basin without, no air in tube at top, quicksilver continue in tube, raised by air on surface in basin below.

6. Usual range barometer in this country from 28 to 31 inches; when air pure and heavy, raises mercury to nearly thirty-one, when light and full of vapours falls to nearly 28. 7. In fine dry weather, air rendered pure, free light vapours, consequently extremely heavy, presses up the quicksilver. 8 In moist rainy weather atmosphere charged with vapours, clouds, fogs, air sensibly lighter, presses on quicksilver less force. 9. When high winds blow, atmosphere light, quicksilver generally low, rises higher in cold weather than warm. 10. During frost, air is purest heaviest, barometer rises highest points.

11. Instrument also serviceable measuring height of moun

tains. 12. Ascending mountains, quicksilver found sink about tenth an inch in ninety feet; that if quicksilver fall an inch, have ascended nearly; but this subject to variations, from change of temperature and other causes, render corrections necessary. 13. General method, however, determining altitudes by barometer and thermometer extremely useful convenient, ingenious rules by Hutton, Gregory, and others, to facilitate computation,

LESSON 150,-Memoriter.

411. MANUFACTURES. - RULE 1. Sometimes the origin and progress of the manufacture may be given, along with the advantages which it has conferred on society. 2. At other times, the attention may be confined to a description of the various processes which the article undergoes. These must be detailed, as before stated, in the order of operation or occurrence.

Memoriter Exercise. 412. 1. Read the following Description two or three times, noticing the sequence of the sentences.

2. Reproduce the Example from recollection.

3. Institute a Comparison between your own and the original, when all deviations either in construction, punctuation, or sequence must be noticed.

413. MODEL.-MANUFACTURE OF Pins. 1. When the brass wire is received at the manufactory, the first operation is winding it off from one wheel to another, and causing it to pass through a circle in a piece of iron of a


smaller diameter. When reduced to its proper dimensions, it is straightened by drawing it between iron pins, fixed in a board in a zigzag manner ; afterwards it is cut into lengths of three or four yards, and then into smaller ones, every length being sufficient for six pins ; each end of these is ground to a point, which is performed by boys, who sit each with two small grindstones before him turned by a wheel. Taking up a handful, he applies the coarser of the two stones, moving the wires round between his fingers, so that the points may not become flat; he then gives them a smoother and sharper point on the other stone : and by these means a lad of twelve or fourteen years of age is enabled to point about 16,000 pins in an hour. When the wire is thus pointed, a pin is taken off from each end, and this is repeated till it is cut into six pieces.

2. The next operation is that of forming the heads, or, as it is termed, head spinning, which is done by a spinning wheel; one piece of wire being thus, with astonishing rapidity, wound round another, and the interior one being drawn out, leaves a (hollow) tube between the circumvolutions. It is then cut with shears, every two circumvolutions or turns of the wire, forming one head; these are softened by throwing them into iron pans, and placing them in a furnace till they are red hot. As soon as they are cold, they are distributed to children, who sit with anvils and hammers before them, which they work with their feet by means of a lathe, and taking up one of the lengths, they thrust the blunt end into a quantity of the heads which lie before them, and catching one at the extremity, they apply them immediately to the anvil and hammer, and by a motion or two of the foot, the point and the head are fixed together in much less time than can be described, and with a dexterity that can only be acquired by practice.

3. The pin is now finished as to its form, but is still merely brass ; it is therefore thrown into a copper containing a solution of tin and the lees of wine. Here it remains for some time, and when taken out assumes a white though dull ap

arance; in order therefore to give a nolish, it is put into a

« AnteriorContinuar »