Графични страници
PDF файл
ePub

people, without being out of countenance, or without embarrassment. A man who is really diffident, timid, and bashful, be his merit what it will, never can push himself in the world, his despondency throws him into inaction, and the forward, the bustling, and the petulant, will always pres cede him. The manner makes the whole difference. What would be impudence in one man, is only a proper and decent assurance in another. A man of sense, and of knowledge of the world, will assert his own rights, and pursue his own objects as steadily and intrepidly as the most impudent man living, and commonly more so; but then he has art enough to give an outward air of modesty to all he does. This engages and prevails, wbilst the very same things shock and fail, from the overbearing and impudent manner only of doing them.

Englishmen, in general, are ashamed of going into company. When we avoid singularity, what should we be ashamed of? And why should we not go into a mixed company with as much ease and as little concern as we would go into our own room ? Vice and ignorance are the only things we ought to be ashamed of; while we keep clear of them we may venture anywhere without fear or concern. Nothing sinks a young man into low company so surely as bashfulness. If he thinks that he shall not, he most assuredly will not please.

Some, indeed, from feeling the pain and inconveniences of bashfulness, have rushed into the other extreme, and turned impudent; as cowards sometimes grow desperate from excess of danger: but this is equally to be avoided, there being nothing more generally shocking than impudence. The medium between those two extremes points

out the well-bred man, who always feels bimself firm and easy in all companies, who is modest without being bashful, and steady without being impudent. A mean fellow is ashamed and embarrassed when he comes into company, is disconcerted when spoken to, answers with difficulty, and does not know how to dispose of his hands ; but a gentleman, who is acquainted with the world, appears in company with a graceful and proper assurance, and is perfectly easy and unembarrassed. He is not dazzled by superior rank; he pays all the respect that is due to it, without being disconcerted; and can converse as easily with a king as with any one of his subjects. This is the great advantage of being introduced young into good company, and of conversing with our superiors. A well-bred man will converse with his inferiors without insolence, and with his superiors with respect and with ease. Add to this, that a man of a gentlemanlike behaviour, though of inferior parts, is better received than a man of superior abilities, who is unacquainted with the world. Modesty, and polite easy assurance, should be united.--Chesterfield.

58. The Air-Pump. AMONG the inventions of the Germans, we ought to take notice of that noble instrument the air-pump, to which the present age is indebted for so many fine discoveries, and which is ascribed to Otho de Guerick, a consul or burgomaster of Magdebourg, who exhibited his first experiments therewith before the emperor and the states of Germany at Ratisbon, in the year 1654. It is true, some bave ascribed the invention to the honourable Mr. Boyle, and the engine has obtained the name of Machina Boyleana; but this is rather on account of the improvements made in it, or the various experiments it was applied to, by that great philosopher : for though he had actually made some attempts of this nature before he knew any thing of what was done in Germany, yet he ingenuously confesses de Guerick to have been beforehand with him, and that it was the information he afterwards received from a book published by Schottus, containing an account of de Guerick's experiments, which enabled him to bring his design to maturity. This machine has been of late years much improved by the ingenious Mr. Hawksbee and others, and seems now to be carried to its utmost perfection.

The use and effect of the air-pump is to exhaust the air out of any proper vessel, and thereby make what is popularly called a vacuum, which in reality is only such a degree of rarefaction, as is sufficient to suspend the ordinary effects of the atmosphere. Hence, therefore, we learn, in some measure, what our earth would be without the mass of air which surrounds it, and how necessary it is to the life, generation, and nutrition, both of animals and vegetables, and to various other purposes, in which we are apt to think it is not at all concerned.— The principle on which this machine is founded is the elasticity or spring of the air ; and the basis or essential part of it is a metallic tube, answering to the barrel of a common pump or syringe, having a valve at the bottom opening upwards, and a moveable piston, answering to the sucker of a pump, furnished likewise with a valve opening upwards ; the whole properly fitted to a vessel as a receiver, from which the air is to be exhausted.

The other

parts of it, chiefly respecting conveniency, have been diversified and improved from time to time, and particularly Mr. Hawksbee, by adding a second barrel and piston to the former, to rise as the other falls, and fall as it rises, which makes the working of the engine much easier than it was before. To this gentleman we are indebted for the structure of the air-pump now in common use; but Mr. Martin has lately contrived one of a different form, which he calls a portable airpump, being so constructed that it may, together with its receivers, be contained in a box of a small size, and sold at a small price in comparison of those before invented.

The reader must observe, as we have already hinted, that an absolute vacuum cannot be obtained by the air-pump, i. e. all the air cannot be exhausted out of the receiver, be the pump ever so good, or worked ever so long; for as the air which is drawn out is only expelled by the spring of that which remains behind, if we suppose every particle to be exhausted, the last must be expelled without an agent, or there would be an effect without a cause, the absurdity whereof is evident. However, the air is dilated by this machine to such a degree, that bodies placed in an exhausted receiver are usually said to be in in vacuo ; and by this means a vast number of curious experiments have been made, which have given great light into the secrets of nature, and of which, if we mention a few of the most remarkable, it perhaps will be an agreeable entertainment to those who are unacquainted with such phænomena.

First then, to demonstrate the spring of the air, if a bottle whose mouth is securely sealed, so that no air can escape from within, be placed under a receiver, and the air exhausted from the surface of the bottle, the spring of the included air will burst it to pieces. 2. If a bladder be almost emptied of air, and tied very tight, the air within will expand itself in vacuo, so as to make the bladder appear full-blown. 3. By making a little bole in the small end of a new-laid egg, and exhausting the receiver, the bubble of air contained between the shell and skin at the great end will expand itself, so as to force the white and yolk of the egg through the hole; and if half the shell of the egg thus emptied be taken off, the said bubble will raise up the skin, so as to resemble an entire egg. 4. A shrivelled apple will become smooth and plump under the exhausted receiver, by the expansion of the air contained in its substance. 5. Fishes are made so light by increasing the spring of the air in their bladders, upon exhausting the receiver, that they rise to the top of the water, and cannot again descend to the bottom. 6. Water heated as hot as the finger can well bear it, will boil vehemently under the exhausted receiver ; but upon letting the air in again the agitation ceases. 7. Fresh small beer placed in

up

in froth, dies, and loses all its taste. 8. Vegetable substances put in water under the receiver emit great quantities of air from all parts of their surface. 9. The weight or pressure of the air is shewn by exhausting two hollow brass hemispheres, which are then compressed together by the external air, so as to require a force to pull them asunder equal to 138 pounds, if the diameter of the hemispheres be four inches. 10. A person placing his hand on the top of an open receiver, will very sensibly feel the pressure of the air on the back of his hand, in proportion as the receiver is exhausted; and the spring of the air contained in the flesh exerting itself at the same

vacuo rises

« ПредишнаНапред »