exercising the weak ear, but in such a manner as not to fatigue it. The one thus made to labour alone will always retain the same force. The child's ear should from time to time be unstopped, in order to make it sing, and to discover whether both ears have the same degree of sensibility."

This natural defect may be then corrected, and any person may be made to acquire a true voice, provided the means pointed out by Dr. Vandermonde be early employed.

Persons who have a false voice, in consequence of some inequality in the ears, may be compared to those who squint: that is to say, who, in order to see an object distinctly, do not turn equally towards it the axis of both eyes, because they have not the same visual powers. It is probable, that the former, if they had early accustomed themselves to make use of only one ear, would hear distinctly different sounds which they would have imitated, and would not have contracted a false voice.

Of the Speaking Trumpêt and Ear Trumpet.

As the sight is assisted by telescopes and microscopes, similar instruments have been devised also for assisting the faculty of hearing. One of these, called the speaking trumpet, is employed for conveying sound to a great distance: the other, called the ear trumpet, serves to magnify to the ear the least whisper.

Sir Thomas Morland, among the moderns, bestowed the most labour in endeavouring to improve this method of enlarging and conveying sound; and on this subject he published a treatise, entitled De Tubáten- S torophonica, a name which alludes to the voice of Stentor, so celebrated among the Greeks for its great strength. The following observations on this subject are in part borrowed from that curious work.

The ancients, it would seem, were acquainted with the speaking trumpet, for we are told that Alexander had a horn, by means of which he could give orders to his whole army, however numerous. Kircher, on the

authority of some passages in a manuscript, preserved in the Vatican, makes the diameter of its greatest aperture to have been seven feet and a half. Of its length he says nothing, and only adds, that it could be heard at the distance of 500 stadia, or about 25 miles.

However this may be, the speaking trumpet is nothing else but a long tube, which at one end is only large enough to receive the mouth, and which goes on increasing in width to the other extremity, bending somewhat outwards. The aperture at the small end must be a little flattened to fit the mouth; and it ought to have two lateral projections to cover part of the cheeks.

Sir Thomas Morland says, that he caused several instruments of this kind to be constructed, of different sizes, viz. one of four feet and a half in length, by which the voice could be heard at the distance of 500

geometrical paces; another 16 feet 8 inches, which conveyed sound 1800 paces; and a third, of 24 feet, which rendered the voice audible at the distance of 2500 paces.

The reason of this phenomenon is as follows:- -As the air is an elastic fluid, so that every sound pronounced in it is transmitted spherically around the sonorous body, when a person speaks at the mouth of the trumpet, all the motion which would be communicated to a spherical mass of air, of four feet radius, for example, is communicated only to a cone, the base of which is the wider extremity of the trumpet. Consequently, if this cone is only the hundredth part of the whole sphere of the same radius, the effect will be the same as if the person should speak a hundred times as loud in the open air: the voice must therefore be heard at a distance a hundred times as great.

The ear trumpet, an instrument exceedingly useful to the deaf, is nearly the reverse of the speaking trumpet: it collects, in the auditory passage, all the sound contained within it; or it increases the sound produced at its extremity, in a ratio which may be said to be as that of the wide end to the narrow end. Thus, for

example, if the wide end be 6 inches in diameter, and the aperture applied to the ear 6 lines, which in surfaces gives the ratio of 1 to 144, the sound will be increased 144 times, or nearly so; for we do not believe that this increase is exactly in the inverse ratio of the surfaces; and it must be allowed, that in this respect acoustics are not so far advanced as optics.

ELECTRICITY.

DEFINITIONS.

1. ELECTRICITY is that property in bodies which enables them, when excited by friction or heat, to attract other light bodies, and produce an effluvium that is sometimes luminous, attended with a snapping noise, and a faint phosphoreal smell.

2. Electricity is called the second of the three species of attraction, gravity being the first, and magnetism the third.

3. Those bodies that produce electricity by friction or heat, are called electrics, and are said to be electric per se.

4. Those bodies that receive and communicate electricity are called conductors, and those that repel or will not suffer it to pass through them, are called nonconductors.

5. All bodies that are made to contain more than their natural quantity of electricity are said to be electrified positively, and those from whom part of their natural quantity is taken away, are said to be electrified negatively. These two electricities being first produced, one of them from glass, and the other from amber, wax, or rosin, the former was called vitreous and the latter resinous electricity.

6. When a quantity of electricity is communicated to any body, it is said to be charged.

7. The effect of the explosion of a charged body, that is, the discharge of its electricity through any other body, is called the electric shock.

8. When any body is prevented from communicating with the earth, by the interposition of an electric body, it is said to be insulated.

9. The residuum of a charged body, as a jar or battery, is that part of the charge which remains in the body after the first discharge, and by which it will give a second shock, though less than the first.

APHORISMS.

1. All substances are distinguished into electrics per se, and non electrics: the latter of which are conductors, and the former non-conductors.

2. All kinds of metals, semi-metals, water, charcoal, and other bodies of a similar nature, are conductors; and all other bodies, whether mineral, vegetable, or animal, are non-conductors: many of the latter, however, may be made to conduct electricity by being heated to a certain degree.

3. Positive electricity is produced by the friction of uninsulated glass tubes or globes; and negative electricity is produced either from the rubber of those bodies, or from the friction of insulated glass bodies; or lastly, from the rubbing of globes or sticks of wax, sulphur, and other bodies of a similar nature.

4. It follows from the last aphorism, that the electricity of the excited body and the rubber, are always opposite, that is, if that of the excited body be positive, that of the rubber will be negative; and the contrary. Those two bodies moreover will act on each other with greater force than any other body.

5. In charging any body, as a coated phial, if one side communicate with the excited body, and the other with the rubber, the electricity of the two sides of the charged body will be opposite.

6. There is a strong attraction between the two electricities on the opposite sides of a glass, so that when they are made to communicate by means of a conductor, they will be both discharged with a flash of light, and a snapping noise.

7. The substance of glass is impervious to electricity; but if the glass be thin, and the electricity on the opposite sides be very strong, that is, if the glass be