Page images
PDF
EPUB

or as the interval of the pulses is greater or less. So that those bodies, which vibrate slowest, and thereby make the greatest intervals between the pulses, have the gravest and deepest tone, as the impression on the ear continues longest; and those bodies which vibrate quickest have the sharpest and shrillest tone. As we have seen already that the times of vibrations, and consequently the tones from musical strings, depend on the lengths, diameters, and tension of the strings; so the same variations will take place in the tones of musical pipes in an organ, arising from the diameter and length of the pipe, and the intensity of the blast, which condenses the air.

7. The velocity and intensity of sound will be altered by the direction of the wind. When the atmosphere is still, the pulses are propagated in all directions from the sounding body. But as the wind is a current of air moving with a certain force, the pulses excited by the sounding body will partake in some measure of its motion. The pulses therefore, which move in the direction of the wind's motion, will be carried to a greater distance on that side, and thereby increase the intensity of the sound in that quarter: so that a person on that side to which the wind carries the sound, will hear it farther, and louder, at a given distance, than he could hear it, in the opposite direction.

From hence it is evident, that if the wind moved with the same velocity that sound does, the pulses, as soon as they were produced, would be all carried off in the direction of the wind, and the sound would not be heard at all against the wind. But the wind, even in storms and hurricanes, seldom moves more than 70 or 80 feet per second, and sound moves at the rate of 1142 feet in the same time. Therefore, as sound moves with a

much greater velocity than wind, the pulses will be propagated, and the sound will be heard, against the wind; though it is weaker and reaches to a smaller distance in this direction, than it does on the other side of the sounding body where it has a fair wind for its conveyance.

Now, if the current of air move the same way with the sound, the sound will partake of its motion, and be accelerated in its velocity by it; but if the sound move in a contrary direction to the wind, it will be retarded by the action of the wind, and therefore will move against the wind with a less velocity than that with which it moves with the wind.

8. The pulses are propagated from the same body, and from different bodies at the same time, without disturbance or confusion. But the capacity of distinguishing different sounds, when the pulses strike the ear in quick succession, depends upon a good ear, perfected by long experience and habit.

9. The particles of air striking against any obstacle, will be reflected back from it, under an angle equal to the angle of incidence; and consequently, new pulses will begin to diffuse themselves every way from that place; and, arriving at the ear, excite in us the idea of a sound, originating from the place of reflection; which secondary sound is called an echo. As the place of the sound is always supposed to be, where the waves begin to diffuse themselves, in all directions; a person, speaking at one end of a tube, is heard, as speaking from the other. As the phonocamptic object is more or less distant from the first place of the sound, the echo will be less or more distinct and audible; as the pulses were more or less languid before they

be

were reflected. If the sound be reflected from various objects, at different distances from the ear, it may often repeated, and thereby constitute the prattling echo. Hence the fragor and rumbling noise of thunder and cannons: hence, also, a number of the last syllables of a sentence will be distinctly heard; as the sound will be returned to the ear, from different objects, at different distances.

Should all the pulses, excited at any place, be collected together in a single point, after various reflections, by any particular contrivance, the sound will be as distinctly heard in this point, as at the place from whence the waves were first diffused. Hence we account for the effect of the whispering gallery in the dome of St. Paul's in London. This dome is a hollow hemisphere, with a gallery in the inside, in which every pulse excited by the lowest whisper is collected at the opposite point of the gallery, where all the circles of the sphere intersect each other; and there the lowest whisper is as distinctly heard, as at the mouth of the speaker.

From this property of sound, we may estimate the breadth of rivers, or the distance of any object that reflects the sound, as easily as from the direct sound; as the reflected sound travels at the same rate with the direct; its velocity being the same both before and after reflection, viz. 1142 feet per second. By this property of sound and the laws of descent in falling bodies, we can also estimate the depths of wells, and the height of steeples, &c. by observing how many seconds elapse, between the time when the heavy body begins to descend, and the time when the reflected sound reaches the ear. Thus,

Let x=the time, in seconds, of the descent of a stone from the top to the bottom.

Let s the seconds elapsed from the beginning of the descent till the return of the sound.

Then s-x-the seconds of the sound's ascent. Let a 16.12 feet, the space a heavy body falls in a second.

And b=1142 feet, the space that sound moves over in a second.

Then as 12: a :: x2: ax2-the height of the steeple or depth of the well.

And as 1" : b:: s-x: bs-bx=the height of the steeple, &c.

[merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

OPTICS is that branch of natural philosophy, which explains the nature of light and colours, the cause of vision, and the construction of optical instruments.

Optics is generally divided, by writers on this subject, into two parts, called dioptrics and catoptrics; by the former is meant the vision occasioned by light transmitted through any transparent substance, and by the latter, the vision by the rays of reflected light.

Before we enter upon this science, it will be necessary to define and explain a few terms, which do not often occur in common life, and are almost peculiar to this branch of philosophy.

A ray of light is a stream of particles issuing from the sun, or any other luminous body, in one and the same direction.

A beam or pencil of rays is a number of rays proceeding parallel to each other from the same luminous body.

The axis of a beam or pencil of light is the middle ray.

A medium, in optics, is any thing that is transparent and affords a passage for the rays of light; and, in this sense, a vacuum through which the rays pass is denominated a medium.

The right course of a ray of light is a straight line, for all rays proceed in right lines from a luminous body; as is evident from the similarity of the shadows, which are projected from opake bodies, to the bodies themselves, being terminated by right lines, from the luminous point, passing through the various corners and sides of the bodies.

The inflexion of a ray of light is its being turned out of its right-lined direction, while it is passing on in the same medium, by the attractive force of any body, near which it passes. If a pencil of rays pass by the edges of two knives set together within a tenth of an inch, they will be so much bent out of their rightlined direction, by the attraction of the knives, that they will be dispersed in the shadows of the knives, which will therefore be in contact.

The reflexion of a ray of light is its being so much turned out of its right course as to be sent back into the same medium again.

The refraction of a ray of light is its being bent out of its course into another direction, when it passes out of one medium into another.

The angle of incidence is that contained between the incident ray, and a perpendicular to the surface of the medium, at the point of incidence.

« PreviousContinue »