cal illusion. From the principles of foreshortening likewise, objects seen from great heights, especially from high precipices, appear much diminished. This effect is very strikingly displayed in the Shetland island of Foula, the celebrated Thule of the ancients, the favourite resort of innumerable flocks of sea birds. Here there is a precipice 1100 or 1200 feet perpendicular, affording to those who have nerve enough to contemplate the scene beneath, a spectacle more stupendous than the wildest fancy could imagine. From this giddy height, the thousands of birds beneath skimming through the air in varied circles, each according to its kind, seem like so many moths and butterflies, and the green expanse of water beyond, with the heaving and curling waves, appears like another sky studded with fleecy clouds.

Habit tends in some measure to dissipate these optical illusions, and persons who are accustomed to view objects from different heights, and to compare them with neighbouring and interposed known objects, are enabled readily to allow for the effects of foreshortening, and obtain correct notions respecting their magnitude and distance. The tall houses and high precipices of Edinburgh and its vicinity, are well fitted for imparting to its inhabitants this kind of knowledge. Still, when a new object is presented to the view, of unusual grandeur and magnitude, previous education fails in enabling the mind to obtain a just conception. On this account, it is not till after some time, and repeated observation and reflection, that the stupendous magnificence of such structures as St. Paul's is justly appreciated. I myself had a very forcible instance of this on visiting St. Paul's last year for the first time. On a very favourable day in August I ascended to the ball, and spent some hours in contemplating the city from the lantern and outer galleries of the dome. I did not, in the slightest degree, observe any diminution of the persons and other objects passing on the streets, which so frequently strikes strangers from the

same situation. This I attributed to my being in the habit of seeing objects from similar heights in and around Edinburgh, as from the Castle, the Calton Hill, and Salisbury Crags, and therefore accustomed to estimate and allow for the influence of relative situation.

Upon descending, however, to the whispering gallery, and looking over upon the people walking on the marble floor beneath, I was not a little surprised to notice that their persons were reduced to the most dwarfish dimensions. The choir also appeared very much smaller than my recollection and judgment knew it to be, and by no effort on my part could I dissipate the illusion. Inquiring of the attendant if the persons beneath appeared to him diminished, he assured me they did not.

Now this illusion on my part, and not on his, depended upon the conceptions which at the time we respectively entertained of the true relative size of that splendid structure, my untutored mind was incapable of grasping at once its grandeur and real magnitude, and consequently a reduced standard was employed when endeavouring to comprehend it; and by the same reduced standard, the persons on the floor were examined, and therefore they seemed dwarfish, while custom had habituated the eye, or rather the mind of the attendant, to the estimation of their just relative proportion.

The painter has recourse to contrast with the greatest advantage. In order to convey an idea of the size and distance of unknown objects, he brings upon the field known objects to serve as a standard of comparison. Contrast therefore is one of the most efficient means by which we judge of size and distance.

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CHAPTER IX.

HEARING.

Of the Nature of Sound-Grave and Acute Tones-Number of Vibrations appreciable by the Human Ear-Transmission of Sounds-Velocity of Sound-Stethoscope-Echo-External Ear adapted for collecting Vibrations-Middle Ear or Drum constructed for the Transmission and Modification of the VibrationsDifferent Apparatus of―Internal Ear or Labyrinth the immediate seat of the Sense of Hearing-Construction and Apparatus of-General Inferences deduced from the Examination of the Structure as a whole.

SOUND has no existence as a distinct substance, but is merely the result of certain conditions of bodies. When a body is struck, its particles are thrown into a state of motion which extends in every direction to a greater or less distance, according to the intensity of the impulse. Thus, if a pebble be cast upon the smooth and tranquil surface of a pool of water, undulating circles are produced; these radiate from the point struck in every direction of the surface, gradually diminishing, till they imperceptibly subside altogether in the distance. The undulations or oscillations in this instance are not confined to the surface of the water, where they are visible, but extend to the whole body of water, in every direction, and likewise to the air in contact with it; consequently the point where the water and pebble come in collision is the centre of a sphere from which the oscillations are communicated to a hemisphere of water in one direction, and to a hemisphere of air in the other. We may take another example: If an elastic wire be fixed at one end, and the other laid hold of and pulled to one side, when set free,

it will move towards the point from whence it was withdrawn, where, however, it will not stop, but proceed beyond it, and again return to the point where it was let go, and thus it will continue to traverse the space, till at last it comes to rest. During these movements it will communicate to the air in contact with it similar vibrations. These motions are similar to the oscillations of a pendulum. They may be performed with such rapidity that the eye cannot follow them, while in the measured movements of the pendulum they are easily noted.

When the vibrations are slow or few in a given time, the individual impulses may be counted; but when the motions are performed with rapidity, their individuality is lost, and a continuous sound or tone is produced. When the vibrations of a sonorous body are numerous in a given time, or follow each other in quick succession, the sound is acute or high; and when fewer during the same period, a grave or low sound is produced. The highest tones appreciable by the human ear consist of 32,000 vibrations in a second, and the lowest 32.

On this point Dr Wollaston observes, (Phil. Trans. 1820), "In the natural healthy state of the human ear, there does not seem to be any strict limit to our power of discerning low sounds. In listening to those pulsatory vibrations of air of which sound consists, if they become less and less frequent, we may doubt at what point tones suited to produce any musical effect terminate; yet all persons but those whose organs are palpably defective continue sensible of vibratory motion, until it becomes a mere tremor, which may be felt and even almost counted.

"On the contrary, if we turn our attention to the opposite extremity of the scale of audible sounds, and with a series of pipes exceeding each other in sharpness, if we examine the effects of them successively upon the ears of any considerable number of persons, we shall find (even within the range of those tones which are produced for their musical effects) a very distinct and striking differ

ence between the powers of different individuals, whose organs of hearing are in other respects perfect, and shall have reason to infer that human hearing in general is more confined than has been supposed with regard to its perception of very acute sounds, and has, probably in every instance, some definite limit, at no great distance beyond the sounds ordinarily heard."

He states further, "The suddenness of transition from perfect hearing to total want of perception, occasions a degree of surprise, which renders an experiment on this subject, with a series of small pipes, among several persons, rather amusing. It is curious to observe the change of feeling manifested by various individuals of the party in succession, as the sounds approach and pass the limits of their hearing. Those who enjoy a temporary triumph, are often compelled, in their turn, to acknowledge to how short a distance their little superiority extends."

Dr Wollaston mentions, that a friend of his could imperfectly hear a note four octaves above the middle E of the pianoforte, and that he could not hear the F next above it, although his hearing in other respects was perfect, and his perception of musical pitch as correct as that of any other person. He likewise gives instances where the chirp of the grasshopper could not be heard; and he observes, that" inability to hear the piercing squeak of the bat seems not very rare, as I have met with several instances of persons not aware of such a sound." He also cites the case of a gentleman who could never hear the chirping of a house sparrow.

The number of vibrations which a string of a musical instrument makes in a given time varies according to its length, tension, and thickness, consequently, according as these conditions vary, so will the tones produced. If a string vibrates a hundred times in a second, when shortened by one-half its length it will vibrate two hundred times, and give out a sound more acute, or higher, by one octave, and so on according to its length. If a string