Dr. Wollaston has given the following scale of sounds, which are scarcely audible by some cars: Cry of the Gryllus compestris.
Piercing squeak of the bat.
Chirping of the house cricket.
Chirping of the house sparrow, or four octaves above l in the middle of the piano forte.
Dr. Wollaston is of opinion, that human hearing ex tends but a few notes above the cry of the Gryllus cam pestris. Ile has met with several persons, who never heard it, nor the squeak of the bat; with some, who never heard the chirping of the house cricket; and with one gentleman, who never heard the chirping of the common house sparrow. This he considers as the lowest limit of acute hearing, and the cases in which it exists to be very rare. He regards the note of the bat as a full octave higher than this, and he believes that some insects may reach as far as one octave more; and the range of human hearing he conceives to he comprised between the lowest notes of the or gan, and the highest known cry of insects, including more than nine octaves, the whole of which are dis tinctly perceptible by most ears, although the vibra tions of a note at the higher extreme are 600 or 700 fold more frequent than those which constitute the gravest audible sound.
Dr: Wollaston concludes his very important paper by the following curious conjecture. " Since there is nothing," says he, "in the constitution of the atmo sphere, to prevent the existence of vibrations incompa rably more frequent than any of which we are con scious, we may imagine that animals like the grylli, whose powers appear to commence nearly where ours terminate, may have the faculty of hearing still sharp er sounds which at present we do not know to exist; and that there may be other insects hearing nothing in common with us, but endowed with a power of excit ing, and a sense that perceives vibrations of the same nature indeed as those which constitute our ordinary sounds, but so remote, that the animals who perceive them may be said to possess another sense, agreeing with our own solely in the medium by which it is ex cited, and possibly wholly unaffected by those slower vibrations of which we are sensible." See Phil. Trans. 1820, part ii. p. 306-314.
4. On the Increase in the Intensity of Sound during the Night.
Every person must have observed, that sounds, such as that of falling water, Exc. which are faintly or not at all heard during the day time, arc distinctly audible at night, even when the direction and force of the wind, and every other general circumstance is the same. This curious fact was remarked even by the ancients. In large cities, or in their neighbourhood, this increase in the distinctness of sound has been ascribed to the cessation of the powers of animated beings, such as men, insects, and birds, and also to the cessation of the action of the wind upon the leaves of the trees. When the celebrated traveller, Baron Humboldt, first heard the noise of the great cataracts of the Orinoco, in the plain which surrounds the Mission of the Apures, his attention was particularly called to this curious fact; and he was of opinion that the noise was three times greater in the night than in the day. The usual explanation of the phenomenon was quite insufficient in the present case, as the humming of insects was much greater in the night than in the day, and the breeze, which might have agitated the leaves of the trees, never rose till after sunset. Humboldt, there fore, was led to ascribe the diminution of the sound during the day to the presence of the sun, which in fluences the propagation and intensity of sound, by opposing to them currents of air of different density, and partial undulations of the atmosphere, produced by unequal heating of the different parts of the ground. In these cases, a wave of sound, when it meets two portions of air of different density, is divided into two or more waves, a part of the primitive wave being propagated with more rapidity through the denser portions, than the parts that pass through air of less density. In this way the wave is broken down into different parts which arrive at the car at different times. These different portions of the wave pass ing again through succeeding portions of the atmo sphere of different density, may be so wasted and frittered down, as to be incapable of affecting the tym pontiff:.