smell. The amount of substance that rubs off from a rabbit’s feet onto the ground at each step cannot be much to begin with; yet this continues for hours to give off gas into the air, and a dog coming along at any time meanwhile will get enough of the gas into his nostrils to smell it. Fortunately for our comfort the sense of smell fatigues very rapidly. An odor that is excessively disagreeable at first presently no longer troubles us. If it were not for this, it would be almost, if not quite, impossible to obtain laborers in those industries where the odor is necessarily bad. There is, however, a source of danger in this quick subsidence of smell perception. About our only method of judging offhand as to the ventilation of a room is by the smell, and this fails as a guide when we have been in the room for a time. Persons coming in from outside are often struck by the bad state of the air in rooms whose occupants are not conscious that there is anything amiss. Because of this the ventilation of schoolrooms usually is not, and never should be, left merely to the judgment of the teacher, but definite rules are laid down as to opening of ventilators or windows.
When the gas that is smelled is part of an inward air current we recognize it as coming from the outside and call it an odor; when it is part of an outward current we call it a flavor. On account of the rapid fatigue of the sense of smell we are unconscious of the smell of our own breath, but can get fresh smells from within, and these come, practically always, from materials that have just been taken into the mouth. In comparison with taste flavor furnishes great variety of perception. As persons become connoisseurs in food their enjoyment depends more and more on flavor and less and less on taste. The sensation from spices is a combination of flavor with irritation of the tongue that is partly pain and partly touch. The sense of smell has evidently a two-fold use; it makes us aware that there is food in the vicinity, or sometimes that disagreeable things are near at hand; and it shares with taste the duty of enabling us to judge of the food as it is being eaten. Agreeable tastes, flavors, and odors add much to the enjoyment of life. Within reasonable limits it is well to cultivate this kind of enjoyment, for while there is no doubt that it can be overdone, as in the excessive lengths to which the decadent Romans went to gratify their taste and smell, neither is there any doubt that bodily health in general, and the bodily function of digestion in particular, benefit definitely from the kind of enjoyment that savory food and delightful odors bring. It is the duty of those charged with the responsibility of preparing and serving food to take pains that full advantage is taken of the possibilities present in what food is to be prepared. This does not mean expensive food; what it does call for is skillful handling of all food, whether cheap or costly.
In introducing the subject of hearing we shall have to say a few words about that which is heard, namely sound. Any object that has any degree of elasticity at all is apt, if struck or rubbed or otherwise set suddenly in motion, to start vibrating back and forth; the vibrations will nearly always be regular, and will occur at a rate that is the same for that particular object whenever it vibrates. The rate depends on the size, the character, and the degree of stretch of the object. Air is to all intents and purposes perfectly elastic; it is set vibrating by any object that is vibrating in it, but since it has no particular size nor degree of stretch it takes the vibration rate of the object that started it going in the first place. The vibrations once started in air spread in all directions, just as waves spread from a stone thrown into a pond, and when these air waves strike upon another object that is free to vibrate they will set it going at the same rate. The human hearing apparatus is a device which is set in vibration by air waves, and the result is called sound. The ear is limited in its ability to respond to vibrations; they must be neither too fast nor too slow; if slower than 16 a second, most people will fail to hear them, and the same is true if they are more rapid than about 40,000 a second. Between these limits vibrations that strike upon the ear are heard as sounds.
Differences in vibration rate between one sound and another can be recognized by the ear; the difference is a matter of pitch. By the pitch of a tone we mean the vibration rate which it has. More rapid vibrations give tones of higher pitch; a slow rate means a low pitch. Middle C on the piano has a rate of either 256 or 261 a second according to the system used by the tuner. The human voice has an extreme range starting with the lowest note that the bass voice can compass with a rate of about 80 vibrations a second, to the highest note that famous sopranos can attain at about 1,400 a second. There is a record of a singer who could achieve a tone with a rate of 2,100 a second, but this has not been duplicated so far as is known. Of course no single voice can cover more than a fraction of this range. Most men produce all their tones at rates of between 90 and 500 a second, and women between 200 and 800 a second. Not every different vibration rate is heard as a tone of different pitch; the ear is not sensitive enough for that. The interval between one note and the next includes several vibrations, more the higher one goes in the scale. A perfectly true tone has exactly the rate called for; a departure of one or two vibrations a second may not be noticed, but if the error is greater the singer is sharping or flatting his tone, according as he is above or below the true rate. A note that has just double the rate of another one is said to be its octave. For convenience the interval of the octave has been split up into twelve tones, and all our music is constructed on that basis.
It is evident that the ear must be a very complicated organ; not only must it perceive differences in pitch, as just indicated, but differences in loudness must also register differently. More than that, the ear has to be able to deal with sounds made up of a great many tones coming into it all at once. When we listen to an orchestra or band, the waves that strike our ears represent the commotion set up in the air by all the instruments together. It is a remarkable fact that in this case, instead of getting a meaningless jumble, we actually get a blend of tones from which, if we are sufficiently musical, we can pick out the individual elements.
The ear consists, in the first place, of a vibrator that will respond accurately to any vibration rate or combination of vibration rates within its range, and secondly of a sensitive apparatus that is acted upon by the vibrator. The vibrator must respond freely to feeble impulses, and, what is of prime importance, to any vibration rate as readily as to any other. Almost all elastic bodies have a preferred vibration rate; that is, they will respond better to some rates than to others. About the only exception to this rule
