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NOTES AND QUERIES,
“ All things are double, one against another.”—JESUS, in Eccl. XLII, 24.
Some Atmospheric Phenomena.
BY PROF. N. B. WEBSTER, Vineland, N. J.
Aristotle attempted to weigh air by weighing a bag when empty, and again when inflated, and because there was no difference in weight under the two conditions he supposed that air had no weight. It is now a frequent school-room experiment to show that a glass or metal flask of ten cubic inches capacity weighs fully three grains more when filled with air than when the air has been drawn from it with an airpump. The weight of the whole terrestrial atmosphere is about that of a solid copper globe sixty-two miles in diameter.
The air in a room 60 feet long, 30 feet wide, and 15 feet high, weighs one ton, or two pounds to the cubic yard.
The weight of the atmosphere must be limited to where gravity will overcome the centrifugal force, and the repulsive force of the ærial molecules. Biot proved that the minimum height of the air must exceed 30 miles, and the maximum height must be less than 21,000 miles. If the air was of equal density throughout, its height would be 27,818 feet, or about 1,200 feet below the summit of Mount Everest in Asia. From this height a falling body would attain a velocity of 1,338 feet a second, which is the velocity with which air rushes into a void space. For this reason a cannon ball fired with greater initial velocity than 1,338 feet a second will leave a vacuum behind it till it is rapidly reduced to a less velocity.
One effect of the atmosphere is to lengthen the day by causing evening and morning twilight. If its height was unlimited the two twitights would blend at midnight, and we should have practically no night.
Without the atmosphere stars would be visible in the day-time from the earth, as they must be from the airless moon. The atmosphere is indispensable to all ordinary sounds.
In condensed air sounds are very loud, and in rarefied air sounds are very feeble. It is true that men, miles above the earth in balloons, hear noises made at the surface with great distinctness, but persons at the surface cannot hear any sounds from such high balloons. The loudness of sounds depends on the density of the air where they originate, and not where they are heard.
There is one reason why people“ hard of hearing" can sometimes hear a sermon, or a play from a gallery, better than in the auditorium below. A few years ago the lower plane in the theaters was, too often, appropriately named the "pit." In the upper part of the room the air is usually lighter than below, and the velocity of sound waves is greater in lighter gases. Hence, sounds are a means of detecting the accumulation of fire-damp in mines, in time to take precaution to prevent explosions in mines. Loudness must not be confounded with velocity. Heavy liquids, unlike heavy gases, transmit sounds faster than those less dense.
Tyndall says that a gun fired from Chamouni is audible on the summit of Mount Blanc, but a gun fired on Mount Blanc cannot be heard at Chamouni.
Experiments have shown that a person speaking in the open air can be heard about equally well at a distance of too feet in front, 75 feet on each side, and 30 feet behind.
Cannon discharge can be heard further than thunder. Thus Campbell wrote truly, as well as poetically, in his “Battle of Hohenlinden,"
" And louder than the bolts of beaven
Far flashed the red artillery." If an elephant were as altisonant, or far-sounding, as a nightingale in proportion to his bulk, his trumpeting could be heard around the world. On like conditions an ox lowing in Australia would be heard in New York about 15 hours after the Australians heard him.
In laying the foundations of the piers of St. Louis bridge over the Mississippi, by means of diving bells, so deep that the pressure was four atmospheres or 60 pounds to the inch, the ticking of a watch was painfully loud, but the occupant found it impossible to whistle.
It has been a question whether 1,000 men shouting at once could be heard farther than the loudest man among them; whether the good and earnest Wesley, who wished " for a thousand tongues to sing,” could have been heard farther with them than with his monoglossol limitation. It is pretty certain that 1,000 boys together can