A Shock Given To The Surface Of A Liquid Will Form Circles Around The Centre Of Percussion, Even When The Liquid Is Agitated.

Several kinds of undulations may cross each other in water, as in air, without being disturbed in their propagation:

Little movements may, as it were, ride over each other, and the real cause of the less intensity of sound during the day appears to be the interpretation of homogeneity in the elastic medium. During the day there is a sudden interruption of density wherever small streamlets of air of a high temperature rise over parts of the soil unequally heated. The sonorous undulations are divided, as the rays of light are refracted and form the mirage wherever strata of air of unequal density are contiguous. The propagation of sound is altered when a stratum of hydrogen gas is made to rise in a tube closed at one end above a stratum of atmospheric air; and M. Biot has well explained, by the interposition of bubbles of carbonic acid gas, why a glass filled with champagne is not sonorous so long as that gas is evolved, and passing through the strata of the liquid.

In support of these ideas, I might almost rest on the authority of an ancient philosopher, whom the moderns do not esteem in proportion to his merits, though the most distinguished zoologists have long rendered ample justice to the sagacity of his observations. "Why," says Aristotle in his curious book of Problems, "why is sound better heard during the night? Because there is more calmness on account of the absence of caloric (of the hottest).* (* I have placed in a parenthesis, a literal version of the term employed by Aristotle, to express in reality what we now term the matter of heat. Theodore of Gaza, in his Latin translation, expresses in the shape of a doubt what Aristotle positively asserts. I may here remark, that, notwithstanding the imperfect state of science among the ancients, the works of the Stagirite contain more ingenious observations than those of many later philosophers. It is in vain we look in Aristoxenes (De Musica), in Theophylactus Simocatta (De Quaestionibus physicis), or in the 5th Book of the Quest. Nat. of Seneca, for an explanation of the nocturnal augmentation of sound.) This absence renders every thing calmer, for the sun is the principle of all movement." Aristotle had no doubt a vague presentiment of the cause of the phenomenon; but he attributes to the motion of the atmosphere, and the shock of the particles of air, that which seems to be rather owing to abrupt changes of density in the contiguous strata of air.

On the 16th of April, towards evening, we received tidings that in less than six hours our boat had passed the rapids, and had arrived in good condition in a cove called el Puerto de arriba, or the Port of the Expedition. We were shown in the little church of Atures some remains of the ancient wealth of the Jesuits.

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