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by the air-pump, but also by observations in the rare atmosphere of the upper regions, it was shown that the intensity of sound depends upon the density. On the top of a mountain the report of a pistol is no louder than that of a cracker in the valley. As to the gradual propagation of sounds, it was impossible to observe fire-arms discharged at a distance without noticing that the flash appears longer before the report in proportion as the distance is greater. The Florentine academicians attempted a determination of the velocity, and found it to be 1148 feet in a second. More accurate and recent experiments made it 1089-42 feet at the freezing-point of water; but the velocity, though independent of the density, increases with the temperature at the rate of 1.14 foot for each degree. For other media the rate is different; for water, about 4687 feet in a second, and in cast iron about 10 times greater than in air. All sounds, irrespective of their note or intensity, move at the same velocity, the medium itself being motionless in the mass. No sound can pass through a vacuum. The sudden aerial condensation attending the propagation of a sound gives rise to a momentary evolution of heat, which increases the elasticity of the air, and hence the velocity is higher than 916 feet in a second, otherwise the theoretical rate.

nomena.

Turning from soniferous media to sounding bodies, it Acoustic phe- was shown that the difference between acute and grave sounds depends on the frequency of vibration. The ear can not perceive a sound originating in less than thirty-two vibrations in a second, nor one of more than 24,000. The actual number of vibrations in a given note was counted by means of revolving wheels and other contrivances. I have not space to relate the investigation of many other acoustic facts, the reference of sounds to phases of condensation, and rarefaction in the elastic medium taking place in a normal direction; the affections of note, intensity, quality; the passage in curved lines and around obstacles; the production of sympathetic sounds; nodal points; the effect of reeds; the phenomena of pipes and flutes, and other wind instruments; the various vibrations of solids, as bells; or of membranes, as drums; visible acoustic lines; the reflexion of undulations

by surfaces of various forms; their interferences, so that, no matter how intense they may be individually, they can be caused to produce silence; nor of whispering galleries, echoes, the nature of articulate sounds, the physiology of the vocal and auditory organs of man, and the construction of speaking machines.

Like the air, the ocean, which covers three-fourths of the earth's surface, when reduced to a proper The ocean; standard of measure, loses very much of its its size. imposing aspect. The varnish that covers a twelve-inch globe represents its relative dimension not inadequately.

On the theory of gravitation, the tides of the ocean were explained as depending on the attractive Tides and force of the sun and moon. Its currents, in a currents. general manner, are analogous to those of the air. They originate in the disturbing action of solar heat, the temperature of the sea varying from 85° in the torrid zone to the freezing-point as the poles are approached. Its specific gravity at the equator is estimated at 1.028; but this density necessarily varies with the rate at which superficial evaporation takes place; the pure vapour rising, leaves a more concentrated salt solution. The effect is therefore, in some degree, to counteract the expansion of the water by warmth, for the sun-rays, being able to penetrate several feet below the surface, correspondingly raise the temperature of that portion, which expands and becomes lighter; but, simultaneously, surface evaporation tends to make the water heavier. Notwithstanding this, currents are established through the preponderance of the dilatation, and of them the Gulf Stream is to us the most striking example.

streams.

The physical action of the sun-rays in occasioning currents operates through the expansion of Effects of water, of which warm portions ascend to the ocean surface, colder portions from beneath setting in to supply their place. These currents, both hot and cold, are affected by the diurnal rotation of the earth, the action being essentially the same as that for the winds. They exert so great an influence as conveyers of heat that they disturb the ordinary climate relation depending on the sun's position. In this way the Gulf Stream, a river of

hot water in a sea of cold, as soon as it spreads out on the surface of the Atlantic in higher latitudes, liberates into the air the heat it has brought from the torrid zone; and this, being borne by the southwest wind, which blows in those localities for the greater part of the year, to the westerly part of the European continent, raises by many degrees the mean annual temperature, thus not only regulating the distribution of animals and plants, but also influencing human life and its pursuits, making places pleasant that would otherwise be inclement, and even facilitating the progress of civilization. Whatever, therefore, can affect the heat, the volume, the velocity, the direction of such a stream, at once produces important consequences in the organic world.

Physical and chemical relations of water.

The Alexandrian school had attained correct ideas respecting the mechanical properties of water as the type of liquids. This knowledge was, however, altogether lost in Europe for many ages, and not regained until the time of Stevinus and Galileo, who recovered correct views of the nature of pressure, both vertical and oblique, and placed the sciences of hydrostatics and hydrodynamics on exact foundations. The Florentine academicians, from their experiments on water inclosed in a globe of gold, concluded that it is incompressible, an error subsequently corrected, and its compressibility measured. The different states in which it occurs, as ice, water, steam, were shown to depend altogether on the amount of latent heat it contains. Out of these investigations originated the invention of the steam-engine, of which it may be said that it has revolutionized the industry of the world. Soon after the explanation of the cause of its three states followed the great discovery that the opinion of past ages respecting its elementary nature is altogether erroneous. It is not a simple element, but is composed of two ingredients, oxygen and hydrogen, as was rigorously proved by decomposing and forming it. By degrees, more correct views of the nature of evaporation were introduced; gases and vapours were found to coexist in the same space, not because of their mutual solvent power, but because of their individual and independent elasticity. The instantaneous formation

of vapours in a vacuum showed that the determining condition is heat, the weight of vapour capable of existing in a given space being proportional to the temperature. More scientific views of the nature of maximum density were obtained, and on these principles was effected the essential improvement of the low-pressure steam-enginethe apparent paradox of condensing the steam without cooling the cylinder.

clature.

In like manner much light was cast on the meteorological functions of water. It was seen that the diurnal vaporization from the earth depends on the amount of Clouds and heat received, the vapour rising invisibly in the their nomenair till it reaches a region where the temperature is sufficiently low. There condensation into vesicles of perhaps 5000 of an inch in diameter ensues, and of myriads of such globules a cloud is composed. Of clouds, notwithstanding their many forms and aspects, a classification was given-cirrus, cumulus, stratus, etc. It was obvious why some dissolve away and disappear when they encounter warmer or drier spaces, and why others descend as rain. It was shown that the drops can not be pure, since they come in contact with dust, soluble gases, and organic matter in the air. Sinking into the ground, the water issues forth as springs, contaminated with whatever is in the soil, and finds its way, through streamlets The return of and rivers, back to the sea, and thus the drainage water to the of countries is accomplished. Through such a returning path it comes to the receptacle from which it set out; the heat of the sun raised it from the ocean, the attraction of the earth returns it thereto; and, since the heat-supply is invariable from year to year, the quantity set in motion must be the same. Collateral results of no little importance attend these movements. Every drop of rain falling on the earth disentegrates and disturbs portions of the soil; every stream carries solid matter into the sea. It is the province of geology to estimate the enormous aggregate of detritus, continents washed away and new continents formed, and the face of the earth remodelled and renewed.

sea.

The artificial decomposition of water constitutes an epoch in chemistry. The European form of this science,

in contradistinction to the Arabian, arose from the Progress of doctrine of acids and alkalies, and their neutrachemistry. lization. This was about A.D. 1614. It was perceived that the union of bodies is connected with the possession of opposite qualities, and hence was introduced the idea of an attraction of affinity. On this the discovery of elective attraction followed. Then came the recognition that this attraction is connected with opposite electrical states, chemistry and electricity approaching each other. A train of splendid discoveries followed; metals were obtained light enough to float on water, and even apparently to accomplish the proverbial impossibility of setting it on fire. In the end it was shown that the chemical force of electricity is directly proportional to its absolute Attraction. quantity. Better views of the nature of chemical The elements. attraction were attained, better views of the intrinsic nature of bodies. The old idea of four elements was discarded, as also the Saracenic doctrine of salt, sulphur, and mercury. The elements were multiplied until at length they numbered more than sixty. Alchemy Theory of merged into chemistry through the theory of phlogiston. phlogiston, which accounted for the change that metals undergo when exposed to the fire on the principle that something was driven off from them-a something that might be restored again by the action of combustible bodies. It is remarkable how adaptive this theory was. It was found to include the cases of combustive operations, the production of acids, the breathing of animals. It maintained its ground even long after the discovery of oxygen gas, of which one of the first names was dephlogisticated air.

But a false theory always contains within itself the germ of its own destruction. The weak point of this was, that when a metal is burnt the product ought to be lighter than the metal, whereas it proves heavier. At length it was detected that what the metal had of the balance gained the surrounding air had lost. This into chemis- discovery implied that the balance had been resorted to for the determination of weights and for the decision of physical questions. The reintroduction of that instrument-for, as we have seen, it had

Introduction

try.

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