voyages to Madagascar are replaced by circumnavigations of the world; the revolving steam-engine of Hero by the double-acting engine of Watt; the great galley of Ptolemy, with its many banks of rowers, by the ocean steam-ship; the solitary watch-fire on the Pharos by a thousand lighthouses, with their fixed and revolving lights; the courier on his Arab horse by the locomotive and electric telegraph; the scriptorium in the Serapion, with its shelves of papyrus, by countless printing-presses; the "Almagest" of Ptolemy by the "Principia" of Newton; and the Museum itself by English, French, Italian, German, Dutch, and Russian philosophical societies, universities, colleges, and other institutions of learning.

So grand is the scale on which this cultivation of science has been resumed, so many are those European engaged in it, so rapid is the advance, and so progress in the acquisigreat are the material advantages, that there is tion of knowno difficulty in appreciating the age of which it ledge. is the characteristic. The most superficial outline enables us to recognize at once its resemblance to that period of Greek life to which I have referred. To bring its features into relief, I shall devote a few pages to a cursory review of the progress of some of the departments of science, selecting for the purpose topics of general interest.

First, then, as respects the atmosphere, and the phenomena connected with it.

From observations on the twilight, the elasticity of aerial bodies, and the condensing action of cold, The atmothe conclusion previously arrived at by Alhazen sphere. was established, that the atmosphere does not extend unlimitedly into space. Its height is considered to be about forty-five miles. From its compressibility, the greater part of it is within a much smaller limit; were it of uniform density, it would not extend more than 29,000 feet. Hence, comparing it with the dimensions of the earth, it is an insignificant aerial shell, in thickness not the eightieth part of the distance to the earth's centre, and its immensity altogether an illusion. It bears about the same proportion to the earth that the down upon a peach bears to the peach itself.

A foundation for the mechanical theory of the atmo

sphere was laid as soon as just ideas respecting liquid pressures, as formerly taught by Archimedes, were restored, the conditions of vertical and oblique pressures investigated, the demonstration of equality of pressures in all directions given, and the proof furnished that the force of a liquid on the bottom of a vessel may be very much greater than its weight.

Such of these conclusions as were applicable were soon Its mechani- transferred to the case of aerial bodies. The cal relations. weight of the atmosphere was demonstrated, its pressure illustrated and measured; then came the dispute about the action of pumps, and the overthrow of the Aristotelian doctrine of the horror of a vacuum. Coincidently occurred the invention of the barometer, and the proof of its true theory, both on a steeple in Paris and on a mountain in Auvergne. The invention of the air-pump, and its beautiful illustrations of the properties of the atmosphere, extended in a singular manner the taste for natural philosophy.

The mechanics of the air was soon followed by its cheIts chemical mistry. From remote ages it had been numrelations. bered among the elements, though considered liable to vitiation or foulness. The great discovery of oxygen gas placed its chemical relations in their proper position. One after another, other gases, both simple and compound, were discovered. Then it was recognized that the atmosphere is the common receptacle for all gases and vapours, and the problem whether, in the course of ages, it has ever undergone change in its constitution arose for solution. The negative determination of that problem, so far as a few thousand years are concerned, was necessarily followed by a recognition of the antagonism of animals and plants, and their mutually balancing each other, the latter accomplishing their duty under the influence of the sun, though he is a hundred millions of miles distant. From this it appeared that it is not by incessant interventions that the sum total of animal life is adjusted to that of vegetable, but that, in this respect, the system of government of the world is by the operation of natural causes and law, a conclusion the more imposing since it contemplates all living things, and

The antagonism of animals and plants.

includes even man himself. The detail of these investigations proved that the organic substance of plants is condensed from the inorganic air to which that of all animals returns, the particles running in ever-repeating cycles, now in the air, now in plants, now in animals, now in the air again, the impulse of movement being in the sun, from whom has come the force incorporated in plant tissues, and eventually disengaged in our fires, shining in our flames, oppressing us in fevers, and surprising us in blushes.

Organic disturbances by respiration and the growth of plants being in the lowest stratum of the air, its uniformity of composition would be impossible The winds; were it not for the agency of the winds and the their origin diffusion of gases, which it was found would and nature. take place under any pressure. The winds were at length properly referred to the influence of the sun, whose heat warms the air, causing it to ascend, while other portions flow in below. The explanation of land and sea breezes was given, and in the trade-wind was found a proof of the rotation of the earth. At a later period followed the explanation of monsoons in the alternate heating and cooling of Asia and Africa on opposite sides of the line, and of tornadoes, which are disks of air rotating round a translated axis with a diameter of one hundred or one hundred and fifty miles, the axis moving in a curvilinear track with a progressive advance of twenty or twenty-five miles an hour, and the motions being in opposite directions in opposite hemispheres of the globe.

The equatorial calms and trade-winds accounted for on physical principles, it was admitted that the winds of high latitudes, proverbially uncertain as they are, depend in like manner on physical causes.

With these palpable movements there are others of a less obvious kind. Through the air, and by reason of motions in it, sounds are transmitted to us.

The Alexandrian mathematicians made sound a favourite study. Modern acoustics arose from the recognition that there is nothing issuing from the sounding of sounds; body, but that its parts are vibrating and their velocity. affecting the medium between it and the ear. Not only

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 а 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.

Effects of

streams.

The physical action of the sun-rays in occasioning currents operates through the expansion 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