of mankind, or which embolden them to expect we should trample on the experience of ages, and abandon a religion which has been attested by a train of miracles and prophecies, in which millions of our forefathers have found a refuge in every trouble, and consolation in the hour of death; a religion which has been adorned with the highest sanctity of character and splendour of talents, which enrols amongst its disciples the names of BACON, NEWTON, and LOCKE, the glory of their species, and to which these illustrious men were proud to dedicate the last and best fruits of their immortal genius?

If the question at issue is to be decided by argument, nothing can be added to the triumph of Christianity; if by an appeal to authority, what have our adversaries to oppose to these great names? Where are the infidels of such pure, uncontaminated morals, unshaken probity, and extended benevolence, that we should be in danger of being seduced into impiety by their example? Into what obscure recesses of misery, into what dungeons, have their philanthropists penetrated, to lighten the fetters and relieve the sorrows of the helpless captive? What barbarian tribes have their apostles visited, what distant climes have they explored, encompassed with cold, nakedness, and want, to diffuse principles of virtue and the blessings of civilization? Or will they rather choose to waive their pretensions to this extraordinary and, in their eyes, eccentric species of benevolence (for infidels, we know, are sworn enemies to enthusiasm of every sort), and rest their character on their political exploits; on their efforts to re-animate the virtue of a sinking state, to restrain licentiousness, to calm the tumult of popular fury, and, by inculcating the spirit of justice, moderation, and pity for fallen greatness, to mitigate the inevitable horrors of revolution? Our adversaries will, at least, have the discretion, if not the modesty, to recede from the test.

More than all, their infatuated eagerness, their parricidal zeal to extinguish a sense of Deity, must excite astonishment and horror. Is the idea of an almighty and perfect Ruler unfriendly to any passion which is consistent with innocence, or an obstruction to any design which it is not

shameful to avow? Eternal God, on what are thine enemies intent! What are their enterprises of guilt and horror, that for the safety of their performers require to be enveloped in a darkness which the eye of heaven must not pierce! Miserable men! Proud of being the offspring of chance; in love with universal disorder; whose happiness is involved in the belief of there being no witness to their designs, and who are at ease only because they suppose themselves inhabitants of a forsaken and fatherless world.-Robert Hall.

LESSON XXI.-MONDAY.

OROGRAPHY.

West of the embouchure of the Arkansas into the Missouri, the level of the prairies rises within a distance of 138 miles to 1,000 feet, and ascends westward very gradually to 5,140 feet at the foot of the Rocky Mountains. The Wind River Mountains are that important portion of the Rocky Mountains whence the Missouri, Yellow Stone, and Platte rivers flow to the Mississippi and the Atlantic ocean; and the Lewis, through the Oregon or Columbia on the one side, and the Colorado on the other, to the Pacific; where Fremont's Peak, the highest summit of the Rocky Mountains, attains a height of 13,569 feet; and where this important watershed may be conveniently crossed by the South Pass at a height of 7,223 feet. The western slope of the Wind River Mountains passes into a plateau which as yet is a terra incognita. It may be regarded as a great basin of 169,484 square miles in extent, of an average height of 4,112 feet, and covered with numerous salt lakes, situated between the Rocky Mountains and the snowy ranges of the western coast of California, amongst which Mounts Jefferson, Hood, and St. Helen's rise to an altitude of 15,000 feet.

The mountain systems of Brazil and Guiana, and of the Alleghanies, are completely detached. Their characteristic form, that of chains running parallel to the coast, is most clearly displayed in North America, where the intersections are so numerous that culture and communication traverse the Alleghanies in every direction. In Guiana, this parallelism is connected with a terrace-like

ascent towards the interior, and a more intimate blending with the lofty mass of the main stem, and consequently with a partial and less favourable system of transverse valleys. In Brazil, on the contrary, the parallel chains of the coast are the highest, and in general the mountains of the interior allow the surrounding lowland to advance along the parallel streams far into the plateau which constitutes their lower base, and to rise gradually to the upper longitudinal river plains. The water-shed between the basins of the Rio de la Plata and Amazon lies close to the foot of the Sierra de la Santa Cruz in the low marshy grounds, and the flat or broken undulating surface of the province of Chiquitos, while in the north the basins of the Amazon and Orinoco are only separated by very low banks, which are not sufficient to prevent the Cassiquiari from flowing as a bifurcation from the Orinoco into the Rio Negro, a tributary of the Amazon. If the level of the sea were a few hundred feet higher, Brazil and Guiana would be islands. It is the same with the small mass of mountains of the Sierra Nevada de Santa Marta, in the low coast near the Magdalena, but not so with the maritime mountains of Venezuela and the northern sierra of Texas, which passes into the lower range of the Ozark mountains. Both these groups of mountains lean against the plateaus of the eastern Andes. The maritime chain of Venezuela is narrow and high where it terminates at the Dragon's Mouth, but rises again out of the sea in the island of Trinidad. The Ozark mountains sink confusedly, and with spreading groups of hills, to the low valley of the Mississippi. Between the two Americas an insular continuation of the sierra of Yucatan may be traced on Cuba, Haiti, and Porto Rico. The orographical relations of Labrador are not known as yet. The Hauteurs des Terres, which send their streams into the small lake Itaska, from which the Mississippi first flows, are not quite 1600 feet high; and southward of this separating ridge, the North Red River, rising from Elbow Lake, within a short distance, sends its waters to Hudson's Bay. No mountain division separates the two lakes; a somewhat higher waterlevel would obliterate all distinction between the basins of the Mississippi and Hudson's Bay. The heights which

limit on the south the streams tributary to Hudson's Bay only mark the boundary of a new basin in the great American lowland, which, accompanying the Cordilleras on the east, extends through North America, from the icy arctic shores to the marshy coasts of the Gulf of Mexico, there sinks beneath the waves of the American Mediterranean Sea, and again forms an enormous extent of low ground, from the delta of the Orinoco to the mouth of the Rio de la Plata, thus establishing an extensive connexion between the eastern coast of America and the eastern foot of the Cordilleras.

LESSON XXII.-TUESDAY.

THE ATMOSPHERE.

By means of the air-pump, it is easy to show, by direct experiment, that air, in common with every form of matter, has weight, and even to measure its weight. We may form some notion of the actual weight of the air by calculating the quantity contained in a given space. Take for example, a room 30 feet long, 28 feet wide, and 19 feet high, offering a content of 15,960 cubic feet; since 100 cubic inches of air weigh 31 grains, 13 cubic feet of air weigh nearly 1 pound. The total weight of air in such a room is, therefore, about 1,220 pounds, a little more than half a ton.

The pressure of the air is the power which raises water in the bore of an ordinary pump. On depressing the pistonrod, air escapes through the upper valve, and on raising it again a fresh portion enters from the pipe attached below the second valve. The weight of the atmosphere upon the surface of water in the well, forces up a portion, the weight of which compensates for the diminished elasticity of the air in the barrel, till on again depressing the piston several times successively, the whole of the air has its place supplied by the water which is thus raised from the well below, the pressure of the atmosphere being removed from the surface of that part of the water contained in the pipe beneath the valves. It is manifest, however, that there must be a limit in the height to which water can be raised in this way. As soon as the column of water in the pump above the level of that in the well is long enough to balance the weight of a

similar column of air extending to the upper limits of the atmosphere, the water will rise no higher. Such a column of water is about 33 feet in height. If a tube 40 feet long be closed at its upper end, filled with water, and then placed mouth downwards in a vessel of water, the water in the tube will fall till it stands about 33 feet above the level of that in the cistern.

If the tube were filled with a heavier liquid than water, a proportionately shorter column of it would be sustained by the pressure of the air, the length of the column being inversely proportioned to the specific gravity of the two fluids. Now, as mercury is rather more than thirteen times as heavy as water, this fluid will rise to a height only about one-thirteenth as great as that of water, or to a height of about 30 inches instead of 33 feet. This result is easily verified; for if a glass tube about three feet long, and closed at one extremity, be completely filled with mercury, the aperture closed with the finger, and it be placed mouth downwards in a basin of mercury, on removing the finger, the column of fluid metal will partially descend, and leave a void space of five or six inches in length in the upper part of the tube. But the most complete demonstration that the mercury is sustained solely by the pressure of the air upon that in the basin, is furnished by placing the whole apparatus under the receiver connected with the air-pump; as the air is exhausted, and consequently the pressure is diminished, the columu sinks, but it recovers its former level on re-admitting the air from without. A simple inverted tube when filled with mercury, with due precautions to exclude every particle of air, and furnished with accurate means of measuring the height of the column above the level of the mercury in the cistern, constitutes one of the most indispensable philosophical instruments-the barometer. The diameter of the tube is of little consequence, but a tube, onethird or half an inch wide, or wider, is preferable to one of smaller bore. The barometer has been constructed in a great variety of forms, but the simple inverted tube is the best for ordinary purposes.

The syphon depends for its operation partly upon the principle of atmospheric pressure. The syphon is a bent tube, by means of which, liquids may be lifted above the