influence, from various causes, is very unequally distributed over the different parts both of the earth and the air; and in the same place, is much greater at one time than it is at another. Heat may affect a portion of the atmosphere by expanding its volume; or, in other words, by causing the same number of airy particles, that is, the same weight of air, to occupy more space. When this spreading takes place in the air over any particular part of the earth's surface, the diminished pressure on the exposed portion of the mercury in a barometer will, of course, be shown by the diminished elevation of that in the tube. But it will occasion other effects also in the air itself. The more condensed air from distant places will soon begin to rush with more or less violence towards the situation of diminished pressure. In other words, winds will occur. But the increase of heat has been in the mean time, also, occasioning a more rapid and plentiful evaporation of moisture from the earth, which rising into the air in the form of an invisible vapour, has of course produced more than the usual accumulation there, and is therefore ready to fall again to the earth as soon as the reduction of temperature takes place. Hence the fall of hail, snow, rain, mists, &c. The process, in short, is this: the heat operating on the air, dilutes it, diminishes its weight, and consequently lowers the mercury in the barometer; operating on the terrestrial moisture, raises it by evaporation into the air, and consequently soon becomes charged with more than its usual quantity of humidity. Then comes the wind, the fall of the temperature, and the rain. A fall in the barometer is found to be generally indicative of these changes.

Q. Do all bodies evaporate at the same temperature?

A. No: some bodies contain so great a portion of caloric, and their particles are held by each other so slenderly, that at the temperature of the atmosphere they will fly off rapidly; as ether, for example.

Q. Has atmospheric pressure great influence in evaporation?

A. Yes, very great. Water does not usually boil until it

180°; and when by an air-pump the atmospheric pressure is removed, it will boil at a very low temperature.

Q. Be so kind as to explain this more fully.

A. Ere liquids can boil, two forces must be met: the first of these is, the attraction of aggregation, by which the particles are more or less firmly held together, and which is generally connected with the caloric they contain. The second is the weight that rests on the liquid, by which the elastic force of the vapour is counteracted. The atmosphere which presses on the surface, is the latter force. As this is lessened by ascent on mountains, by which the length of the superincumbent column is diminished, or by artificial means, the less heat is required to overcome the lessened force, and the elasticity of the vapour is just equal to the actual atmospheric pressure.

Q. What would be the effect of the entire removal of this pressure?

A. In all probability every liquid would evaporate, and be dissipated even quicksilver, it is supposed would in this case become vapour.

Q. If the atmosphere were more dense or more rare than it is, what would be the consequences?

A. If more dense, it would be unfit to take up and hold vapour; if of a high temperature, all the vapour taken up would be a permanently elastic fluid; and in either case there would be no rain, &c. The atmosphere would also be unsuitable to the wants of man and animals. But in its present state it is adapted to its ends, as by weight and mea“O Lord, how manifold are thy works! in wisdom hast thou made them all."

sure.

INVENTION OF LETTERS.

CONCERNING the learning of the Egyptians, much has been said, and little is known. The testimony, however, of

ments of architecture, and traces of literature in the shape of hieroglyphics and symbols, however unintelligible, prove that they were a wonderful people for gigantic enterprise and indefatigable industry, in achieving what were then the highest feats of manual, intellectual, and mechanic power. On these we shall not expatiate here, as another opportunity will be afforded in the next paper of this series, of considering by whom, and by what means, such marvellous works were executed. At present we shall only allude to them generally, in connexion with the discovery of alphabetical writing. When, where, and by whom, letters were invented, it is now vain to imagine. Notwithstanding the pretensions of Hermes Trismegistos, Memnos, Cadmus, and others, the true history, nay, even the personal existence of these supposed claimants, must be ascertained before the unappropriated honour can be conceded to any one of them. It may, meanwhile, be affirmed, as one of those circumstances humbling to human pride that occasionally occur in history, and which, while they strangely stir the imagination, awaken sublime but melancholy reflection in minds given to muse upon the vanity and mortality of all the things that are done under the sun; it may be affirmed, as one of these humbling circumstances, that the man who conquered the greatest trophy ever won from fate and oblivion, lost his own name, after divulging the secret by which others might immortalize theirs. As a figure of speech, one may be allowed to wish that the first letters in which he wrote that name, whether with a pen of iron on granite, or with his finger on sand, had remained indelible. But his own invention is his monument, which, like the undated and uninscribed pyramid, will remain a wonder and a riddle to the end of the world.

It is allowed, I believe, on all hands, that the Egyptians, from time whereof the memory of man knoweth not to the contrary, possessed three kinds of writing,-hieroglyphical, alphabetical, and, probably, as a link between, logographic, of which latter the Chinese is the only surviving example at this day. Indeed, in all countries where society has emerged

180°; and when by an air-pump the atmospheric pressure is removed, it will boil at a very low temperature.

Q. Be so kind as to explain this more fully.

A. Ere liquids can boil, two forces must be met: the first of these is, the attraction of aggregation, by which the particles are more or less firmly held together, and which is generally connected with the caloric they contain. The second is the weight that rests on the liquid, by which the elastic force of the vapour is counteracted. The atmosphere which presses on the surface, is the latter force. As this is lessened by ascent on mountains, by which the length of the superincumbent column is diminished, or by artificial means, the less heat is required to overcome the lessened force, and the elasticity of the vapour is just equal to the actual atmospheric pressure.

Q. What would be the effect of the entire removal of this pressure?

A. In all probability every liquid would evaporate, and be dissipated even quicksilver, it is supposed would in this case become vapour.

Q. If the atmosphere were more dense or more rare than it is, what would be the consequences?

A. If more dense, it would be unfit to take up and hold vapour; if of a high temperature, all the vapour taken up would be a permanently elastic fluid; and in either case there would be no rain, &c. The atmosphere would also be unsuitable to the wants of man and animals. But in its present state it is adapted to its ends, as by weight and mea“O Lord, how manifold are thy works! in wisdom hast thou made them all."

sure.

INVENTION OF LETTERS.

CONCERNING the learning of the Egyptians, much has been said, and little is known. The testimony, however, of

ments of architecture, and traces of literature in the shape of hieroglyphics and symbols, however unintelligible, prove that they were a wonderful people for gigantic enterprise and indefatigable industry, in achieving what were then the highest feats of manual, intellectual, and mechanic power. On these we shall not expatiate here, as another opportunity will be afforded in the next paper of this series, of considering by whom, and by what means, such marvellous works were executed. At present we shall only allude to them generally, in connexion with the discovery of alphabetical writing. When, where, and by whom, letters were invented, it is now vain to imagine. Notwithstanding the pretensions of Hermes Trismegistos, Memnos, Cadmus, and others, the true history, nay, even the personal existence of these supposed claimants, must be ascertained before the unappropriated honour can be conceded to any one of them. It may, meanwhile, be affirmed, as one of those circumstances humbling to human pride that occasionally occur in history, and which, while they strangely stir the imagination, awaken sublime but melancholy reflection in minds given to muse upon the vanity and mortality of all the things that are done under the sun; it may be affirmed, as one of these humbling circumstances, that the man who conquered the greatest trophy ever won from fate and oblivion, lost his own name, after divulging the secret by which others might immortalize theirs. As a figure of speech, one may be allowed to wish that the first letters in which he wrote that name, whether with a pen of iron on granite, or with his finger on sand, had remained indelible. But his own invention is his monument, which, like the undated and uninscribed pyramid, will remain a wonder and a riddle to the end of the world.

It is allowed, I believe, on all hands, that the Egyptians, from time whereof the memory of man knoweth not to the contrary, possessed three kinds of writing,-hieroglyphical, alphabetical, and, probably, as a link between, logographic, of which latter the Chinese is the only surviving example at this day. Indeed, in all countries where society has emerged