of day and night through the year. This is, however, not the case in nature, as has already been fully explained. If, then, the wire be inclined to the table at an angle of 23 degrees, as represented by the circle abcd, and the globe be carried gently round it, the seasons, and increase of day and night, will appear as they are in nature; that is, when the globe is at a, the candle enlightens it no farther northward than the arctic circle, no, all within which, in the middle of our winter, is deprived of a sight of the sun, while all places within the an. tarctic or opposite circle have perpetual day : at this time the candle shines vertically on the tropic of Capricorn. As the earth moves towards b (the vernal equinox), if a small patch be laid on latitude 51° north, it will show how the days increase at London, and how the nights decrease. When it has arrived at b, the candle will then be perpendi. cularly over the equator, and, shining to both poles, equality of day and night will take place as it proceeds towards c (the summer solstice), the days increase, and the candle shines more and more over the north pole: when it has arrived at c, the whole arctic circle, and the countries it includes, will revolve in continual sight of the sun, and all within the antarctic circle will be deprived of that sight. At this time the candle shines vertically on the tropic of Cancer. Moving from midsummer towards d (the autumnal equinox), the days will be found to decrease, and the nights to increase in length, till they come again to equality at d, and thence to the winter solstice, and so on. The particular temperature which distinguishes each of the seasons at any particular place is owing to a difference in the sun's altitude, and the time of his continuance above the horizon of that place. In winter the rays of the sun fall so obliquely, and the sun is such a short time above the hori zon, that his influence in heating the earth is but very little, compared with what it is in summer. For at this season the sun is so much higher than in winter, that his rays not only fall more perpendicularly, but more of them fall on any given space; and as the day is also much longer than the night, the temperature of the earth and the surrounding atmosphere must be much greater than in winter. Since the power of the sun is greater in heating the earth at any particular place, when his rays fall most directly, and when the days are longest at that place, it may be asked how does it happen that the heat is greatest about the end of July, when the sun is highest and the day longest about the 21st of June? The reason of this may easi. ly be discovered by attending a little to the manner in which bodies are heated. The heat which the earth receives is not tran. sient, but is retained by it for some time ; for, like other solid bodies, it receives heat and parts with it gradually. Now, as the earth continues to receive more heat in the day than it gives out in the night, for a considerable time after the 21st of June, its temperature will continue to increase, till the days and nights begin to approach to an equality. But this is not the case till the end of July at least; the earth goes on increasing in temperature, till about this time,* when it is found to be much greater than about the 21st of June, although the sun be then higher at mid-day, and the day longer, than at any other time of the year in the hemisphere in a reverse order, or at six months difference *The same phenomena takes place in the southern of time. northern hemisphere. The heat in July If all parts of the earth were equally atwould be still greater were the sun at his tracted by the moon, it would always retain mean distance from the earth; but this is its spherical form, and there would be no not the case, for he is then at his greatest tides at all. But the action of the moon be. distance. However, the difference between ing unequal on different parts of the earth, his distance at this time and the mean dis- those parts being most attracted that are tance being only one sixty-fourth part of the nearest the moon, and those at the greatest whole, it could not make a great alteration distance least, the spherical figure must sufin the heating power of the rays. But if it fer some change from the moon's action. does operate in any degree in diminishing Now, as the waters of the ocean directly unthe heat in the northern hemisphere in July, der the moon are nearer to her than the the same cause must operate in increasing central parts of the earth, they will be more the heat, but in a double degree, in the south attracted by her than the central parts. For ern hemisphere in January. For the sun is the same reason the central parts will be one sixty-fourth part nearer the earth than more attracted than the waters on the op. his mean distance on the 1st of January. posite side of the earth, and therefore the Consequently the heat must be greater in the distance between the earth's centre and the southern hemisphere in January than in the waters on its surface, both under the moon northern in July, all other circumstances be. and on the opposite side, will be increased; ing the same. The effect of the direct in. or the waters will rise higher, and it will fluence of the sun is, however, greatly mo- then be flood or high water at those places. dified by the transportation of the tempera. But this is not the only cause that produces ture of one region into another, in conse. the rise of the waters at these two points; quence of that disturbance in the equilibri. for those parts of the ocean which are 90° um of the atmosphere which the action of from them will be attracted with nearly the the sun's rays necessarily produce. same force as the centres of the earth, the effect of which will be a small increase of their gravity towards the centre of the earth. Hence, the waters at those places will press towards the zenith and nadir, or the points Of the Tides.-The tides have always where the gravity of the waters is dimin. been found to follow, periodically, the course ished, to restore equilibrium, and thus occaof the sun and moon; and hence it has been sion a greater rise at those points. But, in, suspected, in all ages, that the tides were order to know the real effect of the moon some way or other produced by these bodies. on the ocean, the motion of the earth on its The celebrated Kepler was the first per- axis must be taken into account. For if it son who formed any conjectures respecting were not for this motion, the longest diame. their true cause. But what Kepler only ter of the watery spheroid would point direct. hinted, has been completely developed and demonstrated by Sir Isaac Newton. Thus we see by what simple means the whole variety of the seasons are produced; and also how admirably fitted the means are to accomplish the end. ly to the moon's centre; but by reason of the motion of the whole mass of the earth After his great discovery of the law of on its axis, from west to east, the most elevagravitation, he found it an easy matter to ac- ted parts of the water no longer answer precount for the whole phenomena of the tides; cisely to the moon, but are carried conside. for, according to this law of nature, all the rably to the eastward in the direction of the particles of matter which compose the uni- rotation. The waters also continue to rise verse, however remote from one another, after they have passed directly under the have a continual tendency to approach each moon, though the immediate action to the other, with a force directly proportional to moon begins there to decrease; and they do the quantity of matter they contain, and in. not reach their greatest height till they have versely proportional to the square of their got about 45° farther. After they have distance asunder. It is, therefore, evident passed the point which is 90° distant from from this, that the earth will be attracted both the point below the moon, they continue to by the sun and moon. But although the at- descend, although the force which the moon traction of the sun greatly exceeds that of adds to their gravity begins there to dethe moon, yet, the sun being nearly four crease. For still the action of the moon hundred times more distant from the earth adds to their gravity and makes them than the moon, the difference of his attrac. descend till they have got about 45° far. tion upon different parts of the earth is not ther; the greatest elevations, therefore, do nearly so great as that of the moon; and not take place at the points which are in therefore the moon is the principal cause of a line with the centres of the earth and the tides. moon, but about half a quadrant to the east of these points, in the direction of the motion of rotation. Thus it appears, if the earth were entirely covered by the ocean, as represented by the circle b de c in the preceding figure, that the spheroidal form which it would assume would be so situated that its longest diameter would point to the east of the moon, or the moon would always be to the west of the meridian of the parts of greatest elevation. And as the moon apparently shifts her position from east to west in going round the earth every day, the longer diameter of the spheroid following her motions will occasion two floods and two ebbs in the space of a lunar day, or 24 hours 48 minutes. These are the effects produced by the action of the moon only; but the sun has also a considerable effect on the waters of the ocean. But it is not the action of these bodies on the earth, but the inequalities of their actions, which produce these effects. The sun's action on the whole mass of the earth is much greater than of the moon's; but his distance is so great, that the diameter of the earth is a mere point compared with it; and therefore, the difference between his effects on the nearest and farthest side of the earth becomes on this account vastly less than it would be if the sun were as near as the moon. However, the immense bulk of the sun makes the effect still sensible, even at so vast a distance; and although the ac. tion of the moon has the greatest share in producing the tides, yet the action of the sun adds sensibly to this effect when his ac. tion is exerted in the same direction, as at the time of new and full moon, when these two bodies are nearly in the same straight line with the centre of the earth. this is the case, the effects of these two bodies are united, so that the tides rise high. er than at any other time, and are called spring tides, as represented by the following figure, where S denotes the sun, d ne z the moon, and c the earth. When The action of the sun diminishes that of the moon in the quarters, because his action is opposed to that of the moon; consequently the effect must be to depress the waters where the moon's action has a tendency to raise them. These tides are considerably lower than at any other time, and are called neap tides. The spring tides do not take place on the very day of the new and full moon, nor the neap tides on the very day of the quadratures, but a day or two after; because in this case, as in some others, the effect is nei. ther the greatest nor least when the imme. diate influence of the cause is greatest or least as the greatest heat, for example, is not on the solstitial day, when the immediate action of the sun is greatest, but some time after it. And although the action of the sun and moon were to cease, yet the ocean would continue to ebb and flow for some time, as its waves continue in violent motion for some time after a storm. The high water at a given place does not always answer to the same situation of the moon, but happens sometimes sooner and sometimes later than if the moon alone act. ed on the ocean. This proceeds from the action of the sun not conspiring with that of the moon. The different distances of the moon from the earth also occasion a sensible variation in the tides. When the moon approaches the earth, her action in every part increases, and the difference in that action, upon which the tides depend, likewise increases. For the attraction of any body is in the inverse ratio of the square of its distance; the nearer, therefore, the moon is to the earth, the greater is her attraction, and the more remote, the less. Hence, her action on the nearest parts increases more quickly than it does on the more remote parts, and there. fore the tides increase in a higher proportion as the distance of the moon diminishes. time of full moon, the tide at the following change will be less. The spring tides are highest and the neap tides lowest about the beginning of the year; for the earth being nearest the sun about the 1st of January, must be more strongly attracted by that body than at any other time of the year; hence, the spring tides, which happen about that time, will be greater than at any other time. And should the moon be new or full in that part of her orbit which is nearest to the earth, at the same time the tides will be considerably higher than at any other time of the year. The tide which happens at any time while the moon is above the horizon is called the superior tide, and the other the inferior tide. When the moon is in the equinoctial, other things remaining the same, the superior and inferior tides are of the same height; but when the moon declines towards the elevated pole, the superior tide is higher than the inferior. If the latitude of the place and the declination of the moon are of contrary names, the inferior tide will be the highest. But the highest tide at any particular place is when the moon's declination is equal to the latitude of the place, and of the same name; and the heighth of the tide diminish. es as the difference between the latitude and declination increases; therefore, the nearer any place is to that parallel whose latitude is equal to the moon's declination, and of the same name, the higher will the tide be at that place.* Such would the tides regularly be if the earth were all covered over with the ocean to a great depth; but as this is not the case, it is only at places situated on the shores of large oceans where such tides as above described take place. Sir Isaac Newton has shown that the tides increase as the cube of the distances The tides are also subject to very great decrease, so that the moon, at half her pre- irregularities from local circumstances, such sent distance, would produce a tide eight as meeting with islands, shoals, headlands, times greater. Now, the moon describes an passing through straits, &c. In order that ellipse about the earth, and, of course, must they may have their full motion, the ocean be once in every revolution nearer the earth in which they are produced ought to extend than in any other part of her orbit; consequently, she must produce a much higher tide when in this point of her orbit than in the opposite point. 90° from east to west, because that is the distance between the greatest elevation and the greatest depression produced in the wa. ters by the moon. Hence it is that the tides This is the reason that two great spring in the Pacific Ocean exceed those of the Attides never take place immediately after lantic, and that they are less in that part of each other; for if the moon be at her least the Atlantic which is within the torrid zone distance at the time of new moon, she must be between Africa and America, than in the at her greatest distance at the time of full temperate zones, on either side of it where moon, having performed half a revolution in the ocean is much broader. the intervening time, and therefore the spring tide at the full will be much less than that In comparing the height of the tides at different plaat the preceding change. For the same ces, it is supposed that the sun and moon are at the same distance from the earth, and in the same position with rereason, if a great spring tide happens at the spect to the meridian of these places. In the Baltic, the Mediterranean, and the Black seas, there are no sensible tides; for they communicate with the ocean by so narrow inlets, and are of so great extent, that they cannot speedily receive and let out water enough to raise or depress their surfaces in any sensible degree. The power of the moon to raise the waters, Sir I. Newton has shown to be about 44 times that of the sun, and that the moon raises the waters 8 feet 7 inches, while the sun and moon together raise them 10 feet, when at their mean distances from the earth, and about 12 feet when the moon is at her least distance. These heights are found to agree very well with observations on the coasts of open and deep oceans, but not well on the coasts of small seas, and where the water is shallow. The mean retardation of the tides, or of the moon's coming to the meridian in 24 hours, is 48′ 45′′-7, and the mean interval between two successive tides is 12h. 25' 14"-2; hence the mean daily retardation of high water is 50′ 28′′-4. About the time of new and full moon the interval is least, being only 12h. 19′ 28′′; and at the quadratures the interval is the greatest, being 12h. 30' 7". The common method of calculating the time of high water at any place is to multiply 50′ 28′′, or the mean daily retardation of the tides, by the moon's age, and then to divide the product by 60, which gives the mean time of the moon coming to the meri. dian on that day in hours; to this is added the time of high water on the days of full and change at the given place, and the sum is the time of high water at that place on the afternoon of the given day, if the sum be less than 12 hours; but if greater, 12 hours 25 minutes must be subtracted, in or. der to have the time on the afternoon of the given day; and 25 minutes subtracted from this time will give the time of high water on the morning of the given day.* The Study of Natural History in Common Schools. A revolution of vast importance to the in. tellectual progress of children seems to have commenced in the common school system of the past age, both of this and some other fa. vored countries, which introduces youth to an early acquaintance with the elements, at least, of a series of studies that were once unwisely supposed of no interest or conse This method is far from being exact; but affords an approximation, which may be useful on some occasions. When accuracy is wanted, recourse must be had to other methods. quence to them. That philosophy which provides resources for the active powers of the mind-which contemplates the physical organization of the body, and exerts a mag. netic influence over the first independent inquiries of a human being, and, in its operations, elevates the character, and at the same time lays a broad and sure foundation for usefulness in this, and immortal happiness in another state of existence, must, indeed, recommend itself to every well-wisher of our race. Such seems to be the fact in rela. tion to the new modes of action, which have resulted in a combined effort, certainly in New-England, to raise the standard of what is denominated a common education. Children are by no means so destitute of thought as some may have ignorantly sup. posed; the machinery of their frames is no less in order for movement than their minds are vigorous and expansive. But to reduce the train to an orderly arrangement, and to direct those intellectual rays which, from the very structure of those organs through which external impressions are made, tend to aberrations, to luxuriance, and to monstrosity, calls upon the philanthropic instructor for uncommon exertions in the commence. ment of their pupilage. Every man who has become at all interested in the prodigious efforts new making, for improving or perfecting the plan of early education, so auspiciously commenced, feels the necessity of superadding something else to the catalogue of indispensables to perfect the system, so that there is actual danger of building up an unwieldy and inconvenient edifice, that will, by and by, fall by the weight of its own massive materials. It is possible that we may be actuated by the same spirit of improvement which has been depre. cated in others, as it is the principal object of this essay to call the attention of instructors to the consideration of a particular department of study, which has yet no place in the school. It belongs to one of those departments of knowledge which has employed the devoted talents of the greatest men, and in the sequel, irresistibly conducts the student from earth to things of heaven, and to Him who framed the universe. The subject to which this allusion is made is Natural History, not in its greatest latitude, embrá cing all animated nature, but a particular department of that curious and captivating subject, which explains the laws of animal life, the physiology and functions of individual organs, the habits, instincts, and locali. ties of animals, and lastly, classification, by which is understood the reason for grouping them in tribes or families. |
