Obrázky na stránke
PDF
ePub

schools oft aids the progress of error and Atheism. This ought not longer to be tolerated. Many writers on this subject give us words and phrases that, to common minds at least, convey no meaning. Sir John Herschel says,

“ The Divine Author of the universe cannot be supposed to have laid down particular laws, enumerating all individual contingencies, which his materials have understood and obey-this would be to attribute to him the imperfections of human legislation ; but rather, by creating them endowed with certain fixed qualities and powers, he has impressed them in their origin with the spirit, not the letter, of his law, and made all their subsequent combinations and relations inevitable consequences of this first impression."

We shall not stop to consider this reasoning, in which we can perceive no force, at length; but we ask, in what sense can the materials of the universe, the insensible clod, the rock, the dull

, inert mass of earth, be said to “understand and obeythe laws of God? What intelligible meaning can be given to the terms “qualities” and “powers" in the above quotation? How shall we understand the phrase, “the spirit of his law impressed on matter?" or the assertion, that "the combinations and relations of the materials of the universe, subsequent to creation, are the inevitable consequences of this first impression of the spirit of a law ?

This is a fair specimen of much that is written relative to the phenomena of nature. To us it is perfectly incomprehensible. No consistent meaning can be given to many of the terms used as causes. Use has made them so familiar that at first this may not be perceived. Moreover, there is no propriety in using the terms that express actions of intelligent beings, to explain the changes that take place in the inert earth.

We trust the time is not far distant when the young shall be taught universally that God not only created the universe, but that he upholds it by the immediate and constant exertion of his power. The awe that will result from a knowledge of the nearness of God, as derived from the exhibition of his power in nature, will have a powerful influence to lead the inquirer to that true wisdom, the beginning of which is the fear of the Lord.

ART. II.- THE PRESENT STATE OF ASTRONOMY.

Outlines of Astronomy. By SIR JOHN F. W. HERSCHEL. Philadelphia: Lea & Blan

chard.

ASTRONOMY, which is the oldest of sciences, is emphatically a science of accumulation. The date of its beginning it is impossible to fix. Its first birth is commonly attributed to the shepherds of Chaldea. But there cannot be a doubt that its existence commenced with that of man himself. The heavenly bodies were ever before his eyes, and he must have begun immediately to discern some of the more grossly perceptible laws of their movement. As a science of observation it has been one of successive approximations to truth, so slow and so often repeated, that the immense period during which the process has been conducted has been sufficient to eliminate almost every vestige of error; and, by removing at last the most minute imperfections, to render this a perfect science.

In the chapter upon astronomical instruments, in the work whose title is at the head of the present article, the kind of process alluded to above is incidentally set forth in a very minute and striking manner :

“ The steps by which we arrive at the laws of natural phenomena, and especially those which depend for their verification on numerical determinations, are necessarily successive. Gross results and palpable laws are arrived at by rude observation with coarse instruments, or without any instruments at all, and are expressed in language which is not to be considered as absolute, but is to be interpreted with a degree of latitude commensurate to the imperfection of the observations themselves. These results are corrected and refined by nicer scrutiny, and with more delicate means. The first rude expressions of the laws which imbody them are perceived to be inexact. The language used in their expression is corrected, its terms more rigidly defined, or fresh terms introduced, until the new state of language and terminology is brought to fit the improved state of knowledge of facts. In the progress of this

scrutiny subordinate laws are brought into view which still further modify both the verbal statement and numerical results of those which first offered themselves to our notice; and when these are. traced out and reduced to certainty, others, again, subordinate to them, make their appearance, and become subjects of further inquiry. Now, it invariably happens (and the reason is evident) that the first glimpse we catch of such subordinate laws—the first form in which they are dimly shadowed out to our minds—is that of errors. We perceive a discordance between what we expect, and what we find. The first occurrence of such a discordance we attribute to accident. It happens again and again; and we begin to suspect our instruments. We then inquire, to what amount of error their determinations can, by possibility, be liable. If their limit of possible error exceed the observed deviation, we at once condemn the instrument, and set about improving its construction or adjustments. Still the same deviations occur, and, so far from being palliated, are more marked and better defined than before. We are

now sure that we are on the traces of a law of nature, and we pursue it till we have reduced it to a definite statement, and verified it by repeated observation under every variety of circumstances.”—Pp. 89, 90, $ 139.

Many of the changes detected in the manner here described are so slow, and require so long a period to amount to any appreciable quantity, that ages are sometimes requisite to bring them to light; and it is on this account that length of time is such an important element in astronomical investigations, and that the age of this science, owing to its peculiar nature, is a more than ordinary advantage.

Again:-space being the theatre of astronomic phenomena, the science of space and its relations, namely, geometry in the widest sense of the term, becomes directly applicable, and all the improvements, from the sublime geometry of the ancients to the most refined results of modern analysis, become available, and contribute to the perfection of astronomy. Finally, since the great discovery of the law of gravitation by Newton, astronomy has presented itself in another and new aspect; the movements of the heavenly bodies are brought under the laws of force and motion which constitute the science of mechanics,-a strictly mathematical science, requiring for its full development all, and even more than all, that the highest branches of mathematical calculus in their present imperfect state can furnish.

Astronomy, indeed, may be divided into four distinct branches :

1. Practical Astronomy, which includes the use of the instruments, the calculations necessary to free the observations with them from the effects of atmospheric refraction, parallax, and the aberration of light, and such solutions of spherical triangles as may be necessary to determine the co-ordinates of the places of the heavenly bodies, to wit, their right ascension and declination, or latitude and longitude.

2. Theoretic Astronomy, or the theory of the movements of the heavenly bodies in space, as derived from the results of practical astronomy :-as, for instance, the determination of the nature of the orbits of the planets; their eccentricities; their nodes, or points of intersection with the plane of the earth's orbit; the inclination of the planes of their orbits to that of the earth; the mean distance of the planets from the sun, and their periodic times or terms of revolution.

3. Descriptive Astronomy, which relates to the physical constitution of the planets, and requires for its prosecution the aid of powerful telescopes. This branch of the subject leads to inquiries into the existence of atmospheres, of oceans, mountain-ranges, volcanoes, and other features in the physical phenomena and topography of the planets. In this field the elder Herschel and Schroeter of Lilienthal have been particularly distinguished.

4. Physical Astronomy, which, in the present acceptation of the term, is the investigation of the heavenly movements as a great mechanical problem, setting out from the simple law of gravitation, that the bodies attract each other with a force proportioned directly to the mass, and inversely as the square of the distance, and thence deducing all the grand and minor movements which result from the mutual influences of the numerous bodies, primary and secondary, in our system.

The complete solution of this grand problem, by the aid of that modern invention in mathematics, the calculus, is the boast of the immortal work of Laplace, known as the “Mécanique Céleste.”

When it is considered that the number of bodies in the solar system now discovered, including the satellites, amounts to nearly forty; that of more than seven hundred comets, the orbits of one hundred and fifty have been computed; that of late years the bounds of the solar system have been passed, and the delicacy of modern instruments has detected the minute annual parallax of the fixed stars, thus furnishing the means of determining their distance; that orbitual motion in binary systems, each composed of two of these remote suns, has been discovered and exactly computed; that the motion of the solar system itself in space has been ascertained.-some conception may be formed of the extent and complication of the field which presents itself for the labours of the astronomer.

It is this science, of the nature and extent of which we have thus endeavoured to give a general notion,—a science so vast, so long prosecuted by mankind, so multifarious in its aspects, requiring for its full comprehension such profound knowledge of other sciences --it is this science which the work before us attempts to present in popular form. Demanding but a schoolboy's knowledge of geometry in the reader, it seeks to furnish him with some just conception of every part of this great and varied subject. No easy task. With what success it is accomplished, we shall now endeavour to show.

The nature and difficulty of the undertaking are set forth by the author himself, as follows:

“ After all, I must distinctly caution such of my readers as may commence and terminate their astronomical studies with the present work, (though of such,

at least in the latter predicament -I trust the number will be few,) that its utmost pretension is to place them on the threshold of this particular wing of the temple of science, or rather on an eminence exterior to it, whence they may obtain something like a general notion of its structure; or, at most, to give those who may wish to enter, a ground-plan of its accesses, and put them in possession of the pass-word. Admission to its sanctuary, and to the privileges and feelings of a votary, is only to be gained by one means--sound and sufficient knowledge of mathematics, the great instrument of all exact inquiry, without which no man can ever make such advances in this or any other of the higher departments of science as can entitle him to form an independent opinion on any subject of discussion within their range. It is not without an effort that those who possess this knowledge can communicate on such subjects with those who do not, and adapt their language and their illustrations to the necessities of such an intercourse.”—Pp. 20, 21, § 7.

The author begins with the usual subject of the figure and motions of the earth. Our insensibility to the vast rapidity of the earth's motion through space is happily exemplified.

When, for example, we are carried along in a carriage with the blinds down, or with our eyes closed, (to keep us from seeing external objects,) we perceive a tremor arising from inequalities in the road, over which the carriage is successively lifted and let fall, but we have no sense of progress. As the road is smoother, our sense of motion is diminished, though our rate of travelling is accelerated. Railway travelling, especially by night or in a tunnel, has familiarized every one with this remark. Those who have made æronautic voyages testify that with closed eyes, and under the influence of a steady breeze, communicating no oscillatory or revolving motion to the car, the sensation is that of perfect rest, however rapid the transfer from place to place.

“ But it is on shipboard, where a great system is maintained in motion, and where we are surrounded with a multitude of objects which participate with ourselves and each other in the common progress of the whole mass, that we feel most satisfactorily the identity of sensation between a state of motion and one of rest. In the cabin of a large and heavy vessel, going smoothly before the wind in still water, or drawn along a canal, not the smallest indication acquaints us with the way it is making. We read, sit, walk, and perform every customary action as if we were on land. If we throw a ball into the air, it falls back into our hand; or if we drop it, it alights at our feet. Insects buzz around us as in the free air; and smoke ascends in the same manner as it would do in an apartinent on shore. When we look at the shore, we then perceive the effect of our own motion transferred, in a contrary direction, to external objects—external, that is, to the system of which we form a part." -P. 28, § 15, 16.

The author passes on to the consideration of the atmosphere, which refracts the rays of light passing into it from the heavenly bodies, , bending the rays downward, and making the bodies, which appear always in the last direction of the rays as they enter the eye, to look more elevated than they really are. Thence he goes on to consider the subject of parallax, which is the only correction of importance, besides refraction, to be applied to the observed altitude of a heavenly body, in order to obtain its true altitude. This subject appears to us confused from unnecessary profuseness of illustration. Parallax may be defined to be the difference in direction in which an object appears as viewed from two different points, as from the centre and surface of the earth, (which is diurnal parallax,) or from the centres of the earth and sun, which is called annual parallax. As

« PredošláPokračovať »