immediate effect of the iron composing the vessel and her works, upon the indications of the needle, as the ship's head is turned into different azimuths, being in some cases even more than five points. It is also evident, that in different vessels this will very materially vary, and that little assistance can be derived from experiments made in one vessel towards guiding us in judging of the influence of the iron in another of a different, or even of a similar construction, so far as form and disposition of metal are concerned in rendering them similar. To this, however, we shall speak presently. It is true that at G, C, P, the influence is much less than at any of the other positions where the observations were made; but as these are positions at which, in actual navigation, the observations could not be conveniently made, there can be no inference in favour of the employment of iron steamers deduced from these,—however interesting they may be in reference to some questions concerning the extent of the influence of iron on the magnet, and upon the law of its decrease of force as the distance is increased.

The result of these observations, as well as his first already mentioned, was conclusive to Captain Johnson's mind, that for the purposes of navigation, his observations should be chiefly confined to the positions A and B. Of two series of such observations, the discrepancies are unaccountably great, and especially so, seeing that every possible precaution was taken to secure a sameness of circumstances under which to make the observations. Captain Johnson was not, however, so much surprised at this circumstance as we should have expected; as he had remarked that the "embarrassment in the movement of the needles" after the first series in some degree prepared him to expect it,—or, in other words, that the difference in the magnetic state of the needles themselves, induced by the first series, was so great, that their indications would be materially altered in the second. This we can easily, to a certain degree, but not to the amount which these experiments indicate, conceive; the directive force of the needle may be increased or diminished by induction we admit, and then the power of the iron remaining the same, its influence would be accordingly less or greater upon the position of the needle when out of the magnetic meridian: but still that pure iron should induce so much permanent change in the magnetic state of the hard steel of the needle is to us inconceivable. All experiments go to prove the reverse of this. Captain Johnson's subsequent experiments prove the reverse of this, too; for they actually show that the vessel herself was a permanent magnet, and not a magnet by induction, as cast-iron is generally found to be. Now the needle itself could produce no sensible effect upon the state of the larger magnet; and hence her power upon the needle would be sensibly the same in both series of experiments, whilst that of the needle to resist her influence may be varied in a great degree. Captain Johnson is led to attribute the different effects (in another series of experiments which he afterwards made with the same needles) of the head and stern of the vessel to the needles placed upon quay, to the different disposition of the quantity of iron in those two parts of her; but at the distances at which he observed these deflections, we cannot account for it on these principles, especially keeping in view the very minute influence recorded to have been exercised upon the needles

at G, C, and P, in his former experiments, in the table which we have copied above.

The intensity of the magnetic force in the needle was greatly altered by the iron of the vessel; the dip was also very greatly altered. All this, of course, we should expect; but we wish the intensity of the needles had been observed on shore both before and after the experiments were made on board; and as nearly as possible under the same circumstances. This, we consider an unfortunate omission; but we do not blame Captain Johnson for it. The time of the year was unfavourable, affording so few days fitted for observation, and the period allotted to the whole series was much too short under such unfavourable circumstances. If we are not mistaken, other observations, too, were made which are not here published; and, possibly, when they are made public, they may throw some further light upon the phenomena. However, in this respect, we are not able really to say what those observations were, nor whether they at all bear upon this question. If they do, it was a mistake to withhold them from the scientific public on the present

occasion.

The two following diagrams exhibit visually the several circumstances of these experiments; and require scarcely any description.

When we consider the great number of parts of which a vessel is composed, and the processes by which those parts are formed, we can hardly be justified in considering the vessel as other than an immense magazine of permanent magnets fastened together. Were they all, indeed, so placed in building her, that their axis may be parallel, we may be better able to form some general idea of the magnetic resultant, (so to speak,) and to guess at the intensity and directive force exerted by the whole system; but, even then, the difficulties, in the present state of magnetic knowledge, would be absolutely insurmountable. When, however, we consider the effect of a single and slight stroke with a hammer upon a permanent magnetic bar-the total disregard paid in building the vessel as to the magnetic state of her materials—the utter impossibility of ascertaining it after they are put together, or to alter it, definitely, in any one of the pieces-when we consider that malleable iron can alone be employed, and that all these difficulties stand in the way of even placing the component parts of the compound magnet parallel to one another, we say, we are compelled to affirm, that the real magnetic state of an iron ship is incapable of being ascertained by any series of experiments whatever. Captain Johnson's experiments verify, but do not in our minds strengthen, our convictions on this head. They were formed, à priori, and from the view which, in common with all scientific men, we had before taken of the necessary consequences of the most familiar phenomena, as well as of the diversified experiments of a more refined class carried on by the most eminent philosophers for many years. Instead, therefore, of considering the steamer as a single permanent magnet, we ought to consider her as a vast apparatus of united magnets, distributed perfectly at random; and their relative positions and intensities altogether incapable of estimation. Are we not justified, then, in saying as we have said above, that we are incapable of judging from a series of experiments made on board one vessel, what the effect of one (so far as general form and disposition of material are concerned) similarly built in all respects may be? Surely, we are.

Let us, however, even waive this objection. Have we not seen that on board the same vessel, on two different days not very remote from each other, the deviation of the same needles was very different when all circumstances were alike, except that the magnetic states of the needles were themselves changed by the first series of observations? We do not, indeed know where the needles were placed during the intermediate period,-whether ashore or on board. If on shore, the influence of the short period during which the first observation was made, is only indicative of the intense action of the compound magnet, which could, in so short a time, produce such a permanent change in the needles: if on board, they were probably kept in the positions of observation, and if so, it proves how dangerous it is to trust to the same compass, from one day to another. The compass with which we leave port is a different compass after a single hour's voyage; and after three or four days, has been "deteriorated" to such a degree as to be unworthy of the slightest confidence.

When we look at the matter in this light, we need not inform our readers, that we look also upon all attempts at the correction of the local

attraction, by means of Barlow's plate, as utterly useless on board an iron steamer. Upon this subject, however, as we said before, we intend to speak more fully in a future number: it is sufficient to say here, that Captain Johnson appears to have made but few experiments with it, owing to the "unfavourable state of the weather." It is very true that unfavourable weather is disadvantageous for good experiments for mathematical investigations of the laws which reign through physical phenomena; but we do really think, that, from the circumstance of the knowledge itself being only required in bad weather, the present was an advantageous opportunity thrown away; inasmuch as the amount of discrepancy to be expected in times when the compass is our only trust, is, in a practical point of view, infinitely more important than a knowledge, however accurate, of the laws which prevail when the compass is never recurred to by a properly educated sailor. It is surely of greater consequence to know how far we may safely trust our instruments, at the time when they are our only trust, than to ascertain their laws in a state of comparative repose, and when they are altogether unnecessary to our safety.

Finally, we feel it our bounden duty to our readers and to the public, to express emphatically, our conviction of the extreme danger of this class of vessels, for the purposes of navigation by means of the compass,—or, in other words, for venturing out of the mouth of a river. It is neither our wish, nor our interest, to discourage the progress of the arts and manufactures of our country; but it is both our duty as scientific journalists, and our spontaneous feeling as men, to warn our countrymen against the dangers to which they may expose themselves, by stretching beyond its due limits, the application of any product of our manufacturing ingenuity. We are fully aware of the advantages which belong to the use of iron, in the construction of iron steamers; but we also wish our readers to be fully impressed with a sense of their recklessness in daring all its dangers. We should nor fulfil our duties towards them, did we not distinctly tell them, that the iron vessel is in precisely the same condition with respect to the compass, as if no compass had ever existed,—or, in some respects, even a worse condition,—since they may be led to trust to a guide that cannot guide them aright, and neglect those slight glimpses of indication which may in some small degree assist them. Such, at least, is our view of the matter; and if we are wrong, we hope we need not say, that we shall be extremely glad to be enlightened on the subject, by those who see better than we can.

We do not think it necessary to give here Captain Johnson's determination of the best position for a sailing compass on board the Garryowen, as, even abating all other objections to the use of this kind of vessel, we have given good reasons why it could not be depended on for any other vessel similarly built,-much less for one in which a different disposition of materials may be adopted. On the care bestowed upon the experiments, as well as on the evident honesty of their record, we cannot speak too highly; and we are glad to find such men are found by the Admiralty, to be entrusted with this class of expeditions. We cannot, however, in respect to its scientific value, but regret that a more complete apparatus was not furnished to him, and that a more suitable period, both as to date and extent, was not selected for the purpose.

456

A POPULAR COURSE OF ASTRONOMY.

No. VII.

THE ELLIPTICAL FORM OF THE EARTH'S ORBIT.-KEPLER'S LAWS.

THE earth revolves continually upon an axis within itself, and continually in an orbit about the sun. Hence result the alternations of day and night, the difference of the duration of the solar from that of the sidereal day, and the different times of the rising of the same fixed stars,— of which phenomena the two last constitute an apparent annual revolution of the sun in the heavens, inasmuch as they would result from such a revolution. The axis about which the rotatory motion of the earth takes place remains always parallel to the same line in space; and hence result the phenomena of the seasons. But all these phenomena will be equally well accounted for by a revolution of the earth round the sun, in whatever orbit that revolution may take place, and no inquiry on which we have hitherto entered indicates with any certainty, what is the real form of the earth's orbit. Provided the earth go completely round the sun in the space of a year, it matters not, so far as the facts which have hitherto been stated are concerned, whether the motion of its centre be in a circle, in an ellipse, or in a spiral,—or, in fact, whether it be in a square or an oblong.

We are now about to indicate the means by which the real form of the earth's orbit is ascertained, the nature and law of its motion in that orbit, and the actual dimensions of the orbit. It will then be explained how the apparent motion of the sun, the duration of the seasons, and the length of the year, are modified by these facts.

us.

First, then, as to the form of the earth's orbit. It is not a circle, for then the sun would at all times of the year appear of the same size to We judge of the dimensions of objects by the angles which lines, drawn from the extremities of them, subtend at the eye. The conclusions we thus draw, we modify, however, by that which we know of the distance of the objects. Thus two objects, AB and CD, may sub

tend the same angle, CED.

C

A

E

D

B

Rays of light come to us in straight lines. - If, therefore, an instrument having two arms, which can be made to include any given angle between them, be placed so that one of these arms is in the direction of a ray coming from one point of the edge of the sun's disk, and the other in the direction of a ray coming from a point on the edge diametrically opposite to this, then these two arms of the instrument will include between them precisely the same angle which two lines drawn from opposite points of the sun, or opposite extremities of one of its