Imágenes de páginas

fore described ; so that whatever course is laid down for the line on the diagram annexed to the patent, (and let it be supposed, for example, to be north and south, or east and west,) upon setting the compass in the old marked line on earth, and directing the sights north and south, or east and west, according to the magnetical peedle, the said marked line on earth, originally run by the magnetical aeedte 130 or 140 years ago, has been found by me to be exactly in the line, or direction with that of the compass; consequently no alteration of the variation could have taken place during the whole, or any part, of that period of time in Jamaica.'

Observations on the Variation and on the Dip of the Magnetic Needle, made at the Apartments of the Royal Society between the Years 1786 and 1305 inclusive. By Mr. George Gilpin.This memoir contains, perhaps, the most exact table of the variation of the compass that has been yet submitted to the public. During sixteen months, every day successively, and at several periods of time in each day, Mr. G. observed the com. pass. The hours were 7 A.M., 8 A.M., 10 A.M., 12 A.M., i P.M., 2 P.M., 4 P.M., 6 P.M., 8 P.M., 10 P.M., and 11 P.M.-At 7 A.M. and 8 A.M., the daily yariation was at its minimum : at iP.M. and 2 P.M. at its maximum; and at & P.M. at its maximum again. The daily observations are comprized in tables occupying sixteen pages. A table is also given for the mean monthly variation:

Table IV. (says Mr. Gilpin) contains the differences for 12 years, viz. from 1793 to 1805, between the observations of the variation made in the months of March, June, September, and December, or at the times of the vernal and autumnal equinoxes, and summer and winter solstices; by a mean of these 12 years, the variation appears to increase or go westward, from the winter solstice to the vernal equinox 6.80; diminishes or goes eastward from the vernal equinox, to the summer solstice 1:43 ; increases again from the summer solstice to the autumnal equinox 2'.43 ; and continues nearly the same, only decreasing o':14, from the said equinox to the winter solstice.

• These differences at the times of the equinoxes and solstices have been noticed by M CASSINI, in his observations made at the Royal Observatory at Paris, between the years 1783 and 178.", but the effect was considerably greater in his observations, than in those mentioned above; his results however were, in my opinion, drawn from too few observations, being from only 8 days observations about the times of the equinoxes and solstices, which differ considerably among themselves ; and experience teaches us, that magnetical observations made for a period so limited are not sufficient for minute purposes : I have therefore, in the results here given, taken the mean of the observations made during the whole munth in which the equinoxes and solstices fall, which appear to me likely to furnish

more satisfactory; and all the foregoing observations are to be considered


as the results or mean of a great many, by way of arriving at greater accuracy ihan could be obtained without ; this, however, was found to be more necessary at some times than at others ; sometimes, the peedle would be extremely consistent with itself, so as to return cxactly to the game point, however often it might have been drawn aside ; at other times it varied 2 or 3', sometimes 8, 10, or even more; this uncertainty in the needle arises principally, I believe, from changes in the atmosphere, for a change of wind, from any quarter lo another, almost always produced a change in the needle from steady to insteady, and vice versá, but it was generally more unsteady with an easterly wind, than when it blew from any other quarter, and Hvosi steady when the wind was south or southwesterly. An Aurora Borealis always produced considerable agitation of the necdle.'

The variation of the compass at present, according to Mr. Gilpin, is 24° 8' West, and its annual increase is exceedingly small; so that we may conjecture the needle to have reached is greatest westerly declination.

Mr. Gilpin is eminently intitled to the gratitude of the philosophic world, for the care and assiduity bestowed on this subject, and (as it should seem) for the great nicety with which Lis observations were made..

On the Declinations of some of the principal fixed Stars; with á Description of an Astronomical Circle, and some Remarks on the Construition of circular Instruments. By John Pond, Esq. With a circle constructed by Mr. Troughton, Mr. Pond made observations on the declinations of those stars, the right ascen. sions of which are observed at Greenwich ; and his labours on this subject may be useful to Astronomers.--He suggests an ingenious and a very simple mode of examining the latitudes of places. If the declinations of the same Stars examined at different places do not agree, then the latitudes of the places of observations must have been inaccurately determined; and a correction therefore will be requisite. After having stated his method, and the corrections proposed to be made to the colatitudes of Greenwich, Armagh, Palermo, and Westbury, he makes the following observation, which well merits attention ;

I consider this comparison as interesting likewise on another account; it is an object deserving of curiosity to examine the present state of our best astronomical instruments, and to ascertain what may reasonably be expected from them. The superiority of circular instruments is, I believe, tòo universally admitted, to render it probable that quadrants will ever again be substituted in their place. But the Greenwich quadrant is so intimately counected with the history of astronomy, the observations that have been made with it, and the dedu tions from those observations, are of such infinite importaitce to the science, itat every circumstance relating to it cannot fail of being


interesting Now when it is considered that this instrument has been in constant use for upwards of half a century, and that the center eror, from constant friction, would during this time have a regular tendency to increase, it will not appear at all surprising, if the former accuracy of this instrument should be somewhat impaired, With a view, therefore, of ascertaining more correctly the present state of an instrument on which so much depends, I have exhibited in one view the polar distances as determined by circular instruments alone; the respective co-latitudes being previously corrected by the method above mentioned; and I have compared the mean result with the Greenwich Catalogue, that the nature and amount of the deviations may be seen, and if it be judged necessary, corrected. I should add, that by some observations of the sun at the winter solstice in 1800, the difference between the Greenwich quadrant and the circle vas 10 or 12", the quadrant still giving the zenith distance too little.'

Observations and Remarks on the Figure, the Climate, and the Atmosphere of Saturn and its Ring. By William Herschel, LL.D.F.R.S.-We have already noticed, in a preceding Number, the observations made by Dr. H. on the figure of Saturn It was the Doctor's opinion, last year, that this planet was flattened both at its poles and at the equator; and the prescrit memoir coufirms that idea.

The form of Saturn flattened at the poles, if it be the real form, is not that which, from reasons that obviously suggest themselves, results from the planet's rotation, and from the attraction of the Ring. The figure of Saturn was once differe ently represented by Dr. H.; and this discrepancy of representation having been urged as an objection against the new figure, he thus replies to that remark:

Io the year 1789 I ascertained the proportion of the equatorial to the polar diameter of Saturn to be 22.81 to 20.61, and in this measure was undoubtedly included the effect of the ring on the figure of the planet, though its influence had not been investigated by direct observation. The rotation of the planet was determined afterwards by changes observed in the configuration of the belts, and proper figures to represent the different situation of the spots in these belts were delincated.

In drawing them it was understood that the shape of the planet was not the subject of my consideration, and that consequently a circular disk, which may be described without trouble, would be sufficient to show the configurations of the changeable belts.

Those who compare these figures, and others I have occasionally given, in which the particular shape of the body of the planet was pot intended to be represented, with the tigure which is contained in my last

paper, of which the sole purpose was to express that figure, • See M. Rey. N.S. Vol. xlix. p. 382.

and and wonder at the great difference, have probably not read the meae sures I have given of the equatorial and polar diameters of this planet; and as it may be some satisfaction to compare the appearance of Saturn in 1789 with the critical examination of it in 1805, I have row drawn them from the two papers which treat of the subject ; Fig. 1, Plate XXI. represents the spheroidical form of the planet as observed in 1789, at which time the singularity of the shape since discovered was unknown; and Fig. 2, represents the same as it appeared the 5th of May, 1805. The equatorial and polar diameters that were established in 1789 are strictly preserved in both figures, and the last differs from the first only in having the fattening at the poles á little more extended on both sides towards the equatorial parts. It is in consequence of the increase of the length of this fattening, or from some other cause, that a somewhat greater curvature in the latitudes of 40 or 45 degrees north and south has taken place ; and as these differences are very minute, it will not appear extraordinary that they should have been overlooked in 1789, when my attention was intirely taken up with an examination of the two princi. pal diameters of the planet.'

We qunte also the subsequent passage, as containing a fact whirh is rither remarkable, and for which we see no good: reason. It it be exact, it ought to satisfy those who have used magnifying powers only moderately high, and have not been ahle to discern the pro-oblateness of Saturn:

· The use of various magnifying powers in observing minute objects is not generally understood. A low power, such as 200 or 160, with which I have seen the figure of Saturn, is not sufficient to show it to one who has not already seen it perfectly well with an adequate high power; an observer, therefore, who has not an instru. ment that will bear a very distinct magnifying power of sco, ought not to expect to see the outlines of Saturn, so sharp and well defined as to have a right conception of its figure. The quintuple belt is generally a very good criterion ; for if that cannot be seen, the telescope is not sufficient for the purpose; but when we have intirely convinced ourselves of the reality of the phenomena I have pointed out, we may then giadually lower the power, in order to be assured that the great curvature of the eye glasses giving these high powers, has not occasioned any deceptions in the figure to be investigated, and this was the only reason why I mentioned that I had also seen the remarkable figure of Saturn with low powers.'

A List of Presents received by the Society, and an Index, according to antient and liable customi, terminate this poc lume.


il. is.

Art. IX. Recollections of Paris, in the Years 1802-3-4-5. By

J. Pinkerton. 2 Vols. 8vo. Pp. about goo in each.

Boards. Longman and Co. Is Fashion presided only over articles of personal decoration,

Mr. Pinkerton's idea of representing her as the deified little milliner' would be admissible: but the province of this divinity is of much wider range, extending even to the region of intellect, and influencing men of letters as well as fancy dress-makers. Our thoughts take their hue from those which have been prevalent in the society with which, by choice, we have been most conversant'; and in preparing them for the public, we generally fall into the adopted mode of exhibition, Hence ariees a fashion in book-making; and authors, even while they seem to study variety, preserve in certain traits a kind of family likeness.

Mr. Pinkerton, in his Recollections of Paris, may urge that he has given accounts of the French capital, of its environs, and its inhabitants, which, from his long residence, may be supposed to possess superior accuracy, and some of which are not to be found in any of the several publications with which we have been lately presented; he may plead, moreover, in excuse for his title, that he took no notes;' and that the greater part of these volumes is the result of a happy memory, combined with such observations as were excited by the state of the country and of the society which he explored. So far we have no objection to his proceedings: but we see no necessity for interweaving Essays with his recollected facts; we find too much of modern authorship in the manufacture and complexion of his work, and his considerations on the writings of Rousseau tend to interrupt the narrative, and needlessly to swell the volume. Some individuals, it is probable, will accuse him of being too frenchified; of discovering too great a partiality for the country and usages of our enemies; of being too forward in their vindication, and of arguing with too much energy on the policy of cultivating their friendship, even in the closest bonds, at the very moment when war is raging with all the malignity of the most inflamed passion. We are not, however, of the number of those who upbraid Mr. P. with being a cosmopolitical traveller; and so far are we from being displeased with his endeavours to impress a conviction on the two countries of the superior advantages which would accrue to each from the strenuous cultivation of the arts of peace, in preference to the favourite though unchristian system of war, that we should derive the most heart-felt pleasure from the practical adoption of this principle: but, unfortunately for it


« AnteriorContinuar »