Pagina-afbeeldingen
PDF
ePub

method to measure these two arcs separately; that is to say, the zenith distance of the sun is considered as the immediate object of investigation, and the remaining arc, or as it is usually termed, the co-latitude of the place, (the knowledge of which is presumed) is added to complete the polar distance. But in whatever way we consider the subject, it is evident that the sole object of the practical astronomer is to obtain by some mechanical means the measure of this total arc.

The mural circle at Greenwich has neither level nor plumbline, nor do I conceive that the least advantage could be derived from the application of either: its principle enables the observer to determine this total arc without any intermediate or zenith point, hence the co-latitude, the knowledge of which is so essential from the construction of other instruments, is in this case rather a question of curiosity than of absolute necessity.

I find it, however, convenient (for reasons which I hope to have an opportunity of stating more at large in a future Paper) to assume an intermediate point, which I call the zenith point, without being at all anxious to know whether it is really so or not; because I find myself possessed of the means of determining the position of this imaginary point of departure, on the instrument, to within the tenth of a second, a degree of precision, which I apprehend no level or plumb-line can ever be expected to equal.

From this point of departure, I measure the distance of the sun to the southward, and of the pole to the northward, and the sum of these two measures is evidently the north polar distance of the sun, which in every method is the ultimate object to be attained.

I have, as an example, annexed a computation of the same solstice obtained by direct measurement, from the pole, without the aid of the intermediate point above mentioned; and it will be seen that the results do not in this case differ above a quarter of a second from each other.

There is indeed no other difference between these two methods, but that, in the former case, the part of the arc Z P is obtained rather more accurately, by a great number of observations both before and after the solstice, than could be done in the short interval of time in which the solstice itself is observed.

By sixty observations of y draconis, of which only three differ so much as i" from the mean, it appears that the zenith point which I have assumed is 2', 18”,64, south of the mean position of y draconis for the beginning of the year 1812, which

Υ is the same quantity that is found by the observations with the zenith sector, 1811.

Mr. Troughton is now occupied in making a zenith sector upon a new and very simple principle, with which I have no doubt that this distance may be determined with a much greater degree of precision.

Astronomers will inmediately perceive that this arc, however accurately it may be mechanically determined, must inevitably be subject to whatever uncertainty still exists upon the question of astronomical refractions; the instruinent not having been erected long enough to remove this uncertainty, I have for the present employed Dr. Bradley's refractions, such as they have been used for many years in this Osservatory. Such alterations in this part of the calculation may easily be made in future, as the advanced state of the science may require.

[blocks in formation]

June

12 29,94 64 70 0,30,3 OUL 66,33,33.0 12,93 38,31,34,1015,47,20,17,15,4 28, 1, 1,0 66,32,21,4 1429,70 63 71 10,29,4 OUL66,26 48,512,95 34,16115,47,2 10,32,4

0,58,6

18,9 18/29,77 | 58 62 10,29 6 OU L66,18,20.212.95 34,10 15,46,8 2, 2,6

0,59,9

20,2 1929,311 59 61 10,29 70L260,48,44,610,04 38,31,31,2015,46,8 0,57,0 0,59,3

19,9 2029.28 57 6010,29,2 OUL66,16 31,4 10,04 31,2015,46,8 0,16,3 0,59,9

20,5 2329,78 56 61 10,30,40 LL 66,48,43,5|23,5938,31,44,745,46,6 0,42,7 0,597

21,3 24 29,85 58 62 0,29,8 OUL 56,18,11,3 23,59

44,74 15,46,6
1,41,1 1, 1,9

23,3 25 29,80 58 65 10,30,40 LL 66,51, 5,5 23,59 44,7415,46,6

3, 4,2

I, 0,4 27/29,81 56 | 58 0,29,7 OUL66,23,33, 23:59 44,74 15,46,6

7, 4,4
I, 0,7

22,0 28 29,781 54

54 0,30,7 OLL|66,57,41,5 23,59 44,74 15,46,6 9,41,4 0,59,5 29 30,05 58 64 10,30,1 OUL 66,29,11,423,59

|
4.1,74115,46,6 12,43,0 I, 0,2

21,7 3029,90 58 63 10,30,710 L L/67, 4, 9,5|23,59 44,74115,46,61 16, 8,9 1, 0,1

21,4

21,8

20,8

+, 0,95

Mean of 12 Observations

28, 1, 0,1066,32,21,10 Nutation—8", 4 Parallax of",0

12,401 12,40

28, 0,47,70 Z. P. determined by 30 observations of Polaris above, and 30 below 38,31,21,15 Z. P + O Z or N. P. D.

66,32, 8,83 66 32, 8,70 Solsticial Declination

23,27,51,15 23,27,51,30 Correction for O' Latitude

+, 0,95 Mean Obliquity at Summer Solstice

123,27,52,10 23,27,52,25 Subsequent observations gave the arc Z. P. rather greater than here assumed; but as I attribute this to a change in the refraction arising from a colder temperature, I take the arc as it was measured at the Summer Solstice.

This result may at any future time be rendered more accurate, if it should be found that the arc Z. P. has not been rightly determined ; and in this consists the great advantage of the intermediate point Z., that the arc P. Z. may be determined more and more accurately, almost without limit, the only unalterable error resting on the letermination of o Z.

IV. Observations relative to the near and distant Sight of different

Persons. By James Ware, Esq. F. R. S.

Read November

19, 1812.

THE

he fact that near sightedness most commonly commences at an early period of life, and distant sightedness generally at an advanced age, is universally admitted. Exceptions, however, to these rules so frequently occur, that I flatter myself a brief statement of some of the coincident circumstances, attendant on these different imperfections in vision, may not be found wholly undeserving the attention of the Royal Society. Near sightedness usually comes on between the ages of ten and eighteen. The discovery of it most commonly arises from accident; and, at first, the inconvenience it occasions is so little, that it is not improbable the imperfection would remain altogether unnoticed, if a comparison were not instituted with the sight of others, or if the experiment were not made of looking through a concave glass. Among persons in the inferior stations of society, means are rarely resorted to for correcting slight defects of this nature; and, indeed, I have reason to believe the imperfection in such people is not unfrequently overcome by the increased exertions that are made by the eye to distinguish distant objects. This, however, is not the case, in the present day, with persons in the higher ranks of life. When these discover that their discernment of distant objects is less quick or less correct than that of others,

a

though the difference may be very slight, influenced perhaps by fashion more than by necessity, they immediately have recourse to a concave glass; the natural consequence of whiclı is, that their eyes in a short time become so fixed in the state requiring its assistance, that the recovery of distant vision is rendered afterwards extremely difficult, if not quite impossible. With regard to the proportion between the number of near sighted persons in the different ranks of society, I have taken pains to obtain satisfactory information, by making inquiry in those places where a large number in these several classes are associated together. I have inquired, for instance, of the surgeons of the three regiments of foot guards, which consist of nearly ten thousand inen; and the result has been, that near sightedness, among the privates, is almost utterly unknown. Not half a dozen men have been discharged, nor half a dozen recruits rejected, on account of this imperfection, in the space of nearly twenty years : and yet many parts of a soldier's duty require him to have a tolerably correct view of distant objects; as of the movements of the fugleman in exercise, and of the buil's eye when shooting at the target; the want of which might furnish a plausible apology for a skulker to skreen himself from duty, or to get his discharge from the service. I pursued my inquiries at the military school at Chelsea, where there are thirteen hundred children, and I found that the complaint of near sightedness had never been made among them until I mentioned it; and there were then only three who experienced the least inconvenience from it. After this, I inquired at several of the colleges in Oxford and Cambridge; and, though there is a great diversity in the number of students who make use of glasses in the various

а

« VorigeDoorgaan »