Page:Proceedings of the Royal Society of London Vol 1.djvu/149

Rh many years back; whence it may be inferred that the planes of their orbits are really or nearly at right angles with the visual ray.

Dr. Herschel being aware that the observations he brings forward in this paper are of a nature so delicate and minute as to afford op- portunities for cavil, has been at considerable pains to'point out the principal circumstances that contribute to the perfection of telescopes and micrometers, and the precautions to be used as to the state of the atmosphere. Those ought, he thinks, to be particularly attended to by accurate observers.

In this paper we are presented with a further continuation of the several accounts given, ever since the year 1785, of the trigonometri- cal surveys carried on over various parts of the kingdom. Having now proceeded a great way in these surveys, Major Mudge thought it high time to attempt the measurement of a considerable arc of the meridian in our latitudes. He ﬁrst assigns his reasons for preferring the meridian he has here adopted, which depend chieﬂy on the nature of the country being less hilly and liable to less obstruction than any other tract of the length of this are in the island. He ﬁxed on Dun- nose, in latitude 50° 37’ 8", for the southern extremity, and (in Clif- ton, a small village in the vicinity of Doncaster, latitude 53° 27' 31", for the northern termination of this arc: and near the latter place he found a convenient plain, viz. Misterton Carr, for the measure— ment of a base of veriﬁcation.

As the accuracy of the zenith observations would be most es- sential towards the perfection of this measurement, a new zenith sector was constructed for the purpose by Mr. Ramsden, and ﬁnished by Mr. Berge, the excellence of which is here attested by its being called the ﬁrst instrument of its kind. lts merits .consist chieﬂy in the means of uniting the sector-ial tube to its axis, so as to insure the permanency of the length of its radius when erected for observation; in a more accurate method of adjusting the instrument vertically; an easy way of placing the face of its arch in the plane of the meridian; and a contrivauce by which the plumb-line can be brought precisely over the point marking the centre of the circle, of which the divided arch of the sector should be a part.

Having given a minute description of this instrument, and of the preparatory operations for the series of observations to be made with it, such as the construction of a proper observatory, and other auxiliary requisites, the author proceeds to give us, in different tables,