129 



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. 



An Account of the Measurement of an Arc of the Meridian, extending 

 from Dunnose, in the Isle of Wight, Latitude 50 37' 8", to Clif- 

 ton, in Yorkshire, Latitude 53 27' 31", in course of the Operations 

 carried on for the Trigonometrical Survey of England, in the Years 

 1800, 1801, and 1802. By Major William Mudge, of the Royal 

 Artillery, F.R.S. Read June 23, 1803. [Phil. Trans. 1803, 

 p. 383;] 



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 arc in the island. He fixed on Dun- 

 nose, in latitude 50 37' 8", for the southern extremity, and on 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. Its merits consist chiefly in 

 the means of uniting the sectorial 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 contrivance 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 auxi- 

 liary requisites, the author proceeds to give us, in different tables, 



