362 Proceed inc/s of the British Association for 1850. 



Tuesday^ 6th August. 



Section A. — Mathematical and Physical Science. 



Professor Phillips made a communication on Isoclinal lines 

 in Yorkshire, a contribution to our knowledge of terrestrial 

 magnetism on a point not much investigated, — a point, though 

 small perhaps, yet certainly interesting. There are certain 

 lines, as is well known, on magnetic maps called Isoclinal 

 curves, corresponding to the different angles shown by the 

 needle when freely suspended in different places. The inves- 

 tigations of Professor Phillips related to those lines within a 

 limited track, where the inclination of the needle is the same. 

 The general isoclinal lines have been ascertained, but these 

 are only the mean — the resulting lines, and observations in 

 individual places may, and often do, vary not a little from 

 these, and Professor Phillips wished to find the variation of 

 the lines with reference to the surface of the country. He 

 exhibited a sketch of part of Yorkshire, having elevations to 

 the height of 600 and 400 feet on the east, and rising as high 

 as 2600 feet to the west. It was a portion of country free 

 from trap rocks, and therefore from one great source of irre- 

 gular action of the needle. He had made investigations on 

 the west part sixteen years ago with a dipping needle made by 

 himself, but the Association had thought that the instrument 

 might have been inaccurate in its indications. He had since 

 obtained one of Mr Charles Robinson's dipping needles, whose 

 instruments of this kind he thought the best. In his new 

 investigations, he had taken four parallel lines of observation 

 north and south, and observed at different latitudes, and he 

 had every reason to conclude that his observations were cor- 

 rect. With a good dipping needle, all that is required for 

 accuracy is due time and care, and his instrument he found 

 could generally be depended to within a minute of a degree. 

 His method of observation was to turn the needle in azimuth 

 all round the circle, 30°, 60°, 90°, &c., and read off the dip 

 each time, and do the same with two other needles. Two 

 hours were required for a good observation. He had observed 

 the magnetic dip all the hours of the day, particularly in 



