1823.] Royal Society, 6S 



to 7^ 0', and so on to almost any quantity at pleasure. By 

 approaching his opposing magnets nearer to each other and to 

 the needle, the latter might, moreover, be deflected to any point, 

 and by this means the daily variation observed with the needle 

 in all possible positions. In this way the author found the daily 

 variation, with the north end to the south, to the east, west, &c. 

 &c. and it appeared that the daily change was always greatest 

 with the needle east or west, and least (indeed imperceptible) 

 when the needle pointed any where near NNW and SSE. From 

 the NNW to south, the principal daily motion was shown by the 

 north end approaching the north, and between the SSE and N, 

 the north end still approached the north and NNW, and, there- 

 fore, the motion in the two cases was made in a reverse order. 

 Similar experiments were made on the dipping needle, but the 

 results were not so well marked. 



From a comparison of these experiments, Mr. Barlow is 

 inclined to attribute the cause of the daily variation to a change 

 of magnetic intensity in the earth produced by the action of the 

 solar rays, and depending for its amount upon the declination of 

 that body ; and consequently on its situation with reference to 

 the plane of no attraction as described in his Essay on Magnetic 

 Attractions, where he has stated his reasons for assuming that 

 the cause, whatever it may be, that gives direction to the needle, 

 is resident on its surface only. 



A singular anomaly in the diurnal variation under a reduced 

 directive force, was described in the latter part of the paper : a 

 compass-needle which varied, in Mr. Barlow's house (with the 

 north end of the needle to the east or west), to north, varied, in 

 the garden, from east or west to south. Only three suppositions^ 

 could be made as to the cause of this anomaly ; first, that it 

 might arise from the circumstance that the needle was not 

 exactly in the same relative position with respect to the magnets, 

 &c. in the house as in the garden ; secondly, the window of the 

 room where the compass was placed being on the north side, the 

 light might thence affect the needle ; or, lastly, was it possible 

 that a stove in the room could experience a diurnal increase and 

 diminution of magnetic power ? In order to examine the first of 

 these suppositions, Mr. Barlow carefully measured and deter- 

 mined the position of the needle, &c. in the one situation, and 

 gave them precisely the same in the other, but the discrepancy still 

 remained : he then completely darkened the room for two days, 

 and merely examined the compass with a wax taper, but the 

 former effect was only diminished by this means ; the author is 

 of opinion, however, from the result of this experiment, that the 

 light, and not the heat of the sun, will be found the exciting cause 

 of the diurnal variation: in order to examine the third supposi- 

 tion, Mr. Barlow placed a howitzer shell in the garden in the 

 same position with respect to the needle as the stove was in the 



