NOTES. xxxvii 



middle latitudes of the southern hemisphere, is seen in the graphical representa- 

 tion, with all its striking distinctness, when we compare severally the form and 

 inflexions of the curves of horary variation in the parts of the day from 14 h to 22 h , 

 from 22 h to 4 h , and from 4 h to 14 h . Each inflexion above the line of mean declina- 

 tion corresponds to an almost equal or similar one below it (vol. i. PI. IV. curves 

 AA and BB). Even during the night the opposition is sensible, and it is a very 

 interesting remark that when at St. Helena and the Cape of Good Hope the type 

 is that of the northern hemisphere, there is seen at these very southern places 

 the same anticipation of the turning hours as that which takes place in the same 

 months at Toronto in Canada. Sabine, Observations at Hobarton, vol. i. p. xxxvi. 

 ( I6a ) p. 135. Phil. Trans, for 1847, Pt. I. p. 52 and 57, and Sabine, Observa- 

 tions made at the Magn. and Met. Observatory at the Cape of Good Hope, 1841 

 1846, vol. i. p. xii xxiii. PI. III. (Compare also Faraday's ingenious views on 

 the causes of such phenomena in their dependence on the seasons of the year, in 

 his " Experiments on Atmospheric Magnetism," 3027 3068, and on the ana- 

 logies with St. Petersburg, 3017.) Near the southern shores of the Red Sea, 

 a very diligent observer, M. d'Abbadie, is believed to have observed the same re- 

 markable change of type in the march of the declination in the opposite parts of 

 the year as that found at the Cape, St. Helena, and Singapore. (Airy, On the 

 present State of the Science of Terrestrial Magnetism, 1850, p. 2.) Sabine re- 

 marks, in the Proceedings of the Royal Society, 1849, p. 821 : " It results, from 

 the present position of the four points of maximum force at the surface of the 

 Earth, that the intermediate line of least force departs considerably in the 

 Southern Atlantic from the middle or geographically equatorial portion of the 

 globe, passing between the Cape and St. Helena, and consequently not far from 

 either of those stations. The latitude of the Cape is 33 56' S. It is conse- 

 quently not situated within the tropics, and the sun is throughout the year well 

 to the north of its zenith, and therefore, according to M. de la Rive's thermal 

 theory (Ann. de Chim. et de Phys. t. xxv. 1849, p. 310), the deviation should 

 be in one and the same direction throughout the year. But the fact is not so, for 

 the same contrariety in the direction of the diurnal variation takes place at the 

 Cape as at St. Helena ; the two portions of the year at which the opposite phae- 

 nomena prevail are also identical at the two stations; and at both the change in 

 the direction of the deviation takes place about the time when the sun crosses 

 the equator ; the deviation being to the west at both stations when the sun is 

 north of the equator, and to the east when south of the equator." 



( 167 ) p. 135. Halley, "Account of the late surprising Appearance of Lights in 

 the Air," in the Phil. Trans, vol. xxix. 1714 1716, No. 347, p. 422 428. 

 Unfortunately, Halley's explanation of the Northern Light is connected with the 



