Mr. P. E. Chase on Gravity and Magnetic Inclination. 331 



3. Increasing temperature and increasing solar altitude aug- 

 ment the inclination- disturbance. This is shown both by the 

 diurnal and the semidiurnal curves. 



4. As a corollary of propositions 2 and 3, at St. Helena and 

 Cape of Good Hope the inclination- disturbance is opposed to, 

 and subtracted from the normal dip ; but at Philadelphia, 

 Toronto, and Hobarton the disturbance is added to the dip. 

 Thus the inclination is — 



A minimum at St. Helena ... at 22-23 h . 



„ Cape of Good Hope „ 0- l h . 



A maximum at Philadelphia . . „ 22- h . 



„ Toronto . . . . „ 22-23 h . 



„ Hobarton . . . . „ 23- l\ 



Reasoning either a priori, or from Secchi's postulates, we 

 might naturally infer that the ellipticity of the atmosphere 

 would be increased by the direct action of the sun, and that 

 consequently, from the tendency of magnetic parallelism to the 

 gravitation-currents, the dip would be augmented at noon in all 

 places between the magnetic equator and the magnetic poles. 



Mr. William Perrel, in his paper* which furnished the first 

 satisfactory explanation of the barometric depression at the equa- 

 tor and at the poles, shows that, in consequence of the earth's 

 motion, fluids tend to assume a form similar to his fig. 1 (Math. 

 Monthly, vol. i. p. 215), "the surface of the fluid being slightly 

 depressed at the equator, having its maximum height about the 

 parallel of 35°, and meeting the surface of the earth towards the 

 poles" The direct action of the sun, in increasing the equato- 

 rial ellipticity of the air, may also increase the tendency to equa- 

 torial and polar depression ; and the magnetic parallelism may 

 therefore be manifested in the solar-diurnal inclination-disturb- 

 ance precisely as it is manifested at St. Helena and Cape of Good 

 Hope, by a diminution of dip between the parallels of 35°, and 

 an increase in higher latitudes. 



These coincidences may well suggest the need of more exten- 

 sive observations in different parts of South America, Northern 

 Africa, Southern Asia, the Pacific Ocean, and the Frigid Zones, 

 to furnish the data for determining to what extent mountain- 

 ranges, coast-lines, land and water radiation, winds, and ocean- 

 currents modify the theoretical phenomena of dip and declination. 



By projecting on isoclinal and isogonic charts f the magnetic 

 currents as indicated by the position of the needle in different 



* " The Motion of Fluids and Solids relative to the Earth's Surface." 

 See 'Nashville Journal of Medicine and Surgery ' for 1856, and 'Mathe- 

 matical Monthly' for 1859, vol. i. p. 140 et seq. 



t The charts that I used were the polyconic projection of the " Lines of 

 equal Magnetic Variation for the year 1858," which was constructed from 



