Chase.] J]^2 [May 19. 



the diurnal variations of the inclination at the different observation 

 hours in the four seasons. If we also project, from Gen. Sabine's 

 tables of the mean results, the daily and semi-annual curves at St. 

 Helena and Cape Colony, and compare the curves at the five sta- 

 tions, it will be found that, 



1. The greatest daily disturbance of inclination occurs about noon. 



2. At (magnetically) inter-tropical stations, the dip is diminished, 

 but at extra-tropical stations it is increased in the middle of the day. 



3. Increasing temperature and increasing solar altitude augment 

 the inclination disturbance. This is shown both by the diurnal and 

 the semi-annual 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 sub- 

 tracted from the normal dip; but at Philadelphia, Toronto, and Ho- 

 barton 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— 1 h. 

 ''MAXIMUM" Philadelphia, " 22— Oh. 



" " " Toronto, " 22—23 h. 



" " " Hobarton, " 23— 1 h. 



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

 naturally infer that the ellipticity of the atmosphere would be in- 

 creased 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 Ferrel, in his paper* which furnished the first satis- 

 factory explanation of the barometric depression at the equator 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, I, 215), 

 " the surface of the /laid being sliyhtly depressed at the equator, 

 having its maximum height about the parallel of 35°, and meeting 

 the sur/are of the earth towards the poles." The direct action of 

 the sun, in increasing the equatorial ellipticity of the air, may also 

 increase the tendency to equatorial and polar depression, and the 

 magnetic parallelism may, therefore, be manifested in the solar-diur- 

 nal inclination-disturbance p/Tc;'.sf7y as it is manifested at St. Helena 



* "The motions 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, I, 140, sqq. 



