Proceedings of the British Association. 391 



so these other rays were also absorbed, though possibly in a less degree. 

 The apparent complementary relation between the color of chlorine 

 and that of the ray which seemed to have the highest power, was curi- 

 ous, and excited a wish to know if any thing similar occurred in respect 

 of the vapor of iodine, or if this power was confined to the blue end of 

 the spectrum. 



" On the Regular Variations of the Direction and Intensity of the 

 Earth's Magnetic Force" by Prof. Lloyd. The observations (made at 

 the Dublin Magnetical Observatory) were commenced early in the year 

 1839, and have been continued, almost without interruption. Since the 

 beginning of the year 1840, they have been taken every two hours, day 

 and night. The elements directly observed are the declination and the 

 two components (horizontal and vertical) of the intensity, and from the 

 variations of the latter, those of the total intensity and inclination are 

 readily deduced. The variations were projected in curves, which rep- 

 resented the course of the mean daily changes for the entire year, for 

 the summer and winter half years, and for each month separately. 



Declination. — The mean daily curve of the changes of declination 

 for the entire year exhibits a small easterly movement of the north end 

 of the magnet during the morning hours, which reaches its maximum 

 about 7 A. m. After that hour, the north end moves rapidly westward, 

 and reaches its extreme westerly position at \h. 10m. p. m. It then re- 

 turns to the eastward, but less rapidly, the easterly deviation becoming 

 a maximum about 10 p. M. The mean daily range —9*3 minutes. Du- 

 ring the summer months the morning maximum at 7 A. M. is more mark- 

 ed ; the evening maximum, on the contrary, disappears, there being a 

 slow and regular movement of the north end to the eastward from 7 

 p. M. until 7 a. m. In winter, on the other hand, the evening maximum 

 is well defined, and the morning maximum disappears, there being a slow 

 and regular westerly movement until 9 A. M., after which the move- 

 ment becomes more rapid in the same direction. The epoch of the 

 extreme westerly position of the magnet is nearly the same throughout 

 the year. The greatest daily range, in summer, is about 13 - 7 minutes; 

 the least range, in winter, about 7-2 minutes. 



Horizontal intensity. — The mean daily course of the horizontal force, 

 for the entire year, has two maxima and two minima. The first min- 

 imum occurs between 1 a. m. and 3 a. m., which is followed by a max- 

 imum about 5 a. m., or a little after. These fluctuations are small. A 

 second and principal minimum takes place at lO/i. 10m. a. m. ; and a 

 second, or principal maximum, about 6 p. m. The mean daily range 

 r= - 0024 of the whole intensity. In the summer months the smaller 

 maximum and minimum disappear, the intensity decreasing continually 

 throughout the night, but slowly, until 5 or 6 a. m., after which the de- 

 crease becomes rapid. There are, consequently, but one maximum 





