63 



anything." It is now more than two centuries and a half 

 since Galileo discovered the solar-spots, and astronomers and 

 physicists have speculated, and still speculate much to explain 

 the phenomenon. 



If the body of the sun be observed with a powerful telescope, 

 the eye being properly protected, the enlarged image of the 

 disc will usually appear more or less sprinkled with sun-spots 

 — moveable belongings to the surface of the sun — which spots 

 greatly help the study of the physical constitution of our 

 luminary. The number of these spots follows a certain 

 periodicity, which helps to establish a most interesting cor- 

 relation between sun-spots and terrestrial magnetism. 



When the sun-spots are observed with care during several 

 consecutive days, they are found to vary both in form and 

 position, amidst which variations there is one common and 

 progressive movement in the same direction, from which has 

 been deduced the rotation of the solar globe round an axis 

 passing through its centre. This movement takes place from 

 west to east, and fourteen days is the time during which 

 a spot remains visible, and fourteen days is also the period 

 which elapses between the diasppearance of a spot at the 

 western border, and its re-appearance on the eastern. A point 

 situated on the solar equator travels with a velocity of 4,560 

 miles an hour, or about 1| miles per second. 



Sun-spots consist generally of one or more dark portions 

 called umlrcB, which appear black in comparison with the 

 luminous parts of the sun's disc ; around this dark portion 

 is a grey tint, furrowed with dark striae, this forms the 

 penumbra. The spots are frequently composed of several 

 umbra, inclosed in one penumbra, and sometimes the penum- 

 bra is not provided with umbrae. The penumbra frequently 

 reproduces the principal contours of the umbra, and often 

 presents a great variety of shades when examined with high 

 powers. 



The dimensions of the spots are variable, and are sometimes 

 enormous ; Schroter measured one which he found equivalent 

 to sixteen times the surface embraced by a great circle of the 

 earth, equal to four times the entire superficies of our glob(5, 

 or more than 29,000 miles. Sir William Herschel, in 1779, 

 measured a spot consisting of two parts, the diameter of 

 which was not less than 50,000 miles. In August, 1839, 

 Capt. Davies measured a sun-spot 186,000 miles in its great- 

 est length. 



According to Sir W. Herschel the surface of the sun is made 

 up of " corrugations." Mr. Nasmyth describes the surface as 

 having the appearance of " willow-leaves." Mr, Daws asserts 

 that these interstices differ greatly in form and size — some- 



