The Planets of the Month. 231 



will form a most interesting study, especially should a season of 

 activity be coming on in that frequently-disturbed atmosphere. 

 The transits of the satellites and their shadows across the 

 planet always afford a beautiful spectacle, if the air is favour- 

 able, especially in a telescope which has power enough to show 

 the disc of the entering or emerging satellite distinctly, and 

 the circular form of the ink-spot which marks the place of its 

 shadow. Sometimes more than one satellite, or more than one 

 shadow, may be seen at the same moment, on the face of that 

 noble planet ; at others, the satellite passes away into the clear 

 blue sky, and leaves its shadow behind it, or the shadow breaks 

 in upon the planet's limb, to announce the distant approach of 

 the little globe that casts it. But these are not merely beau- 

 tiful phEenomena — they are attended with circumstances of 

 mystery which yet remain to be investigated. Excepting 

 towards the limbs, where the light of the planet is less vivid, 

 or in front of a dark belt, a satellite is usually invisible upon 

 the face of its primary, from want of contrast ; but instances 

 from time to time occur in which the same satellite — usually 

 III. or IV. — assumes so dark an aspect in that situation, as to 

 be barely distinguishable from its shadow. There can be no 

 question that this arises from spots upon the disc of the satel- 

 lite, which the magnificent telescopes of Lassell, Secchi, and 

 Dawes, have occasionally rendered perceptible in III., even on 

 the background of the blue sky; but as these dark transits 

 are of irregular recurrence, we are obliged to suppose either 

 that these little moons have atmospheres subject to extraordi- 

 nary variation, or that they have rotations upon their axes much 

 more rapid than their revolutions round their primary — each 

 alternative inferring a constitution totally dissimilar to that of 

 our own satellite. These dark transits were noticed from a 

 very early period — I believe 1666 ; Maraldi saw IV. pass as a 

 black spot in 1707; Schroter probably observed them under 

 the mistaken idea that they were spots on the disc, in 1785 

 and 1 786 ; he and Harding recognized their true nature in 

 1796; about this time Sir W. Herschel paid much attention to 

 these objects, and suspected that their forms might not be 

 spherical (which has since been actually shown in the case of 

 III., by the telescopes of Lassell and Secchi) ; but, strange to 

 say, he never alludes to these darkened transits — an omission 

 which, on the part of so accurate an observer, seems to imply 

 that none of them had taken place under his notice. Beer and 

 Madler too have made no mention of them, though it can 

 hardly be supposed that they were ignorant of former observa- 

 tions. But of late years they havo attracted much attention, 

 and the following description by the American astronomer 

 Bond will be the more welcome, as few of our readers are likely 



