2 
POPULAR SCIENCE REVIEW. 
out new items of discovery, and to bring to light new orbs from 
abnost fathomless space. The Jovian planet has always been 
the subject of special attention. His system of attendant 
moons, and the singular belts of shading which diversify his 
globe, have been observed again and again; and one of the 
most important facts gleaned from such observations was that of 
the extreme swiftness with which objects were carried across 
his surface, and the resulting inference that the length of his 
day was very much shorter than ours. Cassini had, as early as 
1665, determined the period as 9 h 56 m , and this conformed with 
remarkable precision with determinations of more recent date 
by Maraldi,W. Herschel, Schroter, Madler, and others, who gave 
slightly differing values, averaging 9 h 55J m . The method of 
proceeding in each case was to observe certain dark spots as they 
were successively presented on the same points of the planet’s 
surface, when, by noting the exact intervals which elapsed 
between their reappearances, the period of rotation became 
readily ascertainable. The slight discordances apparent in the 
values independently assigned by the several observers could 
be explained on ordinary grounds. The spots may not in all 
cases have been subject to the same conditions. Some of them 
may have exhibited proper motion, which must have clearly 
originated such discrepancies. 
In the summer of 1878 a dark elliptical spot of considerable 
dimensions and very striking character made its appearance on 
the surface of Jupiter. It was situated just south of the great 
equatorial belt of the planet, and it still remains visible, forming 
a most frequent and interesting object for telescopic examina- 
tion. It displays the same intensely red colour as the adjoining 
dark belts, and appears likely, from the constancy of form and 
distinctness of outline which it invariably exhibits, to remain 
perceptible for a lengthened period. No one can possibly foresee 
the date of its final disruption and disappearance, because we 
are ignorant of the conditions under which it exists, and of the 
influences to which it is subject. Of its origin we also know 
little or nothing. It was already a conspicuous object, display- 
ing an intense rosy hue, when first discovered by Prof. Pritchett 
at Missouri on July 9, 1878. In fact, the spot does not appear 
to have been traced in the early stages of its formation, so that 
it cannot be said the process was gradual, and that the object, 
first becoming visible as a minute speck, went on enlarging and 
intensifying until it reached its present striking proportions. The 
spot may quite possibly have resulted from a sudden convulsion 
in the Jovian atmosphere, by which a large area was sundered, 
and the planet’s dark surface in part revealed. In any case, 
the seemingly permanent character of the spot and its well-marked 
outline are its most salient features ; and should indications 
