96 KziNSAS ACADEMY OF SCIENCE. 



describe one made upon September 18 of the present year (1894), which is, T 

 think, of interest for several reasons. At 16 hours 34 minutes, the 18-inch 

 Brashear telescope showed that the first and third satellites were very near 

 together, and that both of them presented elongated discs, the elongations 

 being nearly at right angles to one another. The second and fourth satellites 

 were also slightly elongated, but in different directions. The 12-inch Clark 

 •telescope was then turned upon the planet, and the observations repeated 

 without difficulty. I went next to the 6-inch Clark, and was surprised to find 

 that with a power of 400, not only the elongations of the first and third were 

 easily seen, but even that of the fourth could be detected. I especially noted 

 that the elongations had precisely the same position angles relatively to each 

 other as in the larger instruments." Again, "since the elongations occurred 

 in different directions for the different satellites, the appearance clearly could 

 not be due to atmospheric conditions, or to the eye. Since identically the 

 same appearance was seen in three different telescopes, it could not be instru- 

 mental; and, finally, since the elongation has been seen at different times 

 by at least half a dozen different persons, it cannot be a permanent idiosyn- 

 crasy." 



The elongated forms of the satellites of Jupiter, referred to by Mr. Picker- 

 ing, can be accounted for on the tidal theory; and if such be the fact, in all 

 probability their periods of rotation and of revolution must be coincident, 

 unless the third and fourth satellites are at a distance from Jupiter propor- 

 tionally greater than Venus is from the sun, or our moon is from the earth, 

 the masses of Jupiter and his satellites being taken at their estimated values. 

 The diflaculty that presents itself with regard to the elongations of the first 

 and third satellites "being at right angles to one another" can easily be put 

 out of the way, when we consider the many different relative positions of the 

 satellites and Jupiter as observed from our earth standpoint. The new inner 

 satellite discovered by Mr. Barnard, of Lick observatory, will, of course, pre- 

 sent no difficulty, if my statements with regard to the other satellites be 

 allowed. Although Mr. Proctor says that "there are reasons for believing 

 thaj; the moons of Jupiter are (not all of them, at any rate) in the same con- 

 dition as our moon", and that "changes of luster occur such as cannot be 

 wholly explained by the rotation of these bodies on their axes, either like 

 our own moon once in each circuit around their primary"; yet, "if the age of 

 Jupiter's satellites corresponds to that of our own moon, the retarding tidal 

 action caused by a planet whose mass is 315 times that of the earth must have 

 brought about a synchronism between the rotation and revolution periods 

 long ere this." The elongation of each of the four outer satellites discovered 

 or observed by Mr. Pickering, the proximity of all of them to their primary, 

 and the comparative masses and densities of the members of the Jovian sys- 

 tem, all point unmistakably to the correspondence of the rotation and revolu- 

 tion periods of the satellites respectively. 



In like manner it may be affirmed that the satellites of Saturn, Uranus, and 

 Neptune are controlled. 



The perturbations produced among the satellites of the major planets upon 

 one another are not sufficient to destroy the validity of a general law such as 

 has been outlined in the foregoing. 



We have no reason to discredit the conclusions of Schiaparelli, for they are 

 recognized as the best that have so far been reached. As the force of gravity 

 varies inversely as the squares of the distances, and directly as the product 

 of the masses of the attracting bodies, a glance at the following relations will 

 show the scope of the law we are attempting to unfold: 



