84 



SCIENCE. 



[N. S. Vol. XVII. No. 420. 



irradiation, but they will probably better 

 satisfy micrometer work. 



The observations for longitude, latitude 

 and magnitude of objects on the planet 

 Jupiter have all been made with the par- 

 allel-wire micrometer, preferably near the 

 central meridian, but no rigid rule is fol- 

 lowed in this respect. The longitude and 

 latitude are usually determined whenever 

 the spot or marking is wholly on the disk 

 and distinctly visible. 



The longitudes are measured by ascer- 

 taining the distance of the apparent cen- 

 ter of the object from the limb of the 

 planet, according to the method I pointed 

 out some years ago. A determination of 

 longitude or latitude generally consists of 

 three bisections of the object and each 

 limb of the planet. In the case of longi- 

 tude, one half of the difference of the 

 distances at the mean of the times is the 

 distance of the apparent center of the 

 object from the central meridian on the 

 visible disk. This method of determin- 

 ing longitudes has been found to be 

 greatly superior, in point of accuracy, to 

 the method of transits, as well as a great 

 saving of time. 



The error in measurement of objects on 

 a luminous disk is about twice as great as 

 that from the measurement of double stars 

 of equal distance. The ordinary error for 

 location of objects in latitude or longitude 

 on the disk of Jupiter may be placed at 

 about 0.25" arc. 



Twenty-five years ago it was almost the 

 general opinion among astronomers that 

 the phenomena seen on the planet Jupiter 

 were transitory in their nature; that there 

 was no permanency in the spots and mark- 

 ings, but that the aspect of the planet 

 changed from day to day, and even at 

 less intervals of time. Perhaps we shall 

 get a better idea of what was known about 

 the subject by quoting from Grant's 'His- 

 tory of Physical Astronomy': 



"Although generally there appear only 

 three belts upon the disk of the planet, 

 sometimes a greater variety is perceptible. 

 Sometimes only one belt is visible. This 

 is always the principal belt situated on 

 the northern side of the planet's equator. 

 On the other hand, its whole surface has 

 occasionally been seen covered with belts. 

 On the 18th of January, 1790, Sir William 

 Herschel, having observed the planet with 

 his forty-foot reflector, perceived two very 

 dark belts dividing an equatorial zone of a 

 yellowish color, and on each side of them 

 were dark and bright bands alternating and 

 continuous almost to the poles. A similar 

 appearance was once noticed by Messier. 

 These phenomena sometimes undergo very 

 rapid transformations, affording thereby 

 a strong proof that they owe their origin to 

 the fluctuating movements of an elastic 

 fluid enveloping the body of the planet. 

 On the 13th of December, 1690, Cassini per- 

 ceived five belts on the planet, two in the 

 northern hemisphere and three in the 

 southern hemisphere. An hour afterwards 

 there appeared only two belts nearer the 

 center and a feeble trace of the northern 

 belt. The same astronomer frequently wit- 

 nessed the formation of new belts on the 

 planet in the course of one or two hours. 

 The dark spots on the disk of the planet 

 also afforded unequivocal indications of the 

 existence of an atmosphere, for it is im- 

 possible to reconcile their variable velocity 

 with the supposition of their being perma- 

 nent spots adhering to the surface of the 

 planet. Cassini found from his observa- 

 tions that the spots near the equator of the 

 planet revolved with greater velocity than 

 those more distant from it. Sir "William 

 Herschel found that the velocity sometimes 

 underwent a sensible change in the course 

 of a few days. He supposed the spots to be 

 large congeries of cloud suspended in the 

 atmosphere of the planet, and he ascribes 



