maximum of the one belt coinciding with the minimum of the other. [Illustration: PLATE XXII. Jupiter, January 6, 1906--8 hours 20 minutes. Instrument, 9-1/4-inch Reflector. [lambda] = 238° (System 1); [lambda] = 55° (System 2). Rev. T. E. R. Phillips.] These changes in colouring bring us to the fact that the whole system of the Jovian markings is liable to constant and often very rapid change. Anyone who compares drawings made a few years ago with those made at the present time, such as Plates XXII. and XXIII., cannot fail to notice that while there is a general similarity, the details have changed so much that there is scarcely one individual feature which has not undergone some modification. Indeed, this process of change is sometimes so rapid that it can be actually watched in its occurrence. Thus Mr. Denning remarks that 'on October 17, 1880, two dark spots, separated by 20° of longitude, broke out on a belt some 25° north of the equator. Other spots quickly formed on each side of the pair alluded to, and distributed themselves along the belt, so that by December 30 they covered three-fourths of its entire circumference.' The dark belts, according to his observations, 'appear to be sustained in certain cases by eruptions of dark matter, which gradually spread out into streams.' Even the great equatorial belts are not exempt from the continual flux which affects all the markings of the great planet, and the details of their structure will be found to vary to a considerable extent at different periods. At present the southern belt is by far the most conspicuous feature of the surface, over-powering all other details by its prominence, while its northern rival has shrunk in visibility to a mere shadow of what it appears in drawings made in the seventies. Through all the changes of the last thirty years, however, one very remarkable feature of the planet has remained permanent at least in form, though varying much in visibility. With the exception of the canals of Mars, no feature of any of the planets has excited so much interest as the great red spot on Jupiter. The history of this extraordinary phenomenon as a subject of general study begins in 1878, though records exist as far back as 1869 of a feature which almost certainly was the same, and it has been suggested that it was observed by Cassini two centuries ago. In 1878 it began to attract general attention, which it well deserved. In appearance it was an enormous ellipse of a full brick-red colour, measuring some 30,000 miles in length by 7,000 in breadth, and lying immediately south of the south equatorial belt. With this belt it appears to have some mysterious connection. It is not actually joined to it, but seems, as Miss Clerke observes, to be 'jammed down upon it'; at least, in the south equatorial belt, just below where the spot lies, there has been formed an enormous bay, bounded on the following side (_i.e._, the right