188G.] NEW YOKK ACADEMY OF SCIENCES. 251 



them belong to any of tlie late discoveries. One point is note- 

 worthy, that the more recently discovered bodies are much 

 smaller than the earlier ones. The first 25, discovered between 

 May, 1876, and October, 1878, have an average opposition magni- 

 tude of 11.2, while the last 25, discovered since April, 1883, 

 average only 12.2 ; i. e., the first 25 average about 2^ times as 

 bright as the last. Out of the whole 95, two are of the 9th magni- 

 tude (one of them. No. 234, was discovered as recently as August, 

 1883), 14 are of the 10th, 33 of the 11th, 33 of the 12th, and 13 

 of the 13th. Of these last 13, 10 have been found within the 

 past two years ; and of the 12 others found in the same time, 6 

 are of the 11th magnitude, and 6 of the 12th. 



It is clear that there can remain very few to be discovered 

 as large as the 10th magnitude ; but there may be an indefinite 

 number of the smaller sizes. 



The Major Planets. 



As regards the planet Jupiter, the one interesting feature for 

 the past ten years has been "the great red spot." This is an 

 oval spot, some 30,000 miles in length by 6,000 or 7,000 in width, 

 which first attracted attention in 1878. At first, and for three 

 jears, it was very conspicuous, but in 1882 it became rather 

 faint, though still remaining otherwise pretty much unchanged. 

 In 1885 it was partly covered with a central whitish cloud, which 

 threatened to obscure it entirely; but this season the veiling 

 oloud has diminished, and the marking is again as plain as it was 

 in 1882 or 1883, How long it Avill continue no one can say ; nor 

 is there any general and authoritative agreement among astro- 

 nomers as to its nature and cause. 



In connection Avith observations upon this object, several new 

 determinations have been made of the planet's rotation period, 

 and they all show that, as in the case of the sun, the equatorial 

 markings complete the circuit more rapidly than those in higher 

 latitudes ; a white spot near the equator gives 9 h. 50 m. 06 s., as 

 against 9 55 36, for the red spot, which is approximately in lati- 

 tude 30°. 



We must not omit to mention Professor Pickering's new photo- 

 metric method of observing the eclipses of this planet's satellites. 

 Instead of contenting himself with observing merely the mo- 

 ments of their disappearance and reappearance — an observation 

 not susceptible of much accuracy — he makes a series of rapid 

 comparisons between the brightness of the waning or waxing 

 point of light during the two or three minutes of its change, 

 using as the standard one of the neighboring uneclipsed satel- 

 lites. From these comparisons he determines the moment when 

 the satellite under eclipse has just half its normal brightness; 



