68 



ASTRONOMICAL PHENOMENA AND PROGRESS. 



planet Jupiter appeared, in telescopes of mod- 

 erate power, unaccompanied, for the space of 

 about one hour and three-quarters, by the sat- 

 ellites usually seen in attendance upon him. 

 This phenomenon has been seldom observed; 

 the only previous observations known having 

 been made November 12, 1681 (O. S.), by Mo- 

 lyneux; May 28, 1802, by Sir William Herschel ; 

 April 15, 1826, by Wallis; September 27, 1843, 

 by Dawes and Grie'sbach. The planet rose at 

 seven and one-half P. M., almost at the same 

 moment that the sun set. At 7 h 44 m , Green- 

 wich mean time, the shadow of the third sat- 

 ellite passed on to the disk, and the satellite 

 itself followed at 8 h 14. At 9 h 10 m , the 

 second satellite disappeared in the shadow of 

 the planet. At 9 h 28 m , the fourth satellite 

 entered in Jupiter's disk. At 9 h 57, the 

 shadow of the first satellite made its appear- 

 ance, followed, in seven minutes, by the entry 

 of the satellite itself on the disk. At that time, 

 10 h 4 m , Jupiter appeared, entirely without 

 satellites, in telescopes of moderate power, but 

 large telescopes exhibited three satellites on his 

 disk with their three shadows. At ll h 23 m , 

 the shadow of the third satellite passed off the 

 disk, and at 11 h 49 the satellite itself. At 

 12 h 13 m , the second satellite reappeared from 

 behind Jupiter. At 12 h 16 m , the shadow of 

 the first satellite passed off the disk, followed 

 by the satellite itself seven minutes later. Last- 

 ly, at 12 h 59 m , the shadow of the fourth sat- 

 ellite, and at 13 h 54 the fourth satellite itself 

 passed off the disk. A circumstance that tends 

 to render the simultaneous disappearance of the 

 four satellites more uncommon than it other- 

 wise would be, is the fact that the fourth satel- 

 lite is not necessarily eclipsed or occulted at 

 each conjunction with Jupiter. It may pass 

 above or below his disk or shadow ; in fact this 

 happens, on an average, in more than one-third 

 of the revolutions of this satellite. Sir J. Her- 

 schel ascribes the fact to the greater inclination 

 of the satellite's orbit ; but Mr. Proctor says 

 that, in fact, the mean value of the inclination 

 of the fourth satellite's orbit is always less tlian 

 that of the others, and that the true reason why 

 the satellite so often escapes eclipse is its supe- 

 rior distance from Jupiter. The distances of the 

 four satellites from Jupiter's centre are as fol- 

 lows: first, 278,542 miles; second, 442,904 

 miles; third, 706,714 miles; fourth, 1,242,619 

 miles. The appearances of the shadows cast by 

 the satellites wei'e very different. The shadow 

 of the fourth appeared larger than that of the 

 third, though ythe third is the larger satellite. 

 This obscnift*/ oil would show that the apparent 

 dimensions ofthe shadows depend rather on the 

 extent of the penumbra than on that of the true 

 shadows. 



The Eccentricity of the Earth's Orbit, and its 

 Physical Relation to the Glacial Epoch. Mr. 

 James Croll, in an article in the Philosophical 

 Magazine for February, further elaborates his 

 ingenious theory upon this subject (see AXXUAL 

 CYCLOPAEDIA for 1866). He now calculates the 



eccentricity, at epochs of 10,000 years apart, to 

 a period 1,100,000 years before the epoch 1800; 

 and presents, in a tabular form, the eccentricity 

 and the longitude of the perihelion ; the number 

 of days by which the winter was longer than 

 the summer; the number of degrees by which 

 the midwinter temperature was lowered, and 

 the midwinter temperature of the centre of 

 Scotland, on the supposition that the Gulf 

 Stream was affected by the change of eccentri- 

 city. The eccentricity and longitude of the peri- 

 helion, at some of the periods, were calculated 

 by Mr. Stone, of the Greenwich Observatory, 

 others by M. Leverrier, and the remainder by 

 the author. According to these tables, the ec- 

 centricity was at its superior limit 850,000 years 

 ago, at which time the winter was 34.70 days 

 longer than the summer, and the midwinter 

 temperature was lowered 45. 3. Mr. Croll 

 assumes that at that time, the northern hemi- 

 sphere being under glaciation, the Gulf Stream 

 was reduced to one-half its present volume. 

 The midwinter temperature of the centre of 

 Scotland, as is well known, is about 28 higher 

 than it would be were it not for the influence 

 of the Gulf Stream. A reduction of the Gulf 

 Stream to one-half its present volume would 

 lower the temperature of Scotland about 14. 

 Add 14 to the 45.3, and we have 59.3, 

 below the present midwinter temperature, 

 or 20.3, as the midwinter temperature of 

 the centre of Scotland 850,000 years ago. 

 Upon this assumption, that the diminution of 

 the Gulf Stream was proportionate to the extent 

 of the eccentricity, Mr. Croll makes his calcu- 

 lations of the actual midwinter temperature of 

 the centre of Scotland, at intervals of 10,000 

 years. Some of the results are as follows : 100,- 

 000 years previous to the epoch 1800, 4". 6 ; 

 200,000 years, 10.2; 700,000 years, 5. 7; 

 750,000 years, 10.6; 920,000 years, 5.7 (the 

 same as 700,000 years) ; 1,100,000 years, 6., 

 which is the highest temperature noted. The 

 author remarks, that the difference between 

 the mid winter temperature of Scotland at some 

 of the periods given, and at the present, is cer- 

 tainly great ; but he thinks that there is noth- 

 ing extravagant in supposing the existence of 

 so low a temperature during the glacial epoch, 

 when we reflect that at the present day there- 

 are places in the same parallel of latitude as 

 Scotland which have a midwinter temperature 

 nearly as low as any indicated in the table ; 

 e. g., at the Cumberland House, in North Amer- 

 ica, situated in a lower latitude than the south 

 of Scotland, the midwinter temperature is more 

 than 13 below zero. That the cold of the 

 glacial epoch in Scotland was to a considerable 

 extent due to a stoppage or at least great dim- 

 inution of the Gulf Stream, he thinks to be at- 

 tested by the fact pointed out by Rev. Mr. 

 Crosskey, from a comparison of the fossils of 

 the glacial beds of Canada with those of the 

 Clyde, that the change of climate in Canada, 

 since the glacial epoch, has been far less com- 

 plete than in Scotland. It might be thought 



