314 ANNUAL OF SCIENTIFIC DISCOVERY. 



quantity of it in the immediate vicinity of any one place of obse. 

 tion to render its presence manifest. Distance alone, by bringing its 

 particles into more apparent concentration, could give it density 

 enough to become perceptible, just as the same cause converts the 

 unsubstantial and semi-transparent mist into the massive and ponder- 

 ous-looking cloud. It was a more significant fact, and one which may 

 not be generally known, that no electric or magnetic effect whatever 

 was perceptible during its passage ; for such influences have been 

 strongly suspected in cometary phenomena, and might act inde- 

 pendently of any material admixture. 



OBSERVATIONS ON MARS AND THE MOON. 



In 1858, M. Secchi found the appearance of Mars differ consid- 

 erably from the drawings of Maedler, and other astronomers. Now 

 the planet has returned to its former aspect, and instead of exhib- 

 iting large, complicated solar spots, showed them to be reduced to 

 a small circle, as in Maedler. The great spots had given place to 

 rose-colored surfaces, traversed by blue canals, as represented in 

 Secchi's picture of 18.38. From these changes he thinks no doubt 

 can remain that the polar spots consist of snow or condensed clouds, 

 which the summer heat of the planet melts. The red surface he re- 

 gards as land, and the blue canals as water. Comptes Rendus. 



Observations on the Moon. Mr. Lassell, the well-known English 

 astronomer, who has recently established a magnificent four-foot re- 

 flecting telescope at Malta, states, in a letter to the President of the 

 Royal Society, that he observes the details of the moon with a sharp- 

 ness and distinctness which he had never seen before ; and says that 

 if a carpet the size of Lincoln's Inn Fields were laid upon its sur- 

 face, he could tell whether it was round or square. He adds, " I see 

 nothing more than a repetition of the same volcanic texture, the 

 same cold, crude, silent, and desolate character which smaller tele- 

 scopes usually exhibit." 



NEW THEORY OF COMETS. 



In an article contributed to Sillima?i's Journal, Mr. Benjamin Marsh 

 attributes the peculiar character of cometary matter to the extreme 

 and violent changes which it undergoes in its rotation round the sun. 

 llalley's comet, for example, at one time approaches the sun to 

 within fifty-six millions of miles, and then recedes to the enormous 

 distance of thirty-three hundred and seventy millions of miles. At 

 the time of its perihelion, or least distance, it passes through one 

 heliocentric degree of its orbit in 15.7 hours, and receives in a given 

 time three thousand six hundred times as much heat as when it 

 reaches its aphelion, or greatest distance, in which position its motion 

 is so slow that six and one-half years are required for its passage 

 through one heliocentric degree. Thus it will be seen that comets 

 with eccentric orbits are subject to violent changes of temperature 

 and velocity which do not affect planets whose orbits approximate 

 more closely to the circular form, and from this circumstance a 

 greater electrical disturbance and excitation may possibly be pro- 

 duced. Mr. Marsh gives a table of remarkable comets, showing that 

 those whi^h have exhibited the greatest splendor have been distin- 



