SECT. XXXV. DONATI'S COMET. 379 



miles from the earth ; during the month of August its nucleus 

 assumed an almost planetary aspect from the concentration of its 

 light ; on the 27th of September the head appeared almost as 

 bright as Mercury, but smaller ; when near its perihelion passage, 

 on September 30th, its diameter, as ascertained by Signore 

 Donati, was 3" ; during the early part of October it continued 

 to increase in brilliancy, the tail becoming more elongated, and 

 describing a beautiful arc in the heavens, occupying a space of 

 nearly 40, or a length of 40,000,000 miles in the solar system. 

 On the evening of the 5th of October it was seen from most parts 

 of Britain, within 20' of Arcturus, the brightest star in the 

 northern heavens, across which the densest part nearly of 

 the tail passed, and through which notwithstanding the star 

 shone with undiminished brilliancy. On the 30th of October, 

 when in perihelio, the comet was only 55,000,000 miles from 

 the sun ; on the 10th it approached nearest to the earth, from 

 which it was then distant 51,000,000 miles ; and on the 15th 

 of the same month near to Venus, being at that time less than 

 one-tenth the distance of the earth from the Sun ; if the comet 

 had reached its perihelion a few days earlier, Venus might have 

 passed through its nucleus, the consequences of which to the 

 planet it would be very difficult to imagine. The motion of 

 Donati's comet is what astronomer's call retrograde, or from east 

 to west. It ceased to be visible in our northern latitudes in 

 the last week in October, having passed into the southern 

 heavens, where it will traverse the constellations of Sagittarius, 

 Telescopium, and Indus, approaching the large star of Toucan ; 

 after which it will disappear until it has nearly completed its 

 revolution round the sun. The observed orbit of this remark- 

 able comet coincides more nearly with an ellipse than a para- 

 bola ; the longer diameter of the ellipse being 184 times that of 

 the earth's orbit, or the immense distance of 35,100,000,000 

 miles a space which, however great, is less than the thousandth 

 of the distance of the nearest fixed star. According to the cal- 

 culations of M. Loewy, and adopting an elliptic orbit, Donati's 

 comet will not return to the same places in the heavens for 

 2495 years, being 500 less than the period of revolution of the 

 great comet of 1811. 



Signore Donati observed that between the 25th and 30th Sep- 

 tember two concentric, luminous, semicircular envelopes, with a 



