708 PHILOSOPHICAL TRANSACTIONS. [aNNO I672. 



the observation ; who, being come to the Royal Observatory, began to see at 

 4 minutes after 8 o'clock the spot already somewhat removed from the oriental 

 limb, but yet obscure and small. At 47 minutes after 8 o'clock, they saw it very 

 distinctly advancing towards the middle of the belt. From 5 minutes and 40 

 seconds after 9 o'clock until 8 minutes after Q o'clock, they saw it in the midst 

 of the belt. At 15 minutes after Q o'clock it had passed the middle, and was 

 come nearer to the occidental limb. And a little after, the Heavens being over- 

 cast, he could then observe it no further. 



This observation being taken for the epocha, it is easy to find hereafter the 

 times when this spot shall return to the midst of the belt. For you are only to 

 add always Q hours and 56 minutes ; and, for greater exactness, not to omit the 

 ordinary equation of days, that depends on the inequality of the motion of 

 the sun in respect of the equinoctial, nor the particular equation that depends 

 on the inequality of the motion of Jupiter, according to the diversity of the 

 distance of the sun and his apogee. 



This revolution being the swiftest and the most regular that is hitherto known 

 in the Heavens, a traveller alone, even without having any correspondence with 

 other observers, may make use of it to find the longitudes of the most remote 

 places of the earth. We shall hereafter examine to what precision we may arrive 

 by this way. 



Observations of a New Comet, made at Paris in the Royal Observatory, 

 by SiGNon Cjssini. N" 82, p. 4042. 



The mathematicians of la Fleche perceived the comet from March l6, 

 N. S. and gave notice of it at Paris. Those of the college of Clermont saw it 

 March the 25th. 



March id, at half an hour after seven, in the evening, S. Cassini saw it be- 

 tween the head of Medusa and the Pleiades. Without a telescope he appeared 

 as a star of the third magnitude. Its head, seen with a telescope of 17 feet, ap- 

 peared almost round, but well defined, and distinguished from the mistiness 

 which formed a kind of chevelure, or bush of hair, with which it was encom- 

 passed ; and even the middle was a little confused, and seemed to have inequa- 

 lities as are seen in clouds. The tail, which is principally that which distin- 

 guishes comets from stars, was almost imperceptible; yet by the telescope it was 

 seen turned opposite to the sun, and it appeared of the length of two diameters 

 of the head. The whole comet, head, tail, and chevelure, taken together, took 

 up no more than three or four minutes of a degree. 



At 48 minutes after seven it was in a straight line with the lucida in the head 

 of Medusa, and with the most western star of the Pleiades, and above the two 

 clearest stars of the southern foot of Perseus ; so that a straight line drawn 



