It was computed by M. Faye that the _volume_ of the famous Donati's comet (1858) was about 500 times that of the sun! On the other hand, he calculated that its _mass_ (or quantity of matter it contained) was only a fraction of the earth's mass. This shows how almost inconceivably tenuous the material forming the comet must have been--much more rarefied, indeed, than the most perfect vacuum which can be produced in an air-pump. This tenuity is shown by the fact that stars were seen through the tail "as if the tail did not exist." A mist of a few hundred yards in thickness is sufficient to hide the stars from our view, while a thickness of thousands of miles of cometary matter does not suffice even to dim their brilliancy! At the time of the appearance of the great comet of 1843, it was doubtful whether the comet had transited the sun's disc. But it is now known, from careful calculations by Prof. Hubbard, that a transit really took place between 11{h} 28{m} and 12{h} 29{m} on February 27, 1843, and might have been observed in the southern hemisphere. The distance of this remarkable comet from the sun at its perihelion passage was less than that of any known comet. A little before 10 p.m. on February 27, the comet passed within 81,500 miles of the sun's surface with the enormous velocity of 348 miles a second! It remained less than 2¼ hours north of the ecliptic, passing from the ascending to the descending node of its orbit in 2{h} 13{m}·4.[191] The great comet of 1882 transited the sun's disc on Sunday, September 17, of that year, the ingress taking place at 4{h} 50{m} 58{s}, Cape mean time. When on the sun the comet was absolutely invisible, showing that there was nothing solid about it. It was visible near the sun with the naked eye a little before the transit took place.[192] This great comet was found by several computors to have been travelling in an elliptic orbit with a period of about eight centuries. Morrison found 712 years; Frisby, 794; Fabritius, 823; and Kreutz, 843 years.[193] The great southern comet of 1887 may be described as a comet without a head! The popular idea of a comet is a star with a tail. But in this case there was no head visible--to the naked eye at least. Dr. Thome of the Cordoba Observatory--its discoverer--describes it as "a beautiful object--a narrow, straight, sharply defined, graceful tail, over 40° long, shining with a soft starry light against a dark sky, beginning apparently without a head, and gradually widening and fading as it extended upwards."[194] The great southern comet of 1901 had five tails on May 6 of that year. Two were fairly bright, and the remaining three rather faint. Mr. Gale saw a number of faint stars through the tails. The light of these seem to have been "undimmed." Mr. Cobham noticed that the stars Rigel and β Eridani shone through one of the faint tails, and "showed no perceptible difference."[195]