176 SCIENTIFIC RECORD FOR 1884. 



and of tlie 28 comets whose periods are under 100 years, only 3 have 

 retrograde motion. 



An interesting table of comets with similar orbits, which are yet not 

 identical, is also given, and further a table of those comets which are 

 related to each other in groups, so that all their orbits intersect in a 

 line. These comets may be supposed to have had a common origin. 



Definitive determination of comet orhits. — For many years Dr. Bruhns 

 kept a general outlook over the matter of the definitive determination 

 of comet orbits, and his annual papers in the V. J. 8. der Astron. Gesell. 

 were of great value in directing attention to the cases of comets whose 

 orbits needed attention, in indicating the sources from which observa- 

 tions could be taken, and by preventing unnecessary duplication of 

 such work through correspondence. Dr. Weiss, director of the Vienna 

 Observatory, has now agreed to fill the same place, and those intending 

 to occupy themselves with this branch of computation will do well to 

 address themselves to him. 



Periodic comets. — Several periodical comets are expected to return to 

 perihelion in 1885. That of Olbers, discovered on the 6th of March, 

 1815, has been calculated to have a period of somewhat more than seventy 

 years, and will, therefore, probably appear again either in 1885 or 188G. 

 Encke's comet was first discovered in 1786, but its periodicity was not 

 detected till 1819, since which time it has been observed at every return, 

 at intervals of about three and a third years. It will once more be in 

 perihelion on the 7th of March, 1885. A comet discovered by Herr 

 Tempel on the 3d of April, 1867, was found to be moving in an elliptic 

 orbit with a period of about six years ; it was observed in 1873 and in 

 1879, and another return is expected to take place in April of the pres- 

 ent year. Another comet of short period was discovered by the same 

 astronomer on the 27th of November, 1869, but its periodicity was not 

 recognized until after it had been rediscovered by Mr. Swift, at Eoches- 

 ter, IST. Y., in 1880, in consequence of which it is usual to call it Swift's 

 comet. The period is about five and a half years, so that another re- 

 turn to perihelion Avill be due about the end of this year; but like that 

 which must have taken place in 1875, it is likely to pass unseen, the 

 comet being unfavorably placed for observation. Tattle's comet occu- 

 pies a position of its own in having a period amounting to about thirteen 

 and a half years. It was first discovered bj' Mechain, at Paris, on the 

 9th of January, 1790, but its periodicity was not detected until after 

 the second discovery by Mr. Tuttle, at Cambridge, United States, on 

 the 4th of January, 1858, when it was found that it must have made 

 four unobserved returns since Mechain's discovery. It was observed 

 again in the autumn of 1871, passing its perihelion at the end of No- 

 vember, and another return will be due in the month of July, 1885. 

 {Athe7iceum.) 



Comet 1867, II. — M. Eaoul Gautier, of Geneva, has been investigating 

 the perturbations produced by the action of the planets Jupiter, Saturn, 



