Dr. Olbers on the approaching Return of Hallei/s Comet, 45 



and the elements of the water can then be separated by a 

 current of lower intensity. 



914. Advantage may be taken of this principle to inter- 

 polate more minute degrees into the scale of initial intensities 

 already referred to (909. 911.) than is there supposed; for by 

 combining the force of a current constant in its intensity, with 

 the use of electrodes consisting of matter having more or less 

 affinity for the elements evolved from the decomposing elec- 

 trolyte, various intermediate degrees may be obtained. 

 [To be continued.] 



VI. On the approaching Return ofHalleys Comet, By Dr. 

 Olbers. Translated from Schumacher's Astronomische 

 Nachrichten, No. 268, for the Royal Astronomical Society^ 

 by T. Galloway, Esq., and read at the Meeting of the So- 

 ciety, on the I2th of December 1834. 

 T^HE precise day on which Halley's comet will make its 

 ■*■ nearest approach to the sun in the year 1835, cannot be 

 previously determined with certainty, although by the labours 

 of some of the most distinguished astronomers and geometers 

 this point of time has been determined within pretty narrow 

 limits. The complicated, tedious, and wearisome calculations 

 required for computing the perturbations do not give the same 

 results to different computers. Damoiseau found the time of 

 passing the perihelion to be November 4*32 days; Pontecou- 

 lant, November 7*2. The computations of Professor Rosen- 

 berger, which have been made with the utmost possible care, 

 exactness, and accuracy, are not yet completed; but from such 

 parts of them as have hitherto been made public, it appears 

 that the comet will not pass its perihelion before the 11th of 

 November at the soonest, even taking into account the resis- 

 tance of the aether, which, according to his computation, may 

 cause an acceleration of about four days*. But the effect of 

 the resistance of the aether upon Halley's comet cannot be 

 computed. From its effect on Encke's comet, which is known 

 by experience, no conclusion whatever can be formed of its 

 effect on the comet of Halley. 1st. The effect of the resisting 

 medium is necessarily a function of the volume and of the 

 mass of the comet; but the two comets (Encke's and Halley's) 

 differ greatly in respect both of mass and volume, though in 

 what proportions they differ we know not 2nd. The arbitrary 

 hypothesis of Newton, that the density of the resisting medium, 

 or of the aetherin the regions of space, diminishes as the square 



[* Prof. Rosenberger's determination of the elements of Halley 's comet at 

 its last appearance (1759), will be found in Phil. Mag. and Annals, N.S., 

 vol. xi. p. 32.— Edit.] 



