Mars during the Late Opposition. 85 



that of the earth, as would probably be the case from its 

 having nearly the same axial velocity, we could not hope to 

 perceive it, for, under such circumstances, the flattening of 

 the disk of Mars would not exceed twelve miles, and this 

 when the planet is nearest to us, would subtend an angle of 

 only one-tenth of a second. 



Professor Phillips's drawings of Mars, of which three are en- 

 graved in the " Proceedings of the Royal Society," No. 55, were 

 taken when the north pole of Mars was invisible. Allowing 

 for the difference produced by this cause, the drawings agree 

 with mine in the general form of the markings, but the white 

 margin which I only see in the bays, Professor Phillips shows 

 completely fringing the whole of the markings. 



Now half a second is the smallest amount of difference in 

 diameter we could hope to detect, even by the aid of the most 

 delicate micrometric apparatus. The double-image micrometer, 

 devised by the Astronomer Royal, would be the best to employ 

 for this measurement.* 



No satellite attending on Mars has yet been discovered. 

 Pursuing the analogy between the planet and our earth, if 

 such a satellite existed of a size proportionate with our moon, 

 as it would be one-quarter the diameter of the primary, we 

 might expect it to be easily visible, but should a satellite exist, 

 not exceeding in size proportionately the second satellite of 

 Jupiter, it would only be visible in very powerful instruments, 

 still it would not, I think, have escaped the notice of the per- 

 sistent observers who have searched for it hitherto vainly. 



The discovery of a satellite to Mars is looked for with 

 interest, as the effect of the sun and the primary in producing 

 perturbations of a satellite would enable the density and mass 

 of the planet to be accurately calculated. At present these are 

 only imperfectly known, but the density is considered to be 

 almost exactly the same as that of the earth. 



Unfortunately, observations of value can scarcely be made 

 upon this difficult planet with telescopes of less than six inches 

 aperture, and, unless the observer has extremely good sight, 

 eight or ten inches will be found necessary. During the next 

 opposition I hope to be able to continue my observations with 

 an instrument having a silvered glass speculum twelve inches in 

 diameter. 



* In measuring, micrometically, the diameter of Mars, it is very difficult to 

 avoid obtaining too large a result for the polar diameter, the effect of the irradia- 

 tion causing the white spots near the poles to appear to project slightly from the 

 disk of the planet. The best method, probably, of overcoming this difficulty is 

 by using a single reflecting solar eye-piece. 



