VOL. LXXIV.] PHILOSOPHICAL TRANSACTIONS. 53Q 



I shall conclude this subject with a consideration relating to the atmosphere 

 of Mars. Dr. Smith, Optics, § IO96, reports an observation of Cassini's, 

 where " a star in the water of Aquarius, at the distance of 6 minutes from the 

 disc of Mars, became so faint before its occultation, that it could not be seen 

 by the naked eye, nor with a 3-feet telescope." It is not mentioned what was 

 the magnitude of the star; but, from the circumstance of its becoming invisible 

 to the naked eye, we may conclude that it must have been of the 6th or 7th 

 magnitude at least. The result of this observation would indicate an atmosphere 

 of such an extraordinary extent, since at the distance of 36 semi-diameters of 

 the planet it should still be dense enough to render so considerable a star invi- 

 sible, that it will certainly not be amiss to give an observation or two which seem 

 of a very different import. These were 1783, Oct. 26 and 27, when 2 small 

 fixed stars preceding Mars, of different sizes, were viewed with different powers, 

 when they were distinctly seen, on the former day at the distance of 3' Q", and 

 on the latter 2' 56". 



The larger of the 2 stars on which the above observations were made cannot 

 exceed the 12th, and the smaller the 13th or 14th magnitude; and there is no 

 reason to suppose that they were any otherwise affected by the approach of Mars, 

 than what the brightness of its superior light may account for. From other 

 phenomena it appears however, that this planet is not without a considerable at- 

 mosphere; for, besides the permanent spots on its surface, I have often noticed 

 occasional changes of partial bright belts; and also once a darkish one, in a 

 pretty high latitude. And these alterations we can hardly ascribe to any other 

 cause than the variable disposition of clouds and vapours floating in the atmos- 

 phere of that planet. 



Result of the contents of this paper. 



The axis of Mars is inclined to the ecliptic 59 42'. 



The node of the axis is in 17° 47' of Pisces. 



The obliquity of the ecliptic on the globe of Mars is 28° 42'. 



The point Aries on the martial ecliptic answers to our 19 28' of Sagittarius. 



The figure of Mars is that of an oblate spheroid, whose equatorial diameter is 

 to the polar one as 1355 to 127 2, or as 1 6 to 15 nearly. 



The equatorial diameter of Mars, reduced to the mean distance of the earth 

 from the sun, is 9" 8 '. 



And that planet has a considerable but moderate atmosphere, so that its inha- 

 bitants probably enjoy a situation in many respects similar to ours. 



XX. A Description of the Teeth of the Anarrhichas Lupus Linncei, and of those 

 of the Ch&todon Nigricans of the same Author; to ivhich is added, an Attempt 



3z 2 



