VOL. LXXIV.] PHILOSOPHICAL TRANSACTIONS. 53 I 



the former case as masses of the electric fluid repelled or bursting from the 

 great collected body of it in the north ; and in the latter case as masses attracted 

 to that accumulation; a distinction probably much the same in effect, as that of 

 positive and negative electricity near the surface of the earth. 



Tbis tendency toward the magnetic meridian however, seems to hold good 

 only with regard to the largest sort of fire-balls; the smaller ones move more 

 irregularly, perhaps because they become farther within the verge of our atmos- 

 phere, and are thus more exposed to the action of extraneous causes. That the 

 smaller sort of meteors, such as shooting stars, are really lower down in the 

 atmosphere, is rendered very probable by their swifter apparent motion ; perhaps 

 it is this very circumstance which occasions them to be smaller, the electric fluid 

 being more divided in more resisting air. But as those masses of electricity, 

 which move where there is scarcely any resistance, so generally affect the 

 direction of the magnetic meridian, the ideas which have been entertained of 

 some analogy between these two obscure powers of nature, seem not altogether 

 without foundation. 



If the foregoing conjectures be just, distinct regions are allotted to the elec- 

 trical phenomena of our atmosphere. Here below we have thunder and light- 

 ning, from the unequal distribution of the electric fluid among the clouds ; in 

 the loftier regions, whither the clouds never reach, we have the various grada- 

 tions of falling stars ; till beyond the limits of our crepuscular atmosphere the 

 fluid is put into motion in sufficient masses to hold a determined course, and 

 exhibit the different appearances of what we call fire-balls ; and probably at a 

 still greater elevation above the earth, the electricity accumulates in a lighter less 

 condensed form, to produce the wonderfully diversified streams and coruscations 

 of the aurora borealis. 



XIX. On the Remarkable appearances at the Polar Regions of the Planet Mars, 

 the Inclination of its Axis, the Position of its Poles, and its Spheroidical 

 Figure; with a few Hints relating to its Real Diameter and Atmosphere. By 

 William Herschel, Esq., F.R.S. p. 233. 



What Dr. H. offers on the abovementioned subjects is founded on a series of 

 observations delivered in this paper. When he found that the poles of Mars 

 were distinguished with remarkable luminous spots, it occurred to him that we 

 might obtain a good theory for settling the inclination and nodes of the planet's 

 axis, by measures taken of the situation of those spots. But, not to proceed 

 on grounds that wanted confirmation, it became necessary to determine by obser- 

 vation, how far these polar spots might be depended on as permanent ; and in 

 what latitude of the globe of Mars they were situated; for, if they should either 

 be changeable, or not be at the very poles, we might be led into great mistakes 



3 Y 2 



