VOL. LXXI.] PHILOSOPHICAL TRANSACTIONS. 53 



the irregularities of Jupiter's revolutions, deduced from spots on his disc that 

 may have changed their situation; for if we suppose the rotation of Jupiter, ac- 

 cording to Cassini, to be 9 h 56 m , then some spots that Mr. H. had observed 

 must have been carried through about 60° of Jupiter's equator in 22 of his revo- 

 lutions or days. This would certainly be a very great velocity in the clouds, 

 which is however not unparalleled by what has happened in our own atmosphere. 

 But to return: on the planet Mars we see spots of a different nature; their 

 constant and determined shape, as well as remarkable colour, show them to be 

 permanent and fastened to the body of the planet. These will give the revolu- 

 tion of his equator to a great certainty, and by a great number of revolutions, 

 to a very great exactness also. Supposing then, that by a method hereafter de- 

 scribed, we can determine whether a spot on the disc of Mars is, or is not, in 

 the line which joins the centre of the earth and the centre of that planet, to half 

 an hour's time with certainty, probably 10 or 12 minutes will be found sufficient 

 for that purpose, in this case we shall in 30 days have the revolution true to a 

 minute; and, by continuing these observations for 3 months, we shall have it to 

 20 s . When we are so far certain, we can easily arrive to a much greater degree 

 of exactness; for as we now can no longer mistake a whole revolution, if we take 

 the time of any particular spot being in the line which joins the centres of the 

 planets during one opposition of Mars, and take the same again at or near the 

 next opposition, we shall have an interval of about 780 days, which will give the 

 diurnal motion of that planet true to about 2 s . The next opposition will give it 

 to 1, and so forth; by which means, and by taking a proper number of such 

 periods, we may determine the rotation of Mars to as great an exactness as we 

 shall think necessary for the purpose of our comparative view. Had such obser- 

 vations as these been made 2000, or perhaps only so many hundred years ago, 

 we might now, by repeating them, most probably become acquainted with some 

 curious minute changes of the solar system that have hitherto passed unnoticed. 

 There is a certain circumstance which would almost create a suspicion that 

 there has been some retardation in the diurnal motion of the earth. The differ- 

 ence between the equatorial and polar diameters of the earth, by actual measure- 

 ment, has been found to be about 36 English miles and -^-; but, by a calcula- 

 tion wherein the present rotation is made use of, it will only amount to about 33 

 miles and -V; from which it should seem probable, that when the earth assumed 

 the present form, the diurnal rotation was somewhat quicker than it is at present, 

 by which means the centrifugal force bore a greater proportion to the force of 

 gravity, to which it is contrary, and thus occasioned a higher elevation of the 

 equatorial parts. But Mr. H. would not lay much stress on this argument; for, 

 in the calculation it has been supposed, that the earth is nearly of an equal den- 

 sity at the surface and towards the centre, which it seems is not agreeable to 



