BIBLIOGRAPHY OF IIERSCIIKL's WRITINGS. 521 



Syxopsis of the "Wiutixgs OI-— Continued. 



"\Vo have no canse (o suspect any very material ])erio(lical inequality. 

 Aberration would forever liave remained a secret to us if it had not 



been found out by other methods tluiu that of time-keepers. 

 117 No time-keeper will measure such ecjual ])ortions of time as we require 



to compare the diurnal motion of the earth to. 

 117 The, diurnal rotation of the earth l)einy at least tolerably equal) h^, 



that of the other planets is likely to be also ; and this suggested the 



thouglit of estimating tlio diurnal motion of one planet very exactly 



by that of another, malang each the standard of the other. 

 117 Such a test might detect a retardation occasioned by some resistance 



of a very subtle medium in whicli the heav(>nly bodies perhaps move, 



or any acceleration from some cause or other. 



117 Tlie common account of the diurnal rotations of the planets was much 



too inaccurate for this critical purpose, and new observations were 

 required. 



118 Mars was the most suitabI(^ ])lanet for the purpose, as the dark spots 



on Jupiter change their phices. These may be supposed to be large 

 blaclv congeries of vapors and clouds swimming in the atmosphere 

 of Jiipifer. 



118 The bright spots also, though they may aiilK're firmly to the body of J«- 

 2)itcr, may undergo some change of situation by being dilferently cov- 

 ered or uncovered, on one side or the other, by alterations in the belts. 



118 The same bright spot, not suspected of any change of situation, gave 

 by one set of observations 9'' 51"' 45^6, and by another set 9'> 50™ 48'. 



118 The analogy of the trade-wiuds on the earth may account for all the 



irregularities of Jiqnfer's revolutions deduced from spots on the disc 



119 If, with Cassixi, we suppose his rotation-time to be 9'^ !j(]"\ then some 



spots that I have observed must have been carried through about 

 6(P of Jupiter's equator in 22 of his days. This very large vclocitj^ 

 in the clouds is not unparalleled by what has happened in our own 

 atmosphere. 

 119 The spots on ITars are of a ditferent nature. Their constant and de- 

 termined shape, as well as remarkable color, show them to be per- 

 manent, and fastened to the body of the i)lanet. 



119 Suppose that we can determine whether a spot on the disc of Mars is 



or is not in the line which joins the center of the earth and the cen- 

 ter of that planet to half an hour's timi; with certainty, in this case 

 we shall in 'AO days have the revolution true to a minute, and in 

 three mouths to 20^ An interval of about 780 days (the next oppo- 

 sition) will give the diurnal motion true to about 2-', etc. 



120 Had such observations as these been made 20U0 or 200 years ago, we 



might now, by repeating them, most i)i'obal>ly become acquainted 

 with some curious minute changes that may have hitherto passed 

 unnoticed. 



120 The difference between the polar and equatorial diameters of the 



earth is by actual measurement :>5.9 miles, by theory 33.8, 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. But I would not lay much stress upon this argument. 



121 The telescopes used were of my own construction, and are a 20-feet 



Newtonian reflector, a 10-feet reflector of the saine form, and the 7- 

 feet reflector already mentioned {Phil. Trans. 1780, p. 513). 

 121 The time was determined with a brass quadrant of 2 feet radius, car- 

 rying a telescope magnifying 40 diameters. 



