BIBLIOGRAPHY OF IIERSCHEL's WRITINGS. 521 



Hersehel, "W.: Synopsis of tiik Witirixcjs of— ContimnMl. 

 A.D. Vol. r. 



1781 71 nC) "\Vc liavo no cause 1o suspect any very material periodical inequality. 

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



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

 117 No time-keeper will measure such equal portions of time as we require 



to compare the diurnal motion of the earth to. 

 117 The diurnal rotation of the earth being at least tolerably equable, 

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

 thought of estimating tlie diurnal motion of one planet very exactly 

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

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

 of a very subtle medium in which t lie heavenly bodies perhaps move, 

 or any acceleration from some cause or other. 



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



too inaccurate for this critical puri)osc, and new observations were 

 required. 



118 Mars was tho most suitable planet for the ])urj)ose, as the dark spots 



on Jupiter change their places. These may be sup-posed to be largo 

 black congeries of vapors and clouds swinnning in tho atmosphere 

 of Jupiter. 



118 The bright spots also, though they nuiy adhere firndy to the body oi Ju- 

 piter, may undergo some change of situation by being differently cov- 

 ered or uncovered, on one side or the ot her, 1 )\ a Iterat ions in the belts. 



118 Tiie same bright sp^t, not suspected of any change! of situation, gave 

 by one set of observations 9'' .")1"' -loMi, and by another set O'' 50™ 48^ 



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



irregularities of Jupiter s revol ut ions deduced li'om spots on the disc 



119 If, with Cassixi, we suppose his rotal iou-t imc; to be 9'' oG'", thou some 



spots that I have observed must have been carried through about 

 60° of Jupiter's equator in 22 of his days. This very large velocity 

 in the clouds is not unparalleled by what has ha])i)ened in our own 

 atmosphere. 

 119 The spots on Mars are of a different nature. Their constant and de- 

 termined shai)e, as well as remarkable color, show them to be per- 

 manent, and fastened to the body of the planet. 



119 Suppose that we can determine wheiher a spot on the disc ai Mars is 



or is not in the lino which joins the center of the eartli and the cen- 

 ter of that idanet to half an hour's time with certainty, in this case 

 wo shall in 30 days have tho 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 Ilad such observations as these been made 2000 or 200 years ago, we 



might now, by repeating them, most probal)ly become acquainted 

 with some curious minute changes that may have hitherto passed 

 unnoticed. 



120 The dilfcrencc between the polar and equatorial diameters of tho 



earth is Ijy actual measurement :?(!.9 miles, by theory 3:3.8, from 

 which it should seem probable that when tho earth assumed tho 

 present form tho diurnal rotation was somewhat quicker than it is 

 at present. But I would not lay much stress upon this argument. 



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



Newtonian reilector, a 10-feet reflector of the same form, aiid the 7- 

 feet reilector already mentioned {Fhih Traus. 1780, p. 513). 

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

 rying a telescope magnifying 40 diameters. 



