VOL. LXXXVII.] PHILOSOPHICAL TRANSACTIONS. 187 



ner as to be able to convey the water 5 different ways at pleasure, with the power 

 of being able to set any of these pipes dry at will, in order to repair them whenever 

 occasion should require. The water being now entirely at command, I again re- 

 solved on taking out more sand, in order to try what additional quantity of water 

 could be obtained. I cannot exactly ascertain the quantity of sand taken out, but 

 the increase of water obtained was very great ; as instead of the well discharging 

 30 gallons in a minute, the water was now increased to 46 gallons in the same 

 time. 



XV. Observations of the Changeable Brightness of the Satellites of Jupiter, and 

 of the Variation in their Apparent Magnitudes ; with a Determination of the 

 Time of their Rotatory Motions on their Axes. To which is added, a Measure 

 of the Diameter of the Second Satellite, and an Estimate of the Comparative 

 Size of all the Four. By Win. Herschel, LL.D., F.R.S. p. 332. 



It may be easily supposed, when I made observations on the brightness of the 

 5th satellite of Saturn, by way of determining its rotation on its axis, and found 

 that these observations proved successful, that I should also turn my thoughts to 

 the rest of the satellites, not only of Saturn, but likewise of Jupiter, and of the 

 Georgian planet. Accordingly I have from time to time, when other pursuits 

 would permit, attended to every circumstance that could forward the discovery of 

 the rotation of the secondary planets ; especially as there did not seem to lie much 

 difficulty in the way. For since I have determined, by observation, that the 5th 

 satellite of Saturn is in its rotation subject to the same law that our moon obeys, it 

 seems to be natural to conclude that all the secondary planets, or satellites, may 

 probably stand in the same predicament with the two I have mentioned ; conse- 

 quently a {ew observations that coincide with this proposed theory, will go a good 

 way towards a confirmation of it. I had another point in view when I made the 

 observations contained in this paper. It was an attempt to avail myself of the 

 abundant light and high powers of my various telescopes, to examine the nature 

 and construction of the bodies of the satellites themselves, and of their real mag- 

 nitudes. Here phenomena occurred that will perhaps be thought to be remarkable, 

 and even inconsistent or contradictory. So far from attempting to lessen the force 

 of such animadversions, I shall be the first to point out difficulties, in order that 

 future observations may be made to resolve them. 



Perhaps it would have been better to delay the communication of these observa- 

 tions, till I had continued them long enough to be able to account for things 

 which at present must be left doubtful. But as, in final conclusions to be drawn 

 from astronomical observations, we ought to take care not to be precipitate ; so on 

 the other hand I am perhaps too scrupulous in satisfying myself, and should pro- 

 bably require the observations of several years before I could venture to be decisive. 

 It will also be seen by the dates of the first observations, that a further delay in 

 the communication cannot be adviseable , since much information may possibly be 



b b 2 



