ON THE GENERAL DIFFUSION OF KNOWLEDGE, 



nundred years) in which the polar and equato 

 rial diameters of this planet can be measured, 

 and their difference, if any, ascertained, so as 

 to determine whether its fr re, like that of tl 

 earth and several other pickets, be sphere 

 But as this planet may not happen for ; 

 of years to be in the precise position for s 

 observation, the attempt to determine the points 

 now stated, even when the planet happens to be 

 placed in the requisite circumstances, would, in 

 all probability, fail, if a number of observers at 

 the same time, in different places, were not en 

 gaged in the observation ; on account of the 

 uncertainty of enjoying a serene sky at one par 

 ticular place, during the moments when the ob 

 servation behooved to be made. Whereas, by 

 a multitude of observations in different places, 

 the object in view could not fail of being deter 

 mined. The disputes respecting the period of 

 rotation of this planet (whether it be 23 hours 

 20 minutes, or 24 days 8 hours) might also be 

 settled, were a number of persons to observe its 

 surface with equatorial telescopes in the day 

 time ; particularly in those southern climes 

 wnere the air is serene, and the sky exhibits a 

 deep azure, where, in all probability, spots 

 would be discovered, which could be traced in 

 their motions for successive periods of twelve 

 hours or more, which would determine to a cer 

 tainty the point in question. 



The following figure and explanation will per 

 haps tend to show the reason of the dispute 

 which has arisen in reference to this point. Let 

 A represent a spot on the surface of Venus. 



As this planet is seen, by the naked eye, only 

 in the morning a little before sun-rise, or in the 

 evening a short time after sun-set the motion 

 of the spot cannot be traced above an hour or 

 two in succession ; and, consequently, during 

 that time, its progressive motion is almost im 

 perceptible. Suppose the observation to have 

 been made in the evening, after sun-set, the 



nex* jservation cannot be made till about th 



? time, on the following evening, when it is 



1 that the spot has moved from A to B. 



it is s ill uncertain whether the spot has only 



/ed from A to B, since the last observation, 



r has finished a complete revolution, and moved 

 ihe distance A B as part of another revolution 

 round the axis of the planet. This point can 

 only be ascertained by tracing the motion of the 

 spot without interruption for 10, 12, or 14 hours, 

 when, if the rotation is performed in 23J hours, 

 the motion of the spot could be traced without 

 interruption across the whole disk of the planet. 

 But such an observation could only be made in 

 the day-time, in a serene sky, and by means of 

 equatorial instruments, and by numbers of ob 

 servers in different places, where the attention is 

 directed to the same object. But the limits to 

 which I am confined, in throwing out these cur 

 sory hints, prevent me from entering into minute 

 details. 



In regard to comets, it is scarcely necessary 

 to remark, that were the number of those whose 

 attention is directed to a survey of the heavens 

 considerably increased, many of those eccentric 

 bodies, which pass and repass within the orbits 

 of the planets without being perceived, could not 

 fail of being detected. AVere multitudes of such 

 persons engaged in exploring the celestial re 

 gions, on opposite sides of the globe, those co 

 mets which pass within the limits of our view, 

 and which are above our horizon only in the 

 day-time, and consequently invisible, would be 

 detected, during the night, by our antipodes in 

 the opposite regions of the globe. By this 

 means the number of those bodies belonging to 

 our system, the diversified phenomena they pre 

 sent, the form of their trajectories, the periods 

 of their revolutions, the nature of their tails, and 

 their ultimate destination, might be more accu 

 rately determined. With respect to the Jixed 

 stars, particularly those termed variable stars, 

 the results of a multitude of observations made 

 by different persons, might lead us to determine, 

 whether those changes in brightness which they 

 undergo, arise from the transits of arge planets 

 revolving around them, and thus furnish direct 

 evidence of their being the centres of systems 

 analogous to our own, or whether they be oc 

 casioned by large spots which periodically inter 

 pose between our sight, and then disappear in 

 the course of their rotation, or whether the 

 distance of such stars be changed by their re 

 volving in a long narrow ellipse, whose trans 

 verse axis is situated nearly in our line of vi 

 sion. In the several instances now slated, an 

 immense variety of successive observations, by 

 numerous observers at different stations, are re 

 quisite to accomplish the ends in view ; but the 

 limits of this section prevent me from entering 

 into those details requisite for rendering the 

 hints now suggested perspicuous to those whc 



