74 On the Comttarhim. 



vet the instrument is called a planetarium notwithstanding. 

 Nor, indeed, has any system of machinery yet been made 

 " which will correctly exhibit the phasnomena of the solar 

 system." 



" An orrery is a verv fit machine to show the system of 

 the world, and some of them have been made at an enor- 

 mous expense, with a great multiplication of wheels and 

 other parts, by which means they have imitated the prin- 

 cipal movements of the celestial bodies ; but even the best 

 of them fall very short of real accuracy, and of course they 

 are quite unfit for the purposes of calculating the future 

 situations of the celestial bodies." — Cavallo's Elements of 

 Plulosopliy, vol. iv. p. 281. 



My cometarium was never intended to be adapted to any 

 system of machinery made for exhibiting the phenomena of 

 the solar system ; for those phamomena arc so various, that 

 it is, found expedient to exhibit different parts of the system 

 by different machines ; and, perhaps, this is the best way of 

 exhibiting the phaenomena of a comet. The instrument 

 described in my former paper was intended only to represent 

 a comet revolving in a circular orbit, with its tail turned 

 from the sun; but I have since added machinery, constructed 

 on the principle of the elliptical compasses, by which a comet 

 may be represented as describing an elliptical curve, either 

 accompanied with a tail as before, or when deprived of that 

 luminous appendage, by taking away the great lens. 



The sun is now represented by a lamp placed in a lantern 

 with lenses so adjusted as to throw a circular spot of light 

 upon the screen. This lantern stands between the cometa- 

 rium and the screen, but unconnected with either, so that 

 the sun is represented as stationarv in the focus of the ellipse 

 which the comet describes. Although the instrument is 

 turned only by hand, yet by this means the relative motion 

 of a comet may be shown sufficiently near the truth for con- 

 veying a general idea; but those who wish for a more regular 

 motion may easily add machinery for that purpose. The 

 effects of this apparatus are seen to advantage upon a trans- 

 parent screen, and, when used in this manner, it promises 

 to ba useful in a lecture-room. 



The 



