RECEEATIVE SCIENCE. 



THE PLANETS. 



To no otlier subject can tlie oft-repeated 

 assertion of "We live in an age of dis- 

 covery" be more aptly applied, tban to tbe 

 glorious science of Astronomy. Whereas in 

 the scbool-days of our fathers, and indeed of 

 most of us, the number of known planets 

 was only eleven, the last fourteen years have 

 increased the list to sixty-four. It is now 

 difficult for any but the astronomer to keep 

 pace with these rapid discoveries ; our most 

 recent works on the subject are almost sure 

 to be half a dozen planets in arrear with 

 their information, for even whilst the sheets 

 are passing through the press, a number of 

 diligent observers are still exerting them- 

 selves, in order to add new worlds to our 

 solar system. 



It may be asked, how is it that the nine- 

 teenth century has done so much in this par- 

 ticular branch of astronomy ? Many great 

 names had preceded those of the present 

 time, many diligent observers lived and flou- 

 rished in the eighteenth century, yet why 

 had they overlooked so many planets ? 



Fig. 1. 



The systematic plan now adopted of 

 sweeping the heavens with large telescopes, 

 and mapping down all the stars in the neigh- 

 boui'hood of the ecliptic, has mainly contri- 

 buted to these planetary discoveries. All the 

 planets are situated in a small zone of the hea- 

 vens near the ecliptic, so that if such stars are 



mapped down, and the same region of space 

 is again returned to, and carefully compared 

 with the previous maps, it is quite evident 

 that if a certain map had contained ten 

 stars, and on sweeping over the same si)ace 

 a few months later, eleven were found, a new 

 comer must have travelled into this map. An 

 hour's observation with the double-image 

 micrometer is sufficient time to enable the 

 astronomer to ascertain whether this addi- 

 tional body be a planet or a star. 



In short, to make this assertion clear, the 

 double -image micrometer, from its peculiar 

 construction, shows two images of the same 

 object ; and as a planet moves much more 

 rapidly than a star, if the speed of the new 

 body be compared with one of those pre- 

 viously mapped down, an hour's time will 

 amply prove whether both bodies are moving 

 with an equal speed. 



The two images of the new body, and the 

 two images of the body in comparison with 

 it, are, by means of a rackwork adjustment, 

 made to show four objects in one and the 



Fio. 2, 



same straight line, as in Eig. 1. Now, 

 if both the objects are stars, their re- 

 lative positions will remain unaltered, whilst 

 if one is a planet, they may possibly, show 

 themselves as in Fig. 2. Having previously 

 read off the vernier of the position micro- 

 meter when the four images were in a straight' 



