38 Prof. Challis's Observations relative to the New Planet. 



be seen that the plan did not contemphite the use of hour xxi. 

 of the Berlin star-maps, the publication of which was equally 

 unknown at that time to Mr. Airy and myself. It may be 

 proper here to explain, that the construction of a good star- 

 map requires a great amount of time and labour both in ob- 

 serving and calculating, and that precisely this sort of labour 

 must be gone through to conduct a search of the kind [ had 

 undertaken. The stars must first be mapped before the search 

 can properly be said to begin. With a map ready made, the 

 detection of a moving body, as it hafipened in this instance, 

 might be effected on a comparison of the heavens with the 

 map by mere inspection. Not having the advantage of such 

 a map, I proceeded as follows. I noted down very approxi- 

 mately the positions of all the stars to the 11th magnitude, 

 that could be conveniently taken as they passed through the 

 field of view of the telescope, the breadth of the field with a 

 magnifying power of 166 being 9', and the telescope being in 

 a fixed position. When the stars came thickly, some were 

 necessarily allowed to pass without recording their places. 

 Wishing to include all stars to the lltli magnitude, I pro- 

 posed in going over the same region a second time, to avail 

 myself of an arrangement peculiar to the Northumberland 

 equatoreal, the merit of inventing which is due to Mr. Airy. 

 The hour-circle, telescope and polar frame, are moveable by 

 clock-work, which may be regulated to sidereal time nearly. 

 While this motion is going on, the telescope and polar frame 

 are moveable relativelij to the honv circle, by a tangent- screw 

 apparatus, and a handle extending to the observer's seat. This 

 contrivance enables the observer to measure at his leisure dif- 

 ferences of right ascension however small, and therefore meets 

 the case of stars coming in groups. The observations made 

 by this method might include all the stars it was thought de- 

 sirable to take, and therefore might include «//the stars taken in 

 tlie first sweep. The discovery of the planet would result from 

 finding that any star in the first sweep was not in its position in 

 the second sweep. If two sweeps failed in detecting the planet 

 among the stars of the first sweep, it might be among the stars 

 of the second, which would be decided by taking a third sweep 

 of the same kind as the second. It will appear that this plan 

 carried out, would not only detect the planet if it were in the 

 region explored, but would also, in case of failure, enable the 

 observer to pronounce that it was not in that region. The 

 second mode of observing required the aid of my two assist- 

 ants, Mr. Morgan and Mr. Breen, in reading off" and record- 

 ing the obh.ervations. 



1 commenced observing July 29, employing on that day 



