400 Royal Astronomical Society. 



the adjustments of an equatoreal is very trifling, if it be methodically 

 undertaken, and the residual errors much smaller than would at first 

 sight seem possible. With well-turned collars and pivots an error 

 of half a minute, arising from flexure or other causes, must be looked 

 upon as an impossible quantity, in which case the differential effects 

 upon objects in the zodiac might be disregarded. With respect to 

 methods of observing, the author recommends that the telescope be 

 moved in declination like a transit, in order that the star and planet 

 may pass over the same part of the wire. In this case reliance is 

 placed only on the adjustment of the cross-axis ; but when the de- 

 clination is not changed, it is presumed that the position of the wire 

 is correct ; and this can be ascertained with only a moderate degree 

 of certainty. In equatoreals which can be reversed in every position, 

 the observations should be made, one group in one position, and the 

 second in the position reversed. The best wiring for such observa- 

 tions, the author considers to consist of three, five, or seven im- 

 movable wires, at equal distances, and parallel to the meridian, 

 transit- wires, in fact, and seven equidistant wires at right angles to 

 these, at 5' interval, the plate which carries the latter wires being 

 moved by a micrometer-screw. The advantages of this system are 

 a saving of time in screwing the micrometer, less wear of the screw, 

 and less dependence on it for large intervals. 



Thus far it has been shown, that an equatoreal instrument may 

 be made to rival meridian instruments, by the bestowal of a little 

 more time and trouble ; there are, however, many cases where the 

 equatoreal is more convenient, and many where it can, and the 

 others cannot be used. 



A planet which comes to the meridian at a late or inconvenient 

 hour of the night may be observed several hours earlier with the 

 equatoreal. In so variable a climate as ours, it is not too much to 

 say, that the number of good planetary observations might be thus 

 very much increased ; and if an equatoreal were steadily directed to 

 this object in the southern hemisphere, to meet the case where the 

 planet has considerable south declination, we should soon have the 

 materials by which the present sufficiency of theory might be satis- 

 factorily tested. The superior planets cannot always be observed in 

 full daylight with large meridian instruments, yet equatoreals of 

 even a small size might be made to determine their places with great 

 accuracy after sunset. Again, large equatoreals, which are now 

 tolerably abundant, might take charge of the minor planets. Micro- 

 metrical observations only have been taken notice of in the prece- 

 ding remarks, the divided circles of the instrument being considered 

 only as finders, and for performing the adjustment, though in some 

 instruments they are large and good enough to be used in differen- 

 tial observations. Still the proper use of the equatoreal is the as- 

 certaining of small differences by means of the micrometer and 

 time. 



In conducting the observations, the author recommends that there 

 should be made each night two or three transits of the star of com- 

 parison, and of two other stars, one above and one below it a few 



