VB2 



Astronomical Society. 



The first part of the above ephemeris (to the end of the year 1 828) 

 is taken from Mr. Encke's own computations for Paris time d .3, as in- 

 serted in Professor Schumacher's Astronomische Nachrichten, No. 123. 

 That part of it which belongs to the ensuing year 1829, is taken from 

 a letter addressed to the President of this Society by Dr. Olbers, and 

 is computed for the time of midnight at Paris. The positions are 

 computed for every third or fourth day only, at the beginning and end 

 of the ephemeris, and, for every second day towards the middle : this 

 being quite sufficient to enable the observer to find the place of the 

 comet in the heavens. The original computations are extended only 

 to minutes of space ; but the right ascensions are here converted into 

 the nearest second of time, for the convenience of the observer. 



[It being desirable that as many observations should be made of this 

 comet as possible, particularly in the southern hemisphere, where it 

 will probably be seen after its return from the sun, an extra number of 

 copies of the above ephemeris have been struck off for distribution, not 

 only amongst the Members of the Society, but also for circulation in 

 various parts of the world where the comet is likely to be seen. And 

 Capt. Foster has kindly undertaken to distribute them as much as 

 possible in those parts of the southern hemisphere at which he may 

 touch during the scientific voyage in which he is about to embark.] 



There was next read a paper, " On finding the rates of time- 

 keepers s'' by E. Riddle, Esq. In this communication the author ob- 

 serves, that there are many persons fond of astronomy who are not 

 possessed of a transit instrument, or who have not a convenient situ- 

 ation in which to place it : and many others, such as nautical men in 

 particular, who have not the means of using one, who are desirous on 

 many accounts, of knowing the rates of their chronometers, indepen- 

 dent of the absolute time which they indicate. The method of equal 

 altitudes on each side of the meridian is the course usually pursued on 

 such occasions : but Mr. Riddle proposes another mode, often much 

 more convenient in practice and equally correct in its results ; viz. by 

 taking equal altitudes of a fixed star on the same side of the meridian, 

 on successive nights. It is well known that a star will, at the same 

 sidereal hour, arrive at the same altitude, for several succeeding nights : 

 the only difference which occurs, arising from the small change in the 

 aberration, and also from the variation in the refraction. The former 

 is insensible, if the interval between the observations be not too long - } 

 and for the latter, appropriate tables are given. The best time for 

 the observation of such stars is when they are due east or due 

 west ; and any known star may be chosen for the purpose. All 

 we have to do therefore, is to note the difference of the two consecu- 

 tive times at which the star attains the same altitude (whatever it be) 

 on the same side of the meridian ; and if that difference be less than 

 3 m 55%91, the chronometer (presuming that it is regulated to mean 

 solar time) will have gained ; and if more, it will have lost so much in a 

 sidereal day. And, if the observations are made at an interval of n 

 days, the nth part of the difference between the times of observation, 

 compared with 3 m 55 s ,91, will in like manner give the mean rate for 

 that interval j and if this quantity be multiplied by 1,0027, it will give 



the 



