27 



bouring observatories of Europe, than to distant situations 

 and less expert hands. 



I am now convinced that, by attending to the * method of 

 observation, suggested by Dr. Maskelyne, and with reason- 

 able practice, any person who has a moderate turn for mak- 

 ing astronomical observations, may, with a portable transit 

 instrument of three feet length, and from 30 to 50 times mag- 

 nifying power, furnished with a system of five fine wires, and 

 a clock, whose rate is carefully ascertained, make these obser- 

 vations so accurately, that he will soon, from a mean of the 

 five wires, obtain the apparent difference of the AR. of the 

 moon's enlightened limb and a star, true to about oUe-tenth 



of a second of time.-f- 



E 2 If 



* It may not be useless here, to refer such persons as may be induced to pur- 

 sue this method of observing the dift'erences of AR. of the heavenly bodies by 

 the transit instrument and time-piece, to the note at the end of the Greenwich 

 observations of passages of stars, for the 5'ear lldS, where they will find some 

 very delicate and useful observations on the niceties of astronomical observations 

 by the clock and transit instrument ; and, among others, the following precept, 

 on the method of observing, introduced by Dr. Bradley, which Dr. Maskelyne 

 strongly inculcates the necessity of first duly understanding, and then, closely 

 adhering to. " We should observe with all our attention, wiien the star comes 

 near the wire, and^.r (as if we could mark down) tlie apparent places of the 

 star in the field of the telescope, at the two beats of the clock, immediately pre- 

 ceding and following the transit of the star across the wire, and thence estimate 

 and note down the proper second and tenth answering to the actual transit 

 across the wire. If we are not quick in fixing the place of the star, at the time 

 of the beat, we shall be apt to assign it too backward a place in the telescope, 

 and consequently reckon the time of the transit too great. A good ear seems, 

 in this kind of observation, to be almost as useful as a good eye." 



t When the moon has gained 48' of AR. in time, while the earth has revolved 

 once on its axis, a change of AR. of 2' of such time indicates that l"*. or 15°. of 



the 



