to Practical Astronomy. 149 



once, the fillet used in the previous night's work, and with 

 a needle ])oint puncturing in the lines of seconds perpendi- 

 cular and corresponding to each dot made by the observer's 

 circuit. The seconds' dots are then counted from the minute 

 spaces ; the number of the second dot preceding any puncture 

 is noted, and the fraction represented by the position of the 

 puncture with relation to the dot is measured by a small 

 transparent conical scale, by which means any little irregu- 

 larity in the length of the seconds' spaces is rectified. The 

 "reading ott'" is directly transcril^ed into the transit-book. 



The usual mode of observing without the chronographic 

 apparati\s is styled the eyc-and-ear method, and a short 

 description of it will at once make the superiority of ehrono- 

 graphy obvious. 



In observing by the eye-and-ear method, the clock or 

 chronometer must be in such a position that the movements 

 of the hands on the dial shall be visible, and the " beats" of 

 the pendulum or balance distinctly audible to the observer. 

 Shortly before the occurrence of any astronomical phenome- 

 non, such as the transit of a star, he observes the time of the 

 clock, " taking up " the beats, mentally counting them until 

 the star passes the first wire, when he writes down on his 

 observing slip the corresponding seconds indicated ; and if it 

 occurs between any two consecutive seconds, he estimates as 

 nearly as he can the fraction of a second also. For a com- 

 plete transit, this has to be done over several wires, writing 

 down the times between each observation. It has been 

 found that 15 seconds are required to observe and record for 

 each wire, and to " take up " the " beats " again Avithout 

 hurry. The wires, therefore, are generally arranged so that 

 an equatorial star shall take 15 or IG seconds to pass from 

 one to another. The time required to complete a transit 

 observation over seven wires of a star having a declination of 

 45 degrees, is over two minutes ; and the utmost precision 

 that can be attained by the best observers, under the most 

 favorable circumstances, in a mean of seven Avires, is limited 

 to one-tenth of a second, and it is very much to be doubted 

 if anything near this precision is ever attained. 



Two diftereut oliservers will seldom agree as to the exact 

 instant of a transit or occultation, owing to what is known 

 as personal equation, the precision of combining the visual 

 impression of the observations Avith the sound of the clock- 

 beats dei)ending much on the age, temperament, and health 

 of the observer. When several individuals, therefore, are 



