3 88 THE POPULAR SCIENCE MONTHLY. 



vices, to be hereafter spoken of. A little consideration will show, how- 

 ever, that, although every transit observed by our astronomer is too 

 early by 0".2, yet, in ordinary cases, this correction is of no account, 

 provided only that it is constant. If he observes the star too early by 

 3 .2, and the comet also too early by that amount, the difference in the 

 times will be absolutely correct. But suppose one observer to note 

 the transit of the star, and another that of the comet : each may 

 have a peculiar habit, so that where one would note 8 .2 top early, 

 another might note 0\3 too early, and the difference of their absolute 

 personal equations, s . 1, it would be necessary to apply to the observa- 

 tions of A to reduce them to homogeneousness with those of B. This 

 difference of absolute personal equations is relative personal equation, 

 which, when once truly known, enables us to reduce the observations 

 of one skillful astronomer to what they would have been had another 

 made them. * 



We say " skillful," because it is only among skillful observers that 

 the phenomenon in question is ti - uly found. In astronomical obser- 

 vations the senses are trained to a fine delicacy, and old observers 

 acquire a constancy of habit which gives to their work a homo- 

 geneousness that is wanting in that of younger men. 



We have given a brief account of the early method of estimating 

 the time of a star's transit across a spider-line in the field of the 

 telescope by the method of eye and ear; there is yet another method 

 now in common use, which it is necessary to understand before we 

 pass to the consideration of the means of determining personal 

 equation. 



This second method is the American or chronographic method ; 

 this consists, in the present practice, in the use of a sheet of paper 

 wound about and fastened to an horizontal cylindrical barrel, which is 

 caused to revolve by machinery once in one minute of time. A pen 

 of glass which will make a continuous line is allowed to rest on the 

 paper, and to this pen a continuous motion of translation in the direc- 

 tion of the length of the cylinder is given. Now, if the pen is allowed 

 to mark, it is evident that it will trace on the paper an endless 

 spiral line. An electric current is caused to run through the observing 

 clock, through the pen, and through a key which is held in the ob 

 server's hand. 



A simple device enables the clock every second to give a slight 

 lateral motion to the pen, which lasts about a thirtieth of a second. 

 Thus every second is automatically marked by the clock on the chro- 

 nograph-paper. The observer also has the power to make a signal 

 (easily distinguished from the clock-signal by its different length), 

 which is likewise permanently registered on the sheet. In this way, 

 after the chronograph is in motion, the observer has merely to notice 

 the instant at which the star is on the thread, and to press the key 

 at that moment. At any subsequent time he must mark some hour, 



