39Q THE POPULAR SCIENCE MONTHLY. 



minute, and second, taken from the clock on the sheet at its appropriate 

 place, and the translation of the spaces on the sheet into times may be 

 done at leisure. This will be plainer if we examine Fig. 3, which is a 

 fac-simile of a portion of a chronographic record. 



The marks of the clock giving regular signals of seconds are easily 

 distinguished ; the rattles were made by the observer to attract atten- 

 tion to an observation to follow : and the signals of the observer are 

 seen at the times of the transits of various stars. By applying a grad- 

 uated ruler to the sheet when it is unrolled, the exact times of transit 

 can be determined to within a hundredth part of a second, provided 

 we have some hour, minute, and second marked on the sheet as an 

 origin of time. 



It is quite plain that the senses of the observer are not strained to 

 so great a degree in this method as in the method of eye and ear ; the 

 eye has but one thing to do, the ear is not occupied, and the nand has 

 only to press the key at the proper time. 



In this method, we see that the origin of relative personal equation 

 is again in the different times required for different observers to co- 

 ordinate the position of the star in the field and the position of the 

 wire. 



True personal equation, considered physiologically, must arise from 

 the personal differences between observers when they note the same 

 phenomenon. With the chronograph it is the habit of most observers 

 to tap the observing key at the moment at which the star is actually 

 on the wire. There are cases, however, where astronomers of some 

 experience are accustomed to tap the key so that the sound of the tap 

 shall come to the ear at the time when the star is on the w T ire. This 

 seems an utterly wrong habit of observing, as it is really the record 

 of an event which has not yet taken place which such an observer 

 makes. Astronomically, the difference between such an observer and 

 another observer may be treated as a case of personal equation, pro- 

 vided the habit described above remains constant, which it is proba- 

 bly less likely to do than the ordinary one. 



The first case of personal equation on record aj>pears in the " Ob- 

 servations " of the Rev. Nevil Maskelyne, Astronomer Royal for Eng- 

 land ("Observations" for 1796, vol. iii., p. 339). We there find the 

 following note : " I think it necessary to mention that my assistant, 

 Mr. David Kinnebrook, who had observed the transits of the stars 

 and planets very well in agreement with me all the year 1794, and for 

 a great part of the present year, began from the beginning of August 

 last to set them down half a second of time later than he should do 

 according to my observations ; and, in January of the succeeding 

 year, 1796, he increased his error to eight-tenths of a second. As he 

 had unfortunately continued a considerable time in this error before 

 I noticed it, and did not seem to me likely ever to get over it and 

 return to a right method of observing, therefore, though witli reluc- 



