xiii.] MEASUREMENT OF PHENOMENA. 281 



motions are due to a central force coinciding in law with 

 gravity, and doubtless identical with it. In other cases 

 the motions are usually so small that it is exceedingly 

 difficult to distinguish them with certainty. And the time 

 is yet far off when any general results as regards stellar 

 motions can be established. 



The variation in the brightness of stars opens an un- 

 limited field for curious observation. There is not a star 

 in the heavens concerning which we might not have to 

 determine : 



1 . Does it vary in brightness ? 



2. Is the brightness increasing or decreasing ? 



3. Is the variation uniform ? 



4. If not, acording to what law does it vary ? 



In a majority of cases the change will probably be 

 found to have a periodic character, in which case several 

 other questions will arise, such as 



5. What is the length of the period ? -^ 



6. Are there minor periods ? 



7. What is the law of variation within the period ? 



8. Is there any change in the amount of variation ? 



9. If so, is it a secular, i.e. a continually growing 

 change, or does it give evidence of a greater period ? 



Already the periodic changes of a certain number of 

 stars have been determined with accuracy, and the lengths 

 of the periods vary from less than three days up to 

 intervals of time at least 250 times as great. Periods 

 within periods have also been detected. 



There is, perhaps, no subject in which more complicated 

 quantitative conditions have to be determined than ter- 

 restrial ^gnetism. Since the time when the declination 

 of the compass was first noticed, as some suppose by 

 Columbus, we have had successive discoveries from time 

 to time of t|SI-progressive change of declination from 

 century to century ; of the periodic character of this 

 change; of the difference of the declination in various 

 parts of the earth's surface ; of the varying laws of 

 the change of declination; of the dip or inclination of 

 the needle, and the corresponding laws of its periodic 

 changes ; the horizontal and perpendicular intensities have 

 also been the subject of exact measurement, and have been 

 found to vary with place, and time, like the directions of 



