200 Kansas Academy of Science. 



change of declination than a mean annual precession of fifty min- 

 utes would give. But on observing stars near the solstitial colure, 

 the change of declination was found to be less than the quantity 

 due to the same value of precession. During an interval of five 

 years, he found some of the stars near the solstitial colure had 

 changed their declination nine or ten minutes less than the amount 

 due to a precession of fifty minutes, while others near the equinoctial 

 colure had an equal change of declination in the opposite direc- 

 tion. Bradley was led to ascribe the phenomenon to an oscillation 

 of the earth's axis, brought about by the disturbing action of the 

 moon upon the earth's equator. He saw that the apparent move- 

 ments of all stars were in this case the same, and he knew that a 

 movement of this kind can be referred not to the stars themselves, 

 but to the plumb-line from which their directions are measured. He 

 had thought out the possible causes of such a movement of the 

 plumb-line or of the earth itself, and that there might be a nutation 

 which would go through a cycle of about nineteen years, the period 

 of revolution of the moon's nodes. 



At the end of nineteen years of observation Bradley found his 

 conclusions verified, and his second great discovery, that of nuta- 

 tion, was established. Had he lived longer, a third great discovery 

 might have crowned his efforts. He had presented to the astro- 

 nomical world the discovery of the aberration of light ; he had dis- 

 covered the nutation of the earth's axis; he had found the physical 

 causes of these two phenomena ; and with a little more opportunity 

 he might have added another great discovery, namely, the variation, 

 of latitude. The axis of the earth may move in one or two ways : 

 either it may point to a different star, remaining fixed relatively to 

 the earth, as in the case of nutation, or it may actually change its 

 position in the earth. This second movement was not discovered 

 until a century and a half had elapsed after the death of Bradley. 

 But he was on the track that would have led him to its discovery. 

 When he had fully established the principle of the nutation of the 

 earth's axis, he found that there was concealed somewhere in his 

 new-found theory another principle or residuum. 



Now, as to the change of position of the earth's axis in the very 

 body of the earth itself. There is a certain star in the northern 

 heavens called the pole star, and at any particular place in the 

 northern hemisphere of the earth this star is constantly seen at a 

 definite height above the horizon, which is the latitude of the 

 place. With a telescope, the pole star is found to be not absolutely 

 stationary, but describes a small circle in the heavens every twenty- 



