258 ANNUAL OF SCIENTIFIC DISCOVEEY. 



as far as it has any influence, it is evident that it would immediately 

 receive that change of form to which I have adverted. 



" When the position of the axis of rotation within the earth is sen- 

 sibly permanent, its position, excluding the effects of external ac- 

 tions, is also sensibly permanent in the heavens. 



" So far, therefore, as I am at present able to enter into the ques- 

 tion, I entirely doubt the validity of the cause assigned by Sir Henry 

 James for the changes of the earth's climates." 



Mr. W. E. Hicksen, also entering the lists as a disputant on the 

 same subject, writes as follows : 



The sun, in its apparent motion, makes a complete tour of the zo- 

 diac in a period usually estimated a,t 25,868 years, shortened by 

 some later authorities to about 21,000 years, called the Precessional 

 Cycle ; the cause of which is not any real change in the position of 

 the sun, but a change in the position of the observer, from an altered 

 inclination of the earth's axis. 



If an observer were to place himself, on the 21st of March, at the 

 bottom of a dry well, or at the lower end of any long shaft, like that 

 of the inclined passage of the Great Pyramid, and look upwards, he 

 would see, at any given hour, certain stars cross his field of view ; 

 and he would again see the same stars next year on the same day, but 

 not at precisely the same moment. They would cross his field of view 

 some seconds earlier ; and earlier again, and continually earlier, every 

 successive year ; proving, not that the stars had moved, but that the 

 sidereal bearings of the earth had changed. In confirmation of 

 which, if your readers turn to Vyse's Pyramids, they will find a cal- 

 culation of Sir John Herschel's, that the entrance front of the Great 

 Pyramid must, in the year B.C. 2123, have looked towards a Draconis, 

 instead of towards our present polar star, a Ursa Minoris. 



This varying direction of the earth's axis is occasioned by the 

 varying influence of the sun and moon on the protuberant matter of 

 the earth's equator, in necessary correspondence with the earth's 

 variations of distance in different parts of its orbit ; and in respect of 

 which varying influence the earth may be described, if I may use a 

 homely figure, as in the situation of a man held by the collar between 

 two policemen, and swayed to the right or left according to the force 

 exerted by policeman A or policeman B. The monthly swayings to 

 and fro of the earth's axis, which are very slight, thus arising, are 

 called Nutation ; the annual balance of their aggregate result is called 

 the Precession of the Equinoxes ; the meaning of which is, that the 

 equinoxes arrive earlier and earlier every year, and with them, of 

 course, spring and summer, autumn and winter. 



Sir Henry James will have rendered a service to science if he suc- 

 ceeds in inducing geologists to inquire into the effects of this period- 

 ical evagation, the laws and limits of which are known ; but, until 

 they have done so, it would perhaps be wise to defer raising the 

 question, for which we have no astronomical data, of whether or not 

 the axis of the earth was " at one time perpendicular to the plane of 

 the ecliptic." The certain effects of precession in producing changes 

 of climate and modifications of the earth's crust are sufficiently strik- 

 ing to detain us, without at present going further. 



One consequence is, that the summers of unequal length, which 



