262 Rev. E. Hill — On Changes in the Earth's Axis. 



III. — On the Possibility of Changes in the Earth's Axis. 



By EeT. E. Hill, M.A., F.G.S. 



Fellow and Tutor of St. John's College, Cambridge. 



THE possibility of change in the position of the Earth's axis 

 seems periodically to attract attention. This is happening now. 

 Professor Haughton has written upon this subject, Sir "William Thom- 

 son has alluded to it, while two important papers have been read 

 upon it, one by Professor Twisden before the Geological Society, the 

 other by Mr. George H. Darwin before the Eoyal Society. This 

 article is an endeavour to exhibit to the general reader the assump- 

 tions with which they have started, and the results they have ob- 

 tained. 



The phrase "change in the Earth's axis" is susceptible of several 

 meanings which do not coincide in like changes of climate. The 

 Earth may be supposed to be tilted, so that its axis of rotation 

 should make a new angle with the plane of the ecliptic. Or the 

 Earth may retain the same shape, but be caused to rotate about an 

 axis holding a different position in its body. Or — the change most 

 easily conceived by a geologist — the shape may be supposed to 

 change by expansions and contractions, so that the axis of figure 

 shall occupy a new position in the body of the earth, which how- 

 ever shall continue to rotate about an axis the same in direction as 

 before. Or lastly, the shape may thus change, and the axis of rota- 

 tion change also. They are however to separate in their change, 

 for if they kept together this case would agree with the- first men- 

 tioned. Imagine the Earth a huge orange, spinning, impaled by 

 some giant of Hindoo mythology on a correspondingly gigantic 

 knitting needle. The first change would be produced by his tilting 

 the needle, the second by imjmling the orange differently, the third 

 by squeezing it into a new shape, while the fourth would naturally 

 result from the difficulty of holding the needle steady during this 

 operation. 



There are two kinds of alteration of climates which can thus be 

 produced on the Earth. If the axis of rotation were tilted (as in 

 the first case), all places would have the same latitude as they now 

 have, all places on the same latitude would receiA^e heat and light 

 alike, but they would not receive these in the same way as they do 

 now. The lengths of the seasons, and the Sun's midsummer and 

 midwinter altitudes, would be changed. The Arctic Circles and the 

 Torrid Zone would change their breadths. An alteration of climates 

 would result, and the alteration would be of like nature on all sides 

 of the Pole. "What that alteration would be is not so easy to say. 

 Probably a tilt making the axis more nearly perpendicular to the 

 plane of the Ecliptic would render the Polar climate milder. This 

 would agree with the supposed condition of the Miocene age. But 

 it will be seen hereafter that this change is exceedingly difficult to 

 produce. 



The other kind of alteration would result were the axis of rotation to 

 retain its present direction in space but to occupy a different position in 



