VAKIATION OF LATITUDE. 273 



determine latitude, and the large discrepancies disappeared. Some 

 observers found differences between latitudes determined in winter 

 and in summer, and tbey supposed those differences to be due to 

 changes of the pole. 



In the latter part of the seventeenth century J. D. Cassini summed 

 up the state of the problem in his day, and arrived at the conclusion 

 that, notwithstanding the apparent variations in the latitudes, the 

 pole of the earth did not change to any large extent; that most of the 

 apparent changes in latitude were due to errors of observation and 

 defects in theory; but he thought it probable that small changes did 

 occur in tlie position of the pole; he thought the changes were peri- 

 odic, and did not amount to more than 2 minutes of arc, equal to about 

 12,000 feet. "Thus, instead of several degrees wliich were conceded 

 by the astronomers of previous centuries, but a paltry 2 minutes was 

 now allowed ; but with improved instruments, with the discovery of 

 aberration and nutation and the perfection of the theory of refraction 

 even this modest allowance was gradually reduced to a vanishing 

 quantity." 



The geologists in their investigations have found fossil remains in 

 the cold regions of the North belonging to the Miocene, Upper and 

 Lower Cretaceous, Jurassic, and other geological iieriods, which seem 

 to indicate a former temperature much higher than the present. In 

 1876 Dr. John Evans, then president of the British Geological Society, 

 discussed the problem, and concluded that the amount of polar light 

 and lieat in the past must have been much greater than it is now. He 

 invited the attention of the mathematicians to this problem, and 

 asked: "Would a considerable elevation and depression of the sea 

 bottoms and continents produce a change of 15 to 20 degrees in the 

 position of the pole?" 



Sir WiUiam Thomson discussed this problem and gave his conclu- 

 sions in 1876 to the British Association for Advancement of Science. 

 He said: "Consider the great facts of the Himalayas and Andes and 

 Africa, and the depths of the Atlantic, and America and the depths of 

 the Pacific, and Australia; and consider further the ellipticity of the 

 equatorial section of the sea level, estimated by Colonel Clarke at about 

 one-tenth of the mean ellipticity of the meridianal sections of the sea 

 level. We need no brush from a comet's tail to account for a change 

 in the earth's axis ; we need no violent convulsions producing a sudden 

 distortion on a great scale, with change of axis of maximum moment 

 of inertia, followed by gigantic deluges; and we may not merely admit, 

 but assert as highly probable, that the axis of maximum inertia and 

 the axis of rotation, always near one another, may have been in ancient 

 times very far from their x^resent geographical position, and may have 

 gradually shifted through 10, 20, 30, or 40 or more degrees without at 

 any time any perceptible sudden disturbance of either land or water." 



Sir William Thomson gave no account of the calculations made by 

 him as the basis of these conclusions. 

 SM 94 18 



