266 G. W. Bulman — Revised Theory of Glaciation. 



But on page 97 we read: "When all circumstances combine to 

 accentuate as much as possible the difference in the lengths of the 

 seasons, one of them may be 199 days long and the other 166." 



Again, on page 153, we find the statement that seven days is 

 the greatest possible difference between the seasons under present 

 ecceiitricity : '* So long as the eccentricity of the earth's orbit remains 

 at its present value, the difference between the lengths of the seasons 

 will fluctuate between the extreme values of a winter seven days 

 longer than a summer, and a summer seven days longer than a 

 winter." 



Yet, as we have seen, it is expressly stated, on page 95, that the 

 present position of the line of equinoxes is not such as to produce 

 the maximum difference possible for the present eccentricity. And 

 on page 167 it is stated that the present seasonal difference is 

 " near its maximum for the present eccentricity of the orbit." 



The true law is stated on page 152 thus: "The differences 

 between the lengths of the seasons is, as a mathematician would 

 say, a function of two other independent quantities ; it partly 

 depends upon the eccentricity of the earth's orbit, and partly on the 

 longitude of the perihelion, that is to sa,y, on the position of the line 

 of equinoxes with respect to the longer axis of the earth's orbit ; if 

 there were no eccentricity, there could be no difference in the lengths 

 of the seasons, no matter where the line of equinoxes may lie. On 

 the other hand, no matter what the eccentricity may be, there would 

 be no difference in the lengths of the seasons if the line of equinoxes 

 passed through perihelion," That is to say, for each value of the 

 eccentricity the difference in the length of the seasons may vary 

 from zero to a certain maximum value determined by the eccentricity. 

 This maximum difference. Sir R. Ball states, is found by multiplj'ing 

 the eccentricity by 465. Taking -0167922, the figures given by 

 Sir J. Herschel as the present eccentricity, we obtain 7-8 days as 

 the maximum difference. This maximum will occur when the line 

 of the equinoxes is perpendicular to the major axis of the ellipse of 

 the earth's orbit ; and the seasons will be equal when it coincides 

 with the major axis. And by what is called precession of the 

 equinoxes this line describes complete circles round the sun as 

 centre, and thus takes up successively every possible position in 

 relation to the major and minor axes of the earth's orbit. 



With the greatest possible eccentricity the difference between the 

 seasons may attain to 00745x465, that is, 33 days. 



In the above quotations the italics are mine. 



A slip appears to occur on p. 81 in reference to Fig. 2, p. 82. 

 Instead of " the part AB is comparatively near the sun, while the 

 other part XY, is as far as possible from the sun," should we not 

 read, •' the part ^/? is as near the sun as possible, while the other, 

 XY, is comparatively far from the sun " ? 



And on p. 82, when we read, " it therefore sweeps across from 

 X and [to?] Y in much less time than it takes to pass from A to B," 

 the reverse is doubtless intende<1. 



Sir R. Ball has brought forward certain facts in botanical distri- 



