404 M. ADHMAR'S ARGUMENT. 



in perihelion, or nearest the sun, as is the case on the 31st 

 December. 



The perihelion has not always been, nor will it always con- 

 tinue to be, at the same time of year as at present. On the 

 contrary, a constant though slow movement is continually 

 taking place : the time of perihelion takes place in each year 

 a little later than the preceding, so that perihelion which 

 now happens on the 31st December, will in the lapse of time 

 fall on the 1st of January, then on the 2nd, and so on. The 

 interval between the times at which perihelion occurs on the 

 same day of the year, is about 21,000 years. At some future 

 day, about 5000 years hence, the perihelion will occur on 

 the 20th March, and in about 21,000 years it will again be 

 on the 31st Dec. The aphelion changes of course in the 

 same way, and consequently the northern and southern hemi- 

 spheres alternately enjoy a preponderance of summer. The 

 year 1248 A.D. was that in which the first day of winter cor- 

 responded with the passage of the earth into perihelion, and 

 consequently was the period when the balance of summer in 

 favour of the northern hemisphere was greatest. Up to that 

 date the duration of summer was increasing ; it is now, and 

 has been for 630 years, gradually diminishing. 



Astronomers have not, however, generally considered that 

 these changes, or even those which affect the excentricity of 

 our orbit, would produce any material difference between the 

 climates of the two hemispheres, because, whatever the excen- 

 tricity of our orbit may be, the two hemispheres must receive 

 exactly the same amounts of heat, " the proximity of the sun 

 in perigee, or its distance in apogee, exactly compensating 

 the effect of its swifter or slower motion ;" in other words, 

 the southern hemisphere has a shorter summer than ours 

 because it is nearer the sun, and for the same reason it receives 

 in a given time more heat, so that the two differences neutralize 

 one another. 



