NATURAL PHILOSOPHY. 133 



A table is given showing the number of days included in the maxi- 

 mum and minimum portions of each mean period for the years 1818 to 

 18<30, and the number of exceptional days, or tiiose on which during 

 the maximum part of the period the temperature was beioiv, and 

 during the minimum part <tinn\!, the mean value. From this table it 

 appears that out of a total number of 105 days of maximum, only 14 

 were exceptional ; and out of a total of lb'i days of minimum, the 

 number of exceptional days was only 1G. The mean gives a ratio 

 almost exactly as 1 to 11. Considering that the values of the period 

 in the different years are only approximate, this result may be re- 

 garded as affording satisfactory proof of the existence of a variable 

 period of temperature. 



At St. Petersburg the average temperature of the warmer half of the 

 period is not less than 3 greater than that of the cooler hall'; and as 

 this difference of temperature is repeated at least 12 times in every 

 year, it must necessarily exercise a powerful modifying influence over 

 many meteorological phenomena. 



Another period of change having a mean duration of rather over 18 

 months, is then referred to. The author was first led to it from a discus- 

 sion of the Greenwich Magnetical Observations, for the years 1848 to 

 1859 ; and it has been confirmed by the results of a discussion of tem- 

 perature observations, made at Brussels in Europe, and at Yakoutsk, 

 in Asia. It is obvious that this period will, at times, interfere sensi- 

 bly with the shorter one, and it is probable that some of the cases 

 which have been called exceptional may be due to this interference. 



With regard to the probable cause of the variability of the short 

 period the author suggests the following hypothesis : 1st. That a 

 ring of nebulous matter exists differing in density or constitution in 

 different parts, or several masses of such matter forming a discontin- 

 uous ring, circulating round the sun in a plane nearly coincident 

 with the plane of the ecliptic, and at a mean distance from the sun, of 

 about \ of the radius of the earth's orbit. 



2d. That the attractive force of the sun on the matter of this ring 

 is alternately increased and diminished by the operation of the forces 

 which produce the solar spots, being greatest at the times of minimum 

 solar-spot frequency, and least when the spots are most numerous. 



3d. The attractive force being variable, the dimensions of the ring 

 and its period of revolution round the sun will also vary, their maxi- 

 mum and minimum values occurring respectively at the times of 

 maximum and minimum solar-spot frequency. 



In reference to the nature of the varying attractive force, it is not 

 improbable that the matter of the supposed ring may be highly dia- 

 ma< r netic, and being much nearer to the sun than any of the known 



plac 



wheiTinterposed between the earth and the sun, it may act not only 

 by reflecting and absorbing a portion of the light and heat which 

 would otherwise reach the earth, but also by altering the direction of 

 the lines of magnetic force. The changes of temperature at the sur- 

 face of the earth will thus be due partly to differences in the amount 

 of heat received from the sun, and partly to changes in the move- 



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