EARTH'S MAGNETISM TO THE SOLAR AND LUNAR PERIODS. 107 



winter months is 0'001040. It appears to me, however, that it is exaggerated, 

 owing to the curious fact, that the chief negative disturbances in 1844 occmTed 

 about the time of full moon. 



19. It has not appeared to me necessary to verify this law by the result of ano- 

 ther year's observations. Each of the winter months of 1844 shews the facts as 

 completely as the mean ; in the summer months, the result is not so evident. It 

 would appear as if the efifect of phase swallowed up the effect of declination in 

 the winter, while the reverse occurred in summer. I have projected the means 

 of the horizontal intensity for each day from January 4th till April 3d, 1844, 

 including three synodical periods. (See Curves, No. 8, Plate IV.) In each period 

 the curve shews the facts most completely ; and the lunations in September, Oc- 

 tober, November, and December, shew them pei-haps better. The periods of 

 greatest N. and S. dechnation, and of the syzygies, are indicated on the curves, 

 the open being full moon. There are several curious facts, in connexion with 

 the observations projected, which I cannot enter fully into at present; I may 

 remark, however, the appearance of a weeldy period. No observations being made 

 on Sundays, breaks occur in the cm-ve, where the intensity for these days should 

 appear. A great disturbance spoils somewhat the form of the curve in March ; 

 the point belonging to the 29th of March would occur about 1^ inches below 

 the margin. 



20. The law of the variation of the earth's horizontal intensity with the moon's 

 phase, is one productive of many speculations. There is an evident connexion of 

 the great dim-nal variations of the horizontal intensity, with reference to the sun's 

 hour angle ; there is also a strongly mai-ked connexion between the diurnal 

 range and the sun's altitude ; and we have a certain connexion between the sun's 

 declination and the annual period. Are these connected with the heating power 

 of the sun, its light, or its magnetism ? Sir John Hebschel has stated, that, as 

 the sun's rays shine with their whole force on the moon's surface for a fortnight, 

 unstopped by an atmosphere, the heat of the surface must be much more intense 

 than that of a tropical summer ; while, after the next fortnight, the cold must be 

 more severe than that of a polar winter. M. Cournot, the French translator of 

 Sir John Herschel's Treatise on Astronomy, opposes this opinion, and argues 

 that, as there is no atmosphere to prevent radiation, our knowledge of the laws 

 of radiant heat would lead to the conclusion, that the temperature of the 

 moon's surface would differ little at the times of new and full moon.* Supposing 

 Sir John Herschel's opinion accurate, if we could conceive the moon as a mag- 

 netic body acting by induction on the earth, then, according to our knowledge of 

 the effect of heat on magnetic bodies, its intensity would be greatest when it was 

 coldest, and least when warmest : the period of greatest cold we should expect 



* Quoted by M. Francoeuk ; Uranographie, p. 97- 

 VOL. XVI. PART II. 2 E 



