385 



age. The diurnal range is the phenomenon in which the so- 

 lar magnetic action is most conspicuously displayed ; and in it, 

 if anywhere, we should expect to find evidences of the periodi- 

 cal action of another luminary. In fact, if the moon co-operates 

 with the sun, in the course of the day, in its effect upon the 

 position of the free magnet, we should expect the range to be 

 greater in certain portions of the moon's age than in others, 

 the separate actions of the two luminaries, as in the analogous 

 case of the tides, at one time conspiring, and at another being 

 opposed. We have, therefore, only to analyze the diurnal 

 range, in reference to the moon's age, and such joint action, 

 if it exists, will be manifested by a variation in the magnitude 

 of the range, whose period is the synodic month. 



"I have, accordingly, calculated the daily range of the de- 

 clination for eleven years, viz., from 1840 to 1850 inclusive, 

 and arranged the resulting numbers according to the moon's 

 age. As there are two periods of greatest easterly deviation 

 of the magnet in each day, the range is double, viz., from 

 7 a.m. to 1 p.m., and from 1 p.m. to 10 p.m.; the mean of 

 these is here taken. When the twelve lunations of each year 

 are combined, and the means of the ranges corresponding to 

 the same day of the moon's age taken, the resulting numbers 

 exhibit a periodical variation, the range being greatest in the 

 first and third quarters, and least in the second and fourth. 

 But as the law is not exhibited with distinctness for each year, 

 even in these numbers which are the means of 24 separate 

 results, I have again combined them in groups of 7 and 8 al- 

 ternately, corresponding to the four quarterly periods. The 

 following Table gives the results. The numbers are the dif- 

 ferences between the mean ranges in each of the four quarters 

 and in the entire month, for each of the eleven years : — 



