1872.] On the 26-day Period of the Earth's Magnetic Force. 417 



With regard to the application of the photographs, I need not say much 

 at present ; it is evident that the use of gratings would become more general 

 if the cost were reduced in the proportion, say, of 20 to 1, more particularly 

 if there were no accompanying inferiority of performance. 



The specimens sent with this paper are both capable of showing the 

 nickel line and give fairly bright spectra, but they must not be supposed 

 to be the limit of what is possible. From their appearance under the mi- 

 croscope I see no reason to doubt that lines 6000 to the inch can be copied 

 by the same method, a point which I hope shortly to put to the test of 

 experiment. 



II. " On the 26-day Period of the Earth's Magnetic Force." 

 By J. A. Broun, F.R.S. Received June 3, 1872. 



The Astronomer Royal's communication to the Royal Society on this 

 subject (supra, p. 30S) has drawn my attention to the investigation made 

 by the Director of the Prague Observatory. 



Dr. Hornstein having remarked the uncertainty of the result for the 

 time of the sun's rotation as deduced from the movement of the spots in 

 different zones on its surface, thought it would be desirable to consider 

 other phenomena associated with the sun's rotation ; and the apparent 

 connexion of the frequency of the solar spots with the amount of the mag- 

 netic oscillations induced him to seek for a period in the daily mean values 

 of die magnetic elements. For this end he grouped the daily means of 

 observations made at Prague and Vienna in 1870 in periods varying from 

 16 to 28 days; and subjecting the resulting means to calculation for the 

 term 



a sin (0-f c) 



in the usual formula of sines, he considered the most probable of the 

 periods to be that for which a had the greatest value. 



The most carefully calculated result from the declination at Prague 

 gave 26" 7 days nearly, but a graphic interpolation from the same obser- 

 vations indicated 26*2 days; the declination for the same year at Vienna 

 giving by calculation 26 '4 days, while the inclination at Prague showed 

 26 days. 



Dr. Hornstein concludes that the mean of these four values, "26'33 

 days, may be considered provisionally as the most probable value [of the 

 period], and as the result of the first experiment to determine the time of 

 the sun's synodical rotation by means of the magnetic needle, The true 

 time of the sun's rotation derived from this = 24*55 days, almost exactly 

 coinciding with the value found by Spoerer from astronomical observations 

 for the time of rotation of the sun's spots in the equatoreal zone " *. 



In a letter to the late Sir David Brewster, published in the * Philosophical 



* Sitzungsb. der k. Akad. d. Wissensch. zu Wien, Band lxiv. S. 73. 



