1871.] 



On the Computed Lengths of Waves of Light. 



21 



phosphorus indicate that the carbon groups contained in codeia are in an 

 eminently "unsaturated" condition, being capable of taking up several 

 molecules of HI and of H 2 0, forming compounds not decomposed at 100°, 

 2 equivalents of hydrogen for every C i7 being also added on in every case. 



III. " On a Periodic Change of the Elements of the Force of Terres- 

 trial Magnetism discovered by Professor Hornstein." Com- 

 municated by the Foreign Secretary. Received July 22, 1871. 



[From the Anzeiger der k. Akadenrie der Wissenschaften in Wien for June 15, 1871.] 



Professor Hornstein, of Prague, has communicated to the Imperial Aca- 

 demy of Sciences of Vienna a paper entitled " On the dependence of the 

 Earth's Magnetism on the Rotation of the Sun." 



He shows that the changes of each of the three elements of the force of 

 terrestrial magnetism (declination, inclination, and horizontal force) indicate 

 a period of 26^ days. The periodic change of declination for Prague (1870) 

 amounts to 0*705 sin (<r+ 190° 20'), where ^=0°at the commencement of 

 1870, and #=360° at the commencement of 1871. For Vienna the range 

 is a little larger. The range of inclination is nearly one-third of that of 

 declination, that of the intensity nearly 24 units of the 4th decimal (the 

 intensity in June 1870 was nearly 2*0485). 



Professor Hornstein regards these changes of the earth's magnetism as 

 the effect of the sun's rotation, and by a mean of several determinations 

 finds for the duration of the period 26*33 days. This number may con- 

 sequently be regarded as the result of the first attempt to determine the 

 synodic period of the sun's rotation by means of the magnetic needle. 

 The resulting true periodic time of the sun's rotation is 24*55 days, al- 

 most exactly agreeing with the time of rotation of the sun-spots in the 

 sun's equator deduced from astronomical observations (according to Spcirer 

 24*511 days). 



IV. " Corrections to the Computed Lengths of Waves of Light pub- 

 lished in the Philosophical Transactions of the year 1868." 

 By George Biddell Airy, C.B., Astronomer Royal. Re- 

 ceived October 2, 1871. 



(Abstract.) 



The author, after adverting to the process by which in a former paper 

 he had attempted the computation of the Lengths of Waves of Light, for the 

 entire series measured in the Solar Spectrum by Kirchhoff, from a limited 

 number of measured Wave-Lengths, and to the discordances between the 

 results of these computations and the actual measure of numerous wave- 

 lengths to which he subsequently had access, calls attention to his remark 

 that means existed for giving accuracy to the whole. The object of the 

 present paper is so to use these means as to produce a table of corrections 



