140 M. Secchi on Earth-Currents, and their 



tions. The light proceeding from the analyzer was sent through 

 a tine grating [Gitter) instead of through a prism ; the dark bands 

 appeared in the spectra produced by diffraction. The bands are 

 not equidistant, but approach one another quite close at the 

 violet end. If the reciprocal squares of the sines of the devia- 

 tions of the bands be taken, they will be found to be in arith- 

 metical progression. The former law is thereby afresh supported. 



This opportunity was also taken to measure the lengths of 

 undulation of the following Fraunhofer's lines, viz., A, a, B, C, 

 D, E, b, F, G; and the following values in millionths of a milli- 

 metre were found: 759-8, 717-8, 687*2, 655*8, 589'4, 525*3, 

 518-7, 484-3, 430*2. 



For the angles of rotation of the lines B, C, D, E, F, G, H, 

 these values were obtained: 15-55, 17'22, 21*67, 27*46, 32'69, 

 42*37, 50*98 degrees. The constant part in the dispersiou- 

 formukvis — 1*697*, the part divided by the square of the length 

 of undulation is +8*1088. 



The foregoing phenomena are well adapted for exhibition by 

 projection. The following arrangement answers : — Heliostat, slit 

 in window-shutter, polarizing prism, quartz column, analyzing 

 Nicol, lens of l 1 metre focus, prism in minimum deviation, or 

 grating directly against the lens, distance of the latter from the 

 slit 3 metres, screen where the image is distinct. 



XV. On Earth Currents, and their relation to Electrical and 

 Magnetic Phenomena. By Father Secchi f. 



A RECENT communication by M. Matteucci on Earth Cur- 

 rents, in which he mentions my researches on the same 

 subject, affords me an opportunity of presenting the results 

 which I have obtained by comparing observations of magnetized 

 bars and of the atmospheric electricity. The limits of this Note 

 prevent my entering upon the details of these observations, and 

 I shall confine myself to the principal results. 



But before presenting these comparative results, I think it 

 well to resolve some difficulties on the origin of these currents. 

 M. Matteucci's investigation has proved directly that they are 

 not the effect of the chemical action of the terminal plates. I 

 have arrived at the same conclusion in an indirect manner by 

 changing the terminal plates, and finding that the current of the 

 plates, which is strong enough for short circuits, becomes very 

 feeble for the resistance of conductors when the circuit is long 

 enough, and that in other cases the direction is often opposite to 

 that of the electromotive force of the plates. My researches, 

 moreover, were not directed towards the absolute value of these 



* There must be some mistake. The constant term ought assuredly to 

 be positive. — G. G. S. 

 t Comptes Rendus, June 27, 1864. 



