1864.] 429 [Chase. 



only those Days of Great Magnetic Disturbance, in which Records 

 were made by the three Instruments." 



Theoretical value of M, —.000287 



Observed " " —.000257 



Probable error, 000068 



Tables V and VI, exhibit an approximation to the proportion, 

 C : T : : T : M, but the approximation does not come within the 

 limits of probable error. As no attention is paid in these two Tables 

 to the positive and negative signs, we could not reasonably expect so 

 satisfactory results as in Tables II and III. 



"Table VIII. Sums, without regard of sign, of CoefiBcients of 

 Magnetic Irregularity (in terms of Horizontal Force) for each Year, 

 from 1841 to 1857, including all Days of Record of Great Magnetical 

 Disturbance." The proportion C : T : : T : M, gives for M a 



Theoretical value, 001218 



Observed " 00120.3 



Probable error, 000066 



" Table IX. Sums, without regard of sign, of Coefficients of 

 Magnetic Irregularity (in terms of Horizontal Force) for each Year, 

 from 1841 to 1857, including only those Days of Great Magnetic 

 Disturbance, in which Records were made by the three Instru- 

 ments." 



Theoretical value of M, 0011.37 



Observed " " 001150 



Probable error, 000081 



In addition to these numerical cuincidences, the fljllowing points 

 in Mr. Airy's paper appear to me to be specially noteworthy. 



1. " The Aggregate for the Westerly Force .... (taken in com- 

 parison with that for the Northerly Force), appears to show that on 

 the whole, the direction of the Disturbing Force is 10° to the east 

 of south ;" p. 628. This indicates a line of mean disturbance about 

 midway between the magnetic meridian (which, at London, is about 

 N. 24° W.), and the solar meridian, or midway between the meri- 

 dians of decussation in the two sets of principal spirals, to which I 

 have referred. 



2. "Sometimes two waves in one direction correspond nearly with 



one in the other direction A moi'e frequent relation appears to 



be, that the evanescence of one wave corresponds with the maximum 

 of the other;" p. 635. 



3. "The most striking particulars in the last line (of Tables VIII 

 and IX) are the following : 



