80 A. M. Mayer on the Magnetic Decimation 



least principally — an effect of an action which has done its work, 

 we must look more minutely to the changes in area of disturb- 

 ance. For, if the connection exist, it will show itself principally 

 during the periods of rapid increase or decrease in area of solar 

 disturbance and not when a maximum or minimum area has 

 been reached and remains constant. 



These remarks are illustrated by measures on the penumbral 

 areas made before and after the auroral displays of the Uth 

 Oct. and of Oct. 24 and 25. 



The declination range on Oct. 11, three days before the au- 

 rora, was 11' -23 and increased with the area of solar disturbance 

 to the 14th when it reached 18'-43 and the area-number on this 

 day equalled 17075; but on Oct. 15th, the day after the 

 aurora, the declination rsLUge had fallen to ll'-52 while the area- 

 number had reached 18875, showing an increase of 1800 from 

 noon of the 14th to noon of the 15th, during which interval the 

 aurora broke forth. On the 16th the range was ll'-52 and the 

 area-number 15795. 



On Oct. 21, three days before the auroral display of Oct 24, 

 the declination range was only 3'*94 ; the area-number 13748. 

 On Oct. 22, decl. =12' "75 ; area-number =14732. On Oct. 23, 

 decl. =ll'-53 ; area-number =15991. On Oct. 24, the aurora {s& 

 I subsequently learned from the newspapers) was observed at 

 Cincinnati and Cleveland at 5 a. M. I observed myself the 

 great disturbance a few minutes before 8 a. M., when I began 

 my magnetic observations for obtaining the max. B. elongation. 

 At 11 A. M. the measure of solar disturbance gave 169^ 

 and the observed declination range on that day equalled 44'79. 

 On Oct. 25, it reached 54' -63.* On Oct. 26, the declination range 

 equalled 14' -26 and the area-number had declined to 11959. 

 Here also we observe that the aurora appeared during the 

 period of increase in the penumbral surface. , 



These facts, brought forward as illustrations of my remarks ana 

 not as proof of the physical connection of disturbed solar area 

 and magnetic perturbation, — for that will require almost constant 

 coincidences, — show at once wherein lies the difficulty of sucn 

 a research ; a difiiculty which seems not to have been appre- 

 ciated by those who have given their energies more to a mathe- 

 matical than to a physical analysis of the connection o^ j-^^ 

 phenomena; and that is, the frequent impossibility of fixiDo 

 the time at which such a change in area took place. It is very 

 evident, in the first example, that it took place between noo° 

 of the 14th and noon of the 16th, but whether it comcidea 

 with the auroral display and magnetic perturbation could only 

 have been determined by means of solar measurements, 



only slightly deflected. 



