ON “QUIET” DAYS DURING THE ELEVEN YEARS 1890 TO 1900, ETC. 
839 
I observation, by allowing for any change of curve ordinate and for any change ol 
magnetograph room temperature that may have occni'red meantime. Combining the 
observed values of I and of H, as corrected, one obtains an al)Solute value for V, and 
so can institute a comparison with the V curve and determine the value of its 
base line. 
The hourly values of inclination are obtained by combining corresponding hourly 
values of H and V as given by the curves. The same inclinometer has been in use 
throughout the eleven years. The needles originally employed were damaged in 
1899 and had to be replaced. The needles then introduced had been previously 
compared with the discarded pair, and no apprecialde difference was detected. 
§ 5. The methods of reduction just described liad not lieen strictly adhered to in 
some of the earlier years. For instance, no satisfactory allowance had ])een made 
prior to 1894 for change in H between the hours of the H and I ol)servations. This 
had the effect of depressing the values of V and I by something like 30y and F 
respectively. Trouble also arose from an addition made to the Observatory bniilding 
in 1892, introducing various discontinuities into the curves, which liad not been 
altogether successfully dealt with at the time. Keference to the origiiial records, 
however, enabled these defects to be removed fairly satisfactorily, and when this was 
done the more striking irregularities apparent in the secular changes of V and I 
between 1890 and 1894 mainly disappeared. There still remained i-ather a prominent 
discontinuity in the values for 189(3. On examining into the matter, I found that in 
March, 1896, an old weak pair of bar magnets used for stroking the dip needles had 
been replaced by a much stronger pair. The secular change obtained hy comparing 
the results for the six months immediately following the introduction of the new Ijar 
magnets with the results for tlie corresponding six months of the previous year 
amounted to only l', whereas by comparing results from the six months imme¬ 
diately preceding the change with the residts from the corresponding months of the 
previous year, and Ijy comparing results from the six montlis immediately following 
the change with the residts from the coiTesponding months of the suljsequent year, 
one got values for the secular change which were in good agreement and averaged 
2''8. This seemed to point pretty conclusively to a sudden discontinuity amounting 
to about l''8, and simultaneous to all appearance with the introduction of the new 
har magnets. For our present purpose it is immaterial wbetbei' the cause of this 
discontinuity was the introduction of the new bar magnets, or something else occur¬ 
ring approximately at the same time, so long as it is admitted that it is legitimate to 
apply a correction of the amount F'8 indicated above. (hie’s ap})ioval of any such 
correction must largely depend on one’s estimate of the knowledge and judgment of 
the person who decides to fip])ly it. It may be added, however, as evidence in its 
favour that it wiped out discontinuities in the 1896 values alike of A", I, and T. The 
consequence of the several corrections introduced w^as to alter the mean annual values 
of the elements as published in the annual ‘ Ilejjorts ’ to tbe following extent:— 
2 X 
