ON COMPARING AND REDUCING MAGNETIC OBSERVATIONS. 40 
VI. Remarks on Magnetic Reductions. By Senhor Caretto. 
The method of the separation of the disturbances of the readings of 
the bifilar and the vertical force, of which I have sent a réswmé, and the 
examples of the calculation of the last year, seems practical enough to me, 
although it will give some trouble. It has, however, retained the prin- 
cipal fault, the arbitrary nature of the quantity which constitutes the 
disturbance. To practise this method upon the hourly observations of 
the vertical force is not, I think, more difficult than upon the bifilar. 
With respect to the vertical-force instrument of this observatory, I do 
not find it very inferior to the bifilar, except for some months on two or 
three occasions, during which the equilibrium position was not good, for 
the curves had shown a jumping motion; otherwise it has answered 
almost as well as the bifilar, notably in the three or four last years, 
where the coefficient of temperature is very much reduced by the adop- 
tion of a contrivance to compensate the effects of temperature. 
Our photographs already embrace twenty-one complete years. The 
meteorological work, and the care connected with the administration of 
the observatory and the meteorological stations absorb the greater part 
of our time. The reductions of the magnetic observations are very much 
behind, and it would be difficult to advance simultaneously all the 
elements as they should be; therefore I think that it would be convenient 
to establish an agreement upon the work which by preference it is de- 
sirable to accomplish, and for what period for a general comparison. 
With regard to No. 7 of the suggestions of Sir J. H. Lefroy, I am en- 
tirely of his opinion, and I will add my ideas upon some researches that 
I think would throw light upon the causes of the disturbances. 
1. Ina paper by Messrs. Capello and B. Stewart (‘ Proc. R.S.,’ January 
28, 1864) upon a first comparison of the disturbances at Kew and at 
Lisbon, we have recognised that of the little and abrupt disturbances of 
three to five minutes’ duration (which are called peaks and hollows), and 
which are seen simultaneously in the three curves, those of the dech- 
nation and of the vertical force are in the same direction at Kew and in 
the contrary direction at Lisbon; that is to say, while the north end of 
the declination needle at Lisbon goes towards east the same end of the 
vertical-force instrument dips. The contrary happens at Kew, the north 
end of the declination needle going towards east, while the same end of 
the vertical-force raises itself. 
Again, on the other hand, we have also recognised the agreement of 
the behaviour of the peaks and hollows of the declination curves at Kew 
and at Lisbon. Thus one vertical peak at Lisbon corresponds always to 
a hollow at Kew, and vice versd. It would be interesting (1) to extend 
this research upon peaks and hollows further; that is to say, between 
more distant observatories, employing the utmost rigour possible in the 
time-measures, in order to recognise if the times of the appearances of 
the peaks are absolutely the same, or if there is a sensible difference in 
the most distant observatories. (2) Again, we ought to look in some 
observatory immediately between Lisbon and Kew in order to see if the 
vertical-force peaks correspond sometimes to the peaks, sometimes te 
the hollows of the declination. 
2. For the study of the disturbances I think it would be necessary 
that each observatory furnished with magnetographs should make pro- 
