137 
which I offered to the Royal Irish Academy, I should be 
glad to learn that it has proved acceptable, however trifling. 
** As I have always remarked that scientific discoveries 
are long known either in Ireland or in France, before they 
travel from one country to the other, I think it may be 
gratifying to you to become acquainted with a few parti- 
culars of my voyage. 
‘* We made three observations every hour, day and night, 
from the 11th of February to the end of March. The in- 
struments examined were, Ist, the horizontal magnetic 
needle ; 2nd, the thermometer; 3rd, the barometer, @ 
niveau constant; 4th, Saussure’s hair hygrometer. The 
direction and force of the wind, state of sky, &c. were also 
observed. 
“The variations of the needle were far greater than in 
Paris; the hours of maxima and minima agreeing very well 
together, except near the time of the sun’s passage through 
the zenith of Olinda, (lat. 8°. 0’. 58”. long. 2°19" W.) I 
then remarked two important phenomena; Ist, the extreme 
digressions, A. M. in one sense, became P.M. in the same 
direction, when the sun began to culminate in the other 
hemisphere, after passing through the zenith. 2ndly, this 
remarkable alteration was preceded by sudden and perma- 
nent changes in the variation of the needle, amounting to 
more than one degree. The first of these variations took 
place twelve hours after the sun’s centre had reached a 
declination equal to the latitude of the place. All these 
sudden changes were accompanied with feeble storms con- 
fined to one small part of the horizon. Referring to the 
problem as laid down in the Annuaire du Bureau des Longi- 
tudes for last year, it would seem that the transition from 
the daily variations belonging to the northern hemisphere to 
those which characterize the southern part of our globe, is 
not on the magnetic equator, but depends on the sun’s path 
in the heavens ; and the sun acts here not as a source of 
02 
