680 
SIR Gr. B. AIRY ON RESULTS DEDUCED FROM THE MEASURES 
the building, except in 1864, when the ground below the east, west, and south arms, 
was excavated, in order to obtain positions for the three fundamental instruments 
in which the severity of the temperature-changes would be much diminished. Small 
accidental interruptions of observations occurred in 1847, January, and 1861, July. 
The interest taken in the subject of terrestrial magnetism in the first half of this 
century was occasioned principally by the enterprise of Gauss and other German 
philosophers. Magnets were, therefore, established at the Royal Observatory, 
furnished with apparatus adapted to eye-observations corresponding to those of Gauss, 
and some observations were made in concert with the Germans. The observations 
to the end of 1847 with these instruments were made entirely by eye ; the instruments 
(magnets 2 feet in length) being furnished with small plane reflectors, to which 
telescopes were directed, and by which fixed marks were observed. The observations 
were made at every two hours, day and night; proper precautions were taken for 
assurance of the general accuracy of the times of observation ; and I do not doubt 
that the results interpreted from these observations are each as good as those derived 
from the succeeding system; though the intervals of two hours were longer than 
I could wish. But the labour was great, and (as measured by the interruption of 
assistants’ work) was expensive. 
The idea of self-registration by photography of the movements of the instruments 
(an idea little entertained before that time) then suggested itself; and, at the 
Cambridge Meeting of the British Association in 1845, it was proposed for con¬ 
sideration of the Council of that body, that the Government should be requested 
to promote, by offer of a pecuniary reward, the construction of a photographic self¬ 
registering instrument. This proposal was adopted by the Council; letters were 
addressed by Sir John Herschel, President of the Association, to Her Majesty’s 
Treasury, and by myself to the Admiralty ; and, finally, the assistance of Dr. Charles 
Brooke was secured, for forming an efficient apparatus, and making the necessary 
chemical arrangements adapted to our wants. 
I do not propose here to describe the photographic recording apparatus. Allusions 
to the construction will be found in the Introductions to the Greenwich Observations 
for successive years, and especially, and in great detail, in the introduction to the 
volume for 1847. The only alteration that was made in it for several years is the 
following. Mr. Brooke had conceived that advantage would be gained by making 
the recording barrel to revolve in twelve hours. But this caused a doubling of the 
! 
