256 



Sir G. B. Airy. Results from Measures of [Nov. 26, 



observations were made in concert with the Germans. The observa- 

 tions 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 

 thao 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 intruments (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 consideration 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, Pre- 

 sident 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 curves traced on the photographic 

 paper which is wrapped upon the barrel ; and the inconvenience 

 produced by this doubling was soon found to be so great that I 

 thought it necessary to alter the clock-work so as to produce a 

 revolution of the barrel in twenty-four hours. The records of the 

 change of western declination from the north, and of the change of 

 horizontal force, are made on the same barrel ; and by alterations, 

 first suggested by myself about 1881, and carried out by the 

 present Astronomer Royal (then Chief Assistant), the two curves are 

 now so traced that the simultaneous' records of the two instruments at 

 all times are in close juxtaposition. 



While the observations were made by eye, at every two hours, the 

 means of the two-hourly readings were adopted as base for the day, 

 and the excess of each two-hourly reading above the mean was adopted 



