Photography at Greenwich Observatory. 27 



The saving of labour resulting from the introduction of the 

 automatic registration of the instruments is most considerable, 

 involving the release of two assistants at least, and an entire 

 absence of night work. The observations are also more exact, 

 and being continuous instead of intermitting, is, in the case of 

 instruments whose changes are so incessant and capricious 

 in their nature, an advantage that cannot be estimated too 

 highly. 



The system thus inaugurated at Greenwich has been adopted 

 at the observatory of the British Association at Kew, and will 

 no doubt be used in the cases where instruments are supplied 

 from that establishment; and at Oxford the meteorological 

 observations are registered in a similar manner. 



It only remains to mention that, during the last few months, 

 an interesting addition has been made to the photographic 

 recording apparatus at Greenwich. In order to detect electri- 

 cal currents in the earth, two wires are stretched, the one to 

 Dartford and the other to Croydon, passing into the earth at 

 both ends, and having galvanometers included in the circuits ; 

 but no batteries are employed. Each magnet has a mirror 

 mounted, by which a spot of light is reflected to a photographic 

 sheet, mounted on a cylinder made of ebonite, and the curves 

 are thus registered as in the other cases. The apparatus has 

 only been mounted about two months, and it is too soon to 

 infer anything from the results ; but it may be mentioned that 

 the Dartford current, running E. and W., is stronger than the 

 Croydon, which runs from N. to S. ; that both are stronger than 

 was expected; and that the trace of the Dartford current 

 occasionally bears a strong resemblance to that of the declina- 

 tion magnetometer. Such indications cannot fail, when ob- 

 served sufficiently long and properly discussed, to throw light 

 upon the causes of terrestrial magnetism, and add another to 

 the benefits resulting from the operations of Greenwich 

 Observatory. 



