of the Flagstaff Observatory. 95 



showed at once that the spot was not altogether free from 

 local disturbances, and that only the portion towards the 

 northern limit of the hill could be made available as a spot 

 on which to erect a house for determining the absolute values 

 of the magnetic elements. In short, I arrived at the con- 

 clusion that the locality might be made use of, provided an 

 addition were made to the ground of the former station ; at 

 the same time I was aware that, to give the magnetical obser- 

 vations their full value, an additional amount of labour 

 would be required, of a nature calculated to keep a perpetual 

 check on the working of the instruments indicating the 

 horary variations. Taking further into consideration its 

 magnificent position for meteorological observations and the 

 propriety of selecting it as a site for an Observatory con- 

 nected with nautical matters, which site is likely to facilitate 

 the labours having for their aim a successful system of me- 

 teorology at sea, I thought myself justified in making an 

 application for the locality as a site for an Observatory. 



The facts which are calculated to illustrate this matter, and 

 to justify my final choice, do not come within the scope of 

 this paper, but will form part of the first printed report ema- 

 nating from this institution ; still I could not venture to give 

 a description of the Observatory intelligible to every one, 

 without introducing the subject by some remarks bearing 

 upon the position chosen for the establishment. 



Proceeding now to the object of this paper, I shall com- 

 mence with the description and examination of the different 

 buildings and contrivances which form the Observatory. 



The essential parts of the magnetic department are as 

 follows : — 



The House for Observation on the horary motions in the 

 three magnetical elements, declinations, or variation of the 

 needle, inclination or dip of the needle, and horizontal inten- 

 sity, is erected, or more properly speaking, sunk into the 

 ground nearly in the centre of the present enclosure, (vide 

 D, plate No. 1) . The hill inclines towards the south-east. By 

 placing this building in the centre, the object in view was to 

 prevent accidental disturbances as much as possible, and still 

 to facilitate the communication with the different other 

 buildings of the Observatory. The foundation is laid 12 feet 

 below the surface, and consists of strong timber ; upon this 

 are resting the double walls of a room containing the instru- 

 ments for horary variations. The ground plan of the room 

 represents a polygram of 16 sides, with a diameter of 12 feet, 



