of the Flagstaff Observatory. 97 



the system of instruments we have adopted, greatly facilitate 

 the determination of this temperature, as they rapidly and 

 thoroughly follow every change which may take place, thus 

 preventing any serious error arising from sudden changes 

 in which the magnets could not thoroughly participate. 

 Different little openings are made through the walls for the 

 purpose of enabling the observer to take bearings towards 

 well-defined distant objects, the azimuths of which, having 

 been once carefully ascertained, will assist us in arriving 

 sooner at a correct value of the declination than would other- 

 wise be possible. One of those little openings brings this 

 house in connection with 



A Little Brick Toiver (vide E, plate No. 1), situated in the 

 astronomical meridian passing through that pier in the abso- 

 lute house which supports the magnetic theodolite. In the 

 centre of this circular room a stone pillar is erected, upon 

 which is placed a universal instrument, which is principally 

 made use of for ascertaining the astronomical meridian, and 

 transferring it afterwards to the magnetic theodolite. The 

 instrument is placed at such an elevation as to prevent the 

 possibility of the observer's view being obstructed when en- 

 gaged in taking terrestrial objects, and the revolving nature 

 of the roof admits of observations being made in any part of 

 the sky, and towards any direction on the horizon. 



To enable the eye to be brought to the level of this instru- 

 ment, a circular flight of steps leads up to it. A second 

 isolated pillar is erected near the steps, upon which are 

 placed the chronometers, to bring them within reach of the 

 observer. 



The three buildings I have just described form the essen- 

 tial part of the magnetic observatory, in addition to which I 

 have only to mention the computation room, to which pur- 

 pose one of the rooms of the dwelling house is appropriated. 



Proceeding to the further arrangements of the institution, 

 I commence with the description of the house containing the 

 transit room and the room for photometrical measurements 

 (vide B, plate No. 1). In reference to the former I hardly need 

 make any remark, as the construction of a transit room is 

 familiar to every one. With regard to the contrivances for a 

 successful management of photometrical measurements, it is 

 chiefly required that the instrument should be placed in such 

 a position as to admit of observations being made over the 

 whole sky. To fulfil this requisite condition the photometer 

 is raised upon a high stone pillar, by means of which its 



H 



