96 Description and System of Working 



and the door towards N.E. A skylight in the centre of its 

 pyramidal roof throws the light npon the mirrors underneath, 

 and a flight of steps leads to the surface {vide plate No. 2). 



The stands whereupon the instruments are placed consist 

 of sandstone, and are fixed in such a manner as to make it 

 impossible that any motion could be communicated to them, 

 through the floor or the walls of the house, from wind and 

 other accidental vibrations. The principal object in placing 

 the instruments in this underground building is to prevent 

 sudden changes in temperature, which wouldnecessarily have 

 an influence upon the readings, although compensation for 

 temperature is applied to the deflecting magnets. This object 

 is so well attained that, while the open air shows at times a 

 daily range of 35° F., the greatest range as yet registered in 

 the room is 14° F., and on a common day the range is hardly 

 exceeding 4° or 5° F. 



The stone pillar carrying the tubes for the different instru- 

 ments is placed nearly in the centre of the room, and a second 

 pillar is placed outside, for the purpose of putting thereupon 

 a collimator, for the purpose of checking the unchanged po- 

 sition of the tubes and mirrors intended for the registration 

 of the instruments. 



The material of this house, as well as that of the next one, 

 is wood, and the joinings, nails, &c, are of copper and brass, 

 iron having been carefully avoided. 



The House {vide C, plate No. 1) for measuring the 

 absolute values of the magnetic elements, is situated near the 

 northern boundary of the enclosure, in a north-west direction 

 from the former one. 



The ground plan of this building is a parallelogram ; the 

 axis of the same parallel with the longer side is in the mag- 

 netic meridian, and the entrance is towards the east. 



Two square stone pillars are erected in a similar way to 

 those in the house before described. 



The instruments put upon these stones are a magnetic 

 theodolite and a dipping circle, which receive the light neces- 

 sary for reading them by a large skylight in the roof of the 

 building. Although great care was taken in erecting this 

 room, the construction was not a matter of so difficult a na- 

 ture, as changes in temperature are of no consequence in 

 reference to the observations made therein, because such 

 changes must be observed and brought into calculation when 

 reducing the original readings. 



The small dimensions of the magnets in use, according to 



