THE SMITHSONIAN INSTITUTION. 21 



attached to the ceihng or some other support. The plate is then 

 made to revolve through an arc of a circle until, by the force of tor- 

 sion, the bar is deflected from a north and south to an east and 

 west direction. It is thus kept in a state of equilibrium between 

 the foi'ce of torsion of the threads, tending to turn its north end 

 round still further to the south, and the magnetism of the earth, on the 

 other hand, tending to bring it back to its north and south direction. 

 If in this position the magnetism of the earth becomes stronger, it will 

 prevail, and the north end of the needle will turn towards the north ; if 

 the magnetism of the earth diminishes in intensity, the force of torsion 

 will prevail, and the same end will move towards the south. These 

 motions, as in the case of the other magnet, are recorded by a beam of 

 light on the paper surface of the revolving cylinder. 



But beside the change of direction of the horizontal needle, a magnet 

 so supported as to be free to take any position, in this latitude will ar- 

 range itself with its north end dipping down towards the horizon. The 

 amount of this dip, or variation, varies also in different places, and at 

 different times; and to record these changes a bar is supported, in the 

 direction of the magnetic north and south, on two knife edges like the 

 beam of a balance. Any change which takes place in the position of 

 a magnet thus arranged, is recorded by a mirror attached to the pro- 

 longation of the axis on which the bar turns. 



It is proposed to keep these instruments constantly in operation, for 

 the purpose of comparing results with other observations of a similar 

 character in different parts of the world ; and also for the purpose of 

 furnishing a standard to which the observations made at various points 

 by the Coast Survey, and the different scientific explorations which are 

 now in progress in the western portions of the United States, may be 

 referred, and with which they may be compared. 



This establishment ought to be supported by government; but as no 

 provision has been made for it, and as the wants are pressing, in 

 order to rende-r more valuable the observations making in other coun- 

 tries as well as our own, the Smithsonian Institution, in connexion with 

 the Coast Survey, has undertaken to commence it. In accordance, 

 however, with the policy which has thus far governed the acts of the 

 Institution, this observatory will be turned over to other hands as soon 

 as other means are found for its support. 



The other sets of magnetic instruments mentioned which have been 

 imported by the Institution are intended to furnish portable magnetic 

 observatories, in which the dip and the intensity are recorded by the 

 pen from direct and personal observations. 



Besides the facts which will be collected by the Coast Survey along 

 our extended sea-board, those which the various exploring expeditions 

 are furnishing, and those obtained by the instruments belonging to the 

 Smithsonian Institution, a large number of records of observations exist 

 as to the position of the magnetic needle in different parts of the United 

 States in past times. A collection of these, and a comparison of them, 

 with more recent observations, would serve to throw light on the 

 changes which have taken place in the course of years. 



There is, also, on record in the Land Office, an extended series of 



