MAGNETIC ORSERVATORY. 385 



DESCRIPTION OF THE MAGNETIC OBSERVATORY 

 AT THE SMITHSONIAN INSTITUTION. 



[This observatory is supported at the joint expense of the Smithsonian Institution and 

 the Coast Survey. The description h< re given is by J E Hilgard, esq , who had charge 

 of the observations ] 



The instruments of this observatory are designed to give, by means 

 of photographic self registration, a continuous record of the varia- 

 tions in the direction and intensity of the earth's magnetic force. 

 They are similar to those employed for the same purpose at the 

 Greenwich, Paris, and other European observatories, and also at 

 Toronto, on this continent, and consist of a freely suspended decli- 

 nation magnet, a hifiXar magnetometer, after Gauss' design, and a bal- 

 anced magnetometer, after Lloyd; each provided with the apparatus 

 for photographic self-registration, invented by Mr. Chi rles Brooke, 

 under whose direction the instruments were constructed, as has been 

 heretofore stated in the reports of the Secretary of the Smithsonian 

 Institution. 



The general plan of the photographic registry is as follows: each 

 magnet carries a concave mirror, (speculum,) in one of the conjugate 

 foci of which is placed a source of light; a pencil of rays is re- 

 flected by the mirror, and concentrated in the other focus upon a 

 sensitive sheet of paper wrapped about a cylinder, which revolves in 

 a certain time about an axis parallel to the direction of the changes 

 of the magnets. The point of light traces a curve corresponding to 

 the movements of the magnet; the angular value of the ord mates 

 of the curve is, of course, measured by a radius equal to twice the 

 distance of the mirror from the cylinder; or, in other words, we have 

 an index for the movements of the magnet equal to twice that 

 distance; which index or tracer, being without inertia, and acting 

 without flexure or friction, is vastly superior to any mechanical 

 arrangement by which such registry could be effected. 



The magnetic observatory is contained in a small building expressly 

 constructed for the purpose, situated about 150 yards southeast from 

 the south entrance of the Smithsonian Institution. No iron \\i ; -ever 

 has been used in its construction- — copper or brass being employed 

 where metal was necessary. The instrument-room, occupying a - pace 

 of twelve by sixteen feet, is wholly beneath the surface of the j2 1 mind; 

 it is closed in by an nine-inch brick wall, between which aid the 

 outer wall of two feet in thickness a space of two feet intervei ss on 

 three sides. This space is covered in at the surface of the ound 

 by a coping, with a grating at two corners, and communical t the 

 bottom with the instrument-room by small air passages. A nsid- 

 erable uniformity of temperature is thus secured, as the an is to 

 descend about nine feet before it enters the room. A the 



north side of the room, communicating with a chimney, sei n the 

 requisite ventilation. On the fourth (the south) side of the m is 

 the door and an ante-chamber, into which the stairs dese< and 



where there is a trough with supply of water from a cistern per- 



forming the rinsing and washing necessary in the photo oitic 

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