THE SMITHSONIAN INSTITUTION. 395 



tube, and let the tube be taken such that the distance from the centre 

 of the bulb to the point 32° may be k c, and length of one degree 

 2 k c, then at any temperature 32° -j- t°, the statical moment of the 

 mercury displaced by a small change of temperature, d t, will be 

 w (k c -J- 2 k e t) d t, and consequently the statical moment of the 

 mercury displaced between the temperatures of 32° and 32° -f- t° 

 will be (k c t -f- k e t 2 ) w. Let, now, v be the weight which, placed 

 at an unit of distance from the axis of rotation, will represent the 

 temperature for 1° above 32°, it will only remain to obtain the bulb 

 of such a size that k w=zv. 



The value of v is to be determined, experimentally, by observing 

 the displacement of the register line, or change of scale reading, 

 occasioned by a small weight placed on the magnet at a known 

 distance from the axis of rotation, and comparing it with the scale 

 value of the temperature correction obtained in the usual manner, 

 by horizontal vibrations at different temperatures. 



The slit in the screw before the burner for this instrument is, of 

 course, horizontal, the axis of the record cylinder being vertical. 

 The cylinder rests on a horizontal plate, which is supported on 

 friction wheels, and driven by an arm below the stand connected 

 with the train of a time -piece. 



In several papers in the Philosophical Transactions of the Royal 

 Society, from 1847 to 1852, Mr. Chaeles Brooke, F. R. S., has 

 described the apparatus which is the subject of the foregoing paper, 

 in the preparation of which the former have, of course, been largely 

 drawn from. Those wishing to pursue the subject into its practical 

 application should, by all means, read Mr. Brooke's memoirs, and 

 are also referred to an interesting description of the self-registering 

 instruments of the observatory at Toronto, by Captain J. H. Lefkoy, 

 R. A., in Sillimart s Journal, May, 1850. 



Note. — Since the above article was prepared the instruments have 

 been removed from Washington City to Key West, Florida, where they 

 were put into operation in January, 1860, and where observations 

 will be kept up for some years in connexion with the magnetic obser- 

 vations of the United States Coast Survey, and in direct correspond- 

 ence with a system of similar observations set on foot by the British 

 government at various points in their possessions. Toronto, Canada 

 West, being one of those stations, it was thought advisable to remove 

 the Smithsonian station to a point as far south as practicable, as the 

 long series of corresponding observations obtained at Toronto and 

 Girard College, Philadelphia, in the years 1841 to 1845, left nothing 

 further to be desired in the comparison of stations not any distance 

 from each other. The observatory at Key West, and the objects of 

 the system of magnetic observations at present in progress, will be 

 noted in future reports. 



