398 Miscellanies. 



at least to furnish a mode by which analogous effects may be pro- 

 duced. The report was accompanied by illustrations, exhibiting the 

 manner in which the results obtained, had been produced. Mr. 

 Green was requested to continue the investigation. 



Feb. 18. — Dr. Geddings presented to the Library, a copy of 

 " Lea's observations on the genus ' Unio,' and a copy of " Genera 

 Crustaceorum et Insectorum," by Latreille. 



Prof. Ducatel submitted to the Academy, a series of experiments 

 undertaken under the direction of Mr. Nicollet, with a view to deter- 

 mine the magnetic intensity at this meridian. He described the 

 manner in which the experiments were performed, and exhibited 

 one of the instruments employed. This consists of a highly sensi- 

 tive magnetic needle, suspended in a glass vessel, by a single strand 

 of silk, perfectly free from any twist, so that no motion may be pro- 

 duced by the torsion of the silk. The intensity was determined by 

 marking the time, during which any given number of vibrations was 

 accomplished, through a given arc of amplitude ; the temperature, 

 barometrical pressure and direction of the wind, being noted. The 

 consideration and further examination of the subject was referred to 

 section first. — Prof. Ducatel, also presented a memoir on a system 

 of meteorological observations, prepared by Mr. Nicollet, and sub- 

 mitted by him to the secretary of war, being the basis of the obser- 

 vations now being made at the different military and naval stations 

 of the United States, by order of the government. 



Feb. 26. — Donations for the library, were received from Mr. Al- 

 exander and Dr. Geddings. From the former a copy of the " His- 

 toire des Oiseaux d'Afrique," by Levaillant, two vols, folio; from 

 the latter, copies of " Lehmann's Jungermannia," " Bait. Med. and 

 Surg. Journal," and " North American Archives." — Mr. W. R. 

 Fisher, read a short memoir on " Amalgams for electrical machines," 

 and exhibited specimens of Mosaic gold, and Baron Krimayer's mer- 

 curial amalgam, prepared by the processes described in the paper; all 

 referred to section first. — Mr. Green exhibited some electro-magnetic 

 phenomena, and accompanied the experiments with an account of 

 the fact first observed by Mr. Ritchie, that the length of time during 

 which an electro-magnet retains its armature, after the connection is 

 destroyed, depends upon the length of its arms. — A donation was 

 received from Dr. Geddings, of a collection of Southern plants ; re- 

 ferred to section 5th. 



