ON A PHYSICAL OBSEEVATORY. 



By Pkof. Joseph Henry. 



Smithsonian Institution, 



Washington, December 29, 1870. 



My Deah Sir : Yours of the 28tb of November was duly received, but 

 I delayed answering it until the pressure of business which accumulated 

 during my absence should have somewhat subsided, and, also, that I 

 might receive the plans which you mention. I am now gratified in 

 being able to inform you that my visit to Europe was both pleasant 

 and profitable, and that I have returned much iiai^roved in health and 

 with enlarged views as to the present state of science in the Old World. 



While abi-oad I gave special attention to physical observatories, of 

 which there are several in England and on the continent, although 

 there is no one which fally realizes my idea of what such an establish- 

 ment ought to be. 



A physical observatory is one the primary object of which is to in- 

 vestigate the physical piienomena of the earth and the heavenly bodies 

 in contradistinction to an ordinary astronomical observatory, which is 

 principally devoted to the observation and discussion of the motions of 

 the planets, and the determination of the relative positions of the fixed 

 stars. Of the latter kind but one or two are needed in any country, 

 and as these require a numerous corps of observers and computers they 

 can only be supported by appropriations annually from a national gov- 

 ernment. The United States Observatory at Washington is of this 

 character, and, including all expenses, requires an annual appropriation 

 of at least $50,000. The labors of such an observatory are indispens- 

 able to the advancement of the science of theoretical astronomy, and its 

 application to geodesy and geography. 



The establishment I would advise you to found is of the character of 

 the one first mentioned, namely, a i)hysical observatory, the jmncipal 

 object of which would be, as 1 have indicated, to investigate the nature 

 and changes of the constitution of the heavenly bodies; to study the 

 various emanations from these in comparison with the results of experi- 

 ments, and to record and investigate the difterent phenomena which are 

 included under the general term of terrestrial i>hysics. 



A wide field has been opened for the stndy of the nature of the sun 

 and other heavenly bodies by the application of the spectroscope, dif- 

 ferent modifications of the telescope, and other lately invented appliances. 

 We now know thatthesunis undergoing remarkable changes, the charac- 

 ter of which can only be ascertained by the results of accurate observa- 

 tions compared with those of experimental investigation. The observer 



