1870] WRITINGS OF JOSEPH HENRY. 463 



present, in ordinary astronomical observatories. For ex- 

 ample, the celestial phenomena which' address themselves to 

 the sense of sight are studied by means of refracting tele- 

 scopes, as are also those of the photographic ray, although 

 this requires a peculiar form of lens, while the heat-ray of 

 lower intensity and the phosphorogenic ray are not trans- 

 mitted by glass ; the former is readily converged to a focus 

 by a lens of rock-salt, and the latter by one of quartz. They 

 may all however as in the case of light, be concentrated into 

 foci by metallic reflectors. 



In regard to terrestrial physics, the phenomena are also 

 various, and the forces by which they are produced are con- 

 stantly changing both in intensity and in some cases in 

 direction. We now know that the magnetism of the earth 

 scarcely remains the same from one moment to another, and 

 that these changes are connected with the appearance of the 

 aurora borealis and electrical discharges in the atmosphere. 

 They also in all probability may ultimately be referred to 

 disturbances produced by external influences, such as those 

 from the sun, moon, and planets. Furthermore, we may 

 now consider the whole earth as an immense conductor 

 charged with negative electricity, of which the intensity is 

 in a continued state of change, and of the laws of which, 

 as well as those of the changes of magnetism, a knowledge is 

 highly desirable. For the proper study of these, continuous 

 self-recording instruments are necessary. 



There is also an important field of observation in regard 

 to ordinary meteorology, such as the changes of the pressure 

 of the atmosphere, and its connection with other phenomena ; 

 of the normal and abnormal winds; isolated currents of the 

 atmosphere, and especially those of a vertical direction ; the 

 radiation of heat from clouds and different terrestrial sur- 

 faces ; the variation of its intensity in ascending above and 

 penetrating below the surface of the earth, &c. In short, 

 the field is almost boundless, and every year reveals new 

 facts in terrestrial and celestial physics, which never fail to 

 furnish new points for investigation to those who are quali- 

 fied by education and endowed by nature for their proper 

 appreciation. 



