202 THE POPULAR SCIENCE MONTHLY. 



forces what, in its restricted meaning, it did for gravity. That which 

 Borda failed of accomplishing in the measurement of arcs, the pendu- 

 lum realizes in its measurement of time : it multiplies its observations, 

 eliminates its own errors, strikes its own average, and presents to sci- 

 ence the perfect result. In 1851 a crowd of spectators were assembled 

 in the Pantheon of Paris to witness the first performance by the pen- 

 dulum of the new part prepared for it by Foucault, in which, obe- 

 dient to its own inertia, and indifferent to the earth's rotation, it pre- 

 serves the parallelism of its motion an experiment startling, though 

 not wholly unanticipated, and which has made a circuit of the earth. 

 The new contrivance of Zollner promises to indicate changes in the 

 direction of a force as accurately as the common pendulum measures 

 intensity. 



Let us now consider what the physicists of our own day, and their 

 immediate predecessors, have added to their rich inheritance of in- 

 strumental means, remembering all the time that, however impressive 

 from their novelty these additions may be, and however manifold their 

 applications, they have only supplemented the experimental methods 

 which have been described without supplanting them. For the most 

 part, the later devices would be useless without the cooperation of the 

 earlier ones. 



An interesting event in the history of science, which must be known 

 to many of you, has taken place during the current year. In 1824, 

 Poggendorff began to edit the Annalen cler Chemie unci der Physik. 

 Under his supervision 150 volumes have been issued, containing 8,850 

 distinct communications from 2,167 different authors, the 193 papers 

 of H. Rose outnumbering those of any other contributor. The his- 

 tory of physical and chemical discovery during the last fifty years 

 might be written out of the materials treasured up in this single jour- 

 nal. In recognition of the signal service which Poggendorff has 

 hereby rendered to science, his friends assumed the editorship of one 

 volume in 1874, which is called the Jubilee volume (Jubelband). 



In 1826, Poggendorff described, in Volume VII. of his journal, a 

 device of his own invention for observing with exceeding nicety the 

 movements of a magnetized bar. A mirror was attached to the bar 

 and moved with it. From this mirror a beam of light was reflected 

 into a theodolite. This was the origin of the happy thought of am- 

 plifying a trifling motion by making the finger of a long and delicate 

 ray of light serve as a weightless pointer. A few years later, this 

 idea was embodied by the mathematician, Gauss, in an instrument 

 which he called the magnetometer. Since that time it has been con- 

 tinually budding out in new applications, scientific and practical. I 

 need only recall to your recollection the beautiful method of Lissajous 

 for compounding the vibrations of tuning-forks, and tracing in golden 

 lines the curves which are characteristic of different musical intervals 

 and varied phases of vibration. A new chapter has been opened in 



