DISCOURSE OF W. B. TAYLOR. 271 



and afterward independently, as well as mediately in directing his 

 own pupils and assistants. In 1833, he devised an improvement 

 on Wollaston's mechanical scale of the chemical equivalents, for the 

 benefit of his pupils in chemistry : a contrivance which was much 

 used and highly appreciated at the time. 



The suggestion had been thrown out by more than one astron- 

 omer, that carefully timed observations on characteristic meteors 

 or "shooting-stars" might be made available for determining 

 differences of longitude between the stations of observation.* For 

 many years however the proposition had been generally regarded 

 as offering rather a speculative than a practical method of solving 

 a problem of so great nicety. Henry in concert with his brother- 

 in-law, Professor Alexander, and with his friend Professor Bache, 

 determined to ascertain by actual trial the availability and value of 

 the system. On the 25th of November, 1835, Professor Bache 

 observing at his residence in Philadelphia (assisted by Professor J. 

 P. Espy,) simultaneously with Professor Henry and Professor 

 Alexander, at the Philosophical Hall at Princeton, they obtained 

 seven co -incidences : the instant of disappearance of the meteor 

 being in each case selected as the most accurately attainable epoch. 

 These seven observations (whose greatest discrepancies amounted to 

 but a trifle over 3 seconds) gave a mean result of 2 minutes 0.61 

 second (time longitude), differing only one second and two-tenths 

 from the mean estimate of relative longitude arrived at by other 

 methods, f 



In 1840, Henry gave an account of "electricity obtained from a 

 small ball partly filled with water, and heated by a lamp.' 



"t 



*"The merit of first suggesting the use of shooting-stars and fire-balls as signals 

 for the determination of longitudes is claimed by Dr. Olbers and the German 

 astronomers for BENZENBERG, who published a work on the subject in 1802. Mr. 

 Bailey however has pointed out a paper published by Dr. MASKELYNE twenty 

 years previously, in which that illustrious astronomer calls attention to the sub- 

 ject, and distinctly points out this application of the phenomena." This was 

 dated Greenwich, November 6th, 1783. (L. E. D. Phil. Mag. 1841, vol. xix. p. 554.) 



^Proceed. Am. Phil. Soc. Dec. 20, 1839, vol. i. pp. 162, 163. "This appears to have 

 been the first actual determination of a difference of longitude by meteoric obser- 

 vations." (L. E. D. Phil. Mag. 1841, vol. xix. p. 553.) Several years later (in 1838) 

 similar meteoric observations were made between Altona and Breslau; and also 

 between Rome and Naples. 



\ Proceed. Am. Phil. Soc. Dec. 18, 1840, vol. i. p. 323. 



