338 



Connaissance des Terns, to be 28' 59" west of Paris; which added 

 to 1 51' 18"'73, its east longitude from Greenwich, resulting from 

 the preceding work, gives 2 30' 17"'73 for the difference of longitude 

 between Paris and Greenwich; which is equivalent to 9 2P-18 in 

 time, differing only s - 28 in defect from the results obtained with fire 

 signals as reported in the Phil. Trans, for 1826, by Mr. Herschel. 

 As the accuracy of the preceding work wholly depends upon the de- 

 gree of reliance that may be placed upon the base at Hounslow 

 Heath, which is somewhat questionable, the author recommends that 

 a new base be measured to connect in the most unexceptionable man- 

 ner the stations of Leith Hill and Wrotham; to the successful accom- 

 plishment of which, the arrangements so happily devised by Colonel 

 Colby for compensating expansion would eminently contribute. 



An Appendix is subjoined, containing miscellaneous observations 

 with respect to various objects connected with the proceedings, ac- 

 companied with tables relating to the computations. The methods 

 employed for securing the permanence of the positions of the micro- 

 scopes of the theodolites are pointed out, and the influence of various 

 minute causes of inaccuracy is inquired into, especially that of lateral 

 refraction, which frequently occasioned a sensible variation in the 

 same angles observed in different states of the weather. 



The original observations relating to the work are deposited with 

 the Royal Society, for the purpose of being consulted whenever oc- 

 casion may require. All the angles employed in the work, with the 

 name of the observer, and the manner in which they were derived, 

 are given at the end of the present paper. Tables are also given 

 detailing the observations of the pole star. 



On the Phenomena of Volcanoes. By Sir Humphry Davy, Bart . F.R.S. 

 Read March 20, 1828. [Phil Trans. 1828, p. 241.] 



In a paper on the Decomposition of the Earths, published in the 

 Philosophical Transactions for 1812, the author offered it as a con- 

 jecture that the metals of the alkalies and earths might exist in the 

 interior of the globe, and on being exposed to the action of air and 

 water, give rise to volcanic fire and to the production of lavas ; by 

 the slow cooling of which, basaltic and other crystalline rocks might 

 subsequently be formed. Vesuvius, from local circumstances, pre- 

 sents peculiar advantages for investigating the truth of this hypo- 

 thesis ; and of these, the author availed himself during his residence 

 at Naples in the months of December 1819, and of January and Fe- 

 bruary 1820. A small eruption had taken place a few days before 

 he visited the mountain, and a stream of lava was then flowing with 

 considerable activity from an aperture in the mountain a little below 

 the crater, which was throwing up showers of red-hot stones every 

 two or three minutes. On its issuing from the mountain, it was per- 

 fectly fluid, and nearly white-hot ; its surface appeared to be in vio- 

 lent agitation, from the bursting of numerous bubbles, which emitted 

 clouds of white smoke. There was no appearance of more vivid ignition 



