I 



* 





r 

 J 



I 



British Association. 417 



w 



refraction or absorption of an atmosphere. As respects the sea, the mode 

 of examination long since suggested by Sir David Brewster is probably 

 the most effective. If water existed on the moon's surface, the sun's 

 light reflected from it should be completely polarised at a certain elonga- 

 tion of the moon from the sun. No traces of such light have been ob- 

 served; but I am not aware that the observations have been repeated 

 recently with any of the larger telescopes. 



It is now well understood that the path of astronomical discovery is 

 obstructed much more by the earth^s atmosphere than by the limitation 

 of telescopic powers. Impressed with this conviction, the Association has, 

 for some time past, urged upon Her Majesty's Government the scientific 

 importance of establishing a large reflector at some elevated station in 

 the southern hemisphere- In the mean time, and to gain, as it were, a 

 sample of the results which might be expected from a more systematic 

 search, Prof. Piazzi Smyth undertook, last summer, the task of transport- 

 ing a large collection of instruments — meteorological and magnetical, as 

 well as astronomical — to a high point on the Peak of Teneriffe. His sta- 

 tions were two in number, at the altitudes above the sea of 8,840 and 

 10,700 feet respectively; and the astronomical advantages gained may 

 be inferred from the fact, that the heat radiated from the moon, which 

 has been so often sought for in vain in a lower region, was distinctly per- 

 ceptible, even at the lower of the two stations. 



Figure of the Earth and the Tides. — The researches relative to the Fig- 

 ure of the Farth and the Tides are intimately connected with Astronomy, 

 and next claim our attention. The results of the Ordnance Survey of 



I Britain, so far as they relate to the earth's figure and mean density, have 



] been lately laid before the Royal Society by Col. James, the Superintend- 



ent of the Survey, The ellipticity deduced is 2~?i'T3'' The mean spe- 

 cific gravity of the earth, as obtained from the attraction of Arthur's Seat, 

 near Edinburgh, is 5*316 ; a result which accords satisfactorily with the 

 mean of the results obtained by the torsion balance. Of the accuracy of 

 this important work, it is suflScient to observe, that when the length of 

 each of the measured bases (in Salisbury Plain and on the shores of 

 Lough Foyle) was computed from the other, through the whole series of 

 intermediate triangles, the difference from the measured length was only 

 five inches in a length of from five to seven miles. Our knowledge of the 

 laws of the Tides has received an important accession in the results of 

 the tidal observations made around the Irish coasts in 1851, under the 

 direction of the Royal Irish Academy. The discussion of these observa- 

 tions was undertaken by Prof. Haughton, and that portion of it which 

 relates to the diurnal tides has been already completed and published. 

 The most important result of this discu.ss?on, is tlie separation of the ^S^qXb 

 I of the sun and moon in the diurnal tide — a problem which was proposed 



ty the Academy as one of the objects to be attained by the contemplated 

 observations, and which has been now for the first time accomplished. 



From the comparison of these effects, Prof. Haughton has drawn some 

 i^emaikable conclusions relative to the mean depth of the sea in th« At- 

 lantic. In the dynamical theory of the tides, the ratio of the solar to 

 the lunar effect depends not only on the masses, distances, asd periodic 

 times, of the two luminaries, but also on the depth of the sea; and this, 



! SECOND SERIES, VOL. XXIV, NO. 72. NOV., 1857, 



53 



I 



