British Association for the Advancement of Science. 3 
travel from one place to another at the rate of fifteen miles an 
‘hour, while one of ten feet deep would proceed at the rate of only 
twelve miles an hour; so that if the places were thirty miles apart, 
the one would receive the former tide two hours later, and the 
Jatter tide two and a half hours later than the other. The crea- 
tion of a tidal Bore in some places was also accounted for on the 
same principles; and it was evident, that the means of improving 
the navigation of tidal rivers might be satisfactorily deduced from 
these principles. 
Similar observations had been made on the tidal wave of the 
River Clyde, which was found to move in strict conformity with 
the laws of the great wave of translation, as determined by Mr. 
Russell’s previous experiments. 
Maguetical Observatory at Dublin.—The Magnétical Observa- 
tory now in progress at Dublin, is situated in an open space in the 
gardens of Trinity College, sant sufficiently remote from all dis- 
turbing influences. 'The building is forty feet in length, by thirty 
in depth. It is constructed of the dark-colored argillaceous lime- 
stone, which abounds in the valley of Dublin, and which has been 
ascertained to be perfectly devoid of any influence on the needle. 
This is faced with Portland stone ; and within, the walls are to be 
studded, to protect from cold and damp. No iron whatever will 
be used throughout the building. With reference to the materi- 
als, Prof. Lloyd mentioned, that in the course of the arrangements 
now making for the erection of a Magnetical Observatory at Green- 
ie Mr. Airy had. rejected bricks in the construction of the 
uilding, finding that they were in all cases magnetic, and some- 
aes even polar. Mr. Lloyd has since confirmed this observation, 
by the examination of specimens of bricks from various localities ; 
and though there appeared to be great diversity in the amount of 
their action on the needle, he met with none entirely free from 
such influence. 
The building consists of one principal room, and two smaller 
rooms,—one of which serves as a vestibule. The principal room 
is thirty-six feet in length, by sixteen in breadth, and has projec- 
tions in its longer sides, which increase the breadth of the central 
part to twenty feet. This room will contain four principal instru- 
ments, suitably supported on stone pillars: viz. a transit instru- 
ment, a theodolite, a variation instrument, and a dipping circle. 
The transit instrument (four feet in focal length, ) will be stationed 
