486 
ACCOUNT OF A LENS OF ROCK SALT. 
Et tlie time when the list of the Com- 
mittee was printed. 
The President then called upon Sir 
David Brewster to an account of a lens 
of rock salt, upon the construction of 
which he had been authorized by the 
Association assembled at Edinburgh to 
expend a sum of SOI. ; as certain opti- 
cal researches upon the nature and qua- 
lities of that sid)stance, had induced 
men well skilled in the subject to anti- 
cipate the most favourable results to 
astronomy from its construction. 
Sir David Brewster stated, that 
through the kindness and activity of 
Dr. Traill, he had procured from Che- 
shire several splendidly transparent 
and hom.jgeueous crystals of rock salt; 
and that he had little doubt that these 
would in every way answer the desired 
end ; but that, as a lens, when con- 
structed of this material, would re- 
q’'ire to be adapted to a certain glass 
lens or lenses, — and as the construc- 
tion of each of these and their mu- 
tual adaptation was a matter requiring 
not only the nicest mechanical mani- 
pulation, but also a skill and knowledge 
of principles wiiich was not to be ex- 
pected in workmen of an ordinary class, 
— he had most reluctantly been com- 
pelled to abstain from an attempt at the 
actual construction, but he hoped very 
soon to have it in his power to accom- 
plish thi^ most desirable object. 
The President requested Mr. Baily 
to inform the Section, what steps had 
been taken towards procuring from the 
French olisewiatories, the reduction of 
the Observations respecting certain fix- 
ed stars, fur which a grant of money 
had been already made by the Associa- 
tion.. 
Mr. Bally replied, that as he w^as in 
daily e?:ppctacion of receiving from M. 
Arago information respecting these re- 
ductions, which vvoi Id be more precise 
and interesting than any thing he was 
at present pi’enared to say respecting 
them, he tru.sted the Section v/ould jier- 
mit him to defer his notice of them to a 
future occasion. 
To this request, the President, in the 
name of the Section, assented, and cal- 
led upon Mr. Lubbock to give an ac- 
count of the recent discussions of tide 
observations made at the ports of Liver- 
pool and London, and for which a 
liberal grant of money had also been 
made by the Association. 
Mr. Lubbock rose and stated, that 
through the indefatigable exertions of 
Mr. Dessiou, considerable progress had 
been made in the reductions of the ob- 
servations made at Liverpool by Mr. 
Plutchinson. 
The diurnal inequality or difference 
between the superior and inferior tide 
of the same day, v'hich in the Thames 
was very inconsiderable, if not insen- 
sible, was found at Liverpool to amount 
to more than a foot ; a matter upon 
which the learned gentleman laid con- 
siderable stress, as calculated to lead to 
important practical results. The object 
of these reductions was to compare the 
results of theory with these observa- 
tions, and with those of Mr. Jones and 
Mr. Russell made at the port of Lon- 
don. The principal objects of comfiari- 
son were the heights of the several tides, 
and the intervals between tide and tide ; 
and these were examined in their rela- 
tions to the parallax and declination of 
the Moon and of the Sun, and in re- 
ference to local, and what may in one 
sense he called accidental causes, as 
storms, &c. Of this latter, one of the 
most curious, as well as important, is 
the effect of the pressure of the atmo- 
spheric column. The learned gentle- 
man stated, that M. Daussy had ascer- 
tained, that at the harbour of Brest a 
variation of the height of high water was 
found to take place, which was inverse- 
ly as the rise or fall of the barometer, 
and that a fall of the barometer of 0.622 
parts of an inch, was found to cause an 
increase of the height of the tide, etmal 
to 8.78 inches in that port. To conffrm 
this interesting and hitherto unsuspect- 
ed cause of variation, had been one 
piincipal object of the researches of tho 
learned gentleman, and at his request, 
Mr. Dessiou had calculated the heights 
and times of high water at Liverpool for 
the year 1/84, and compared them with 
the heights of the barometer, as re- 
corded by Mr. Hutchinson for the 
same year ; and by a most careful 
induction, it nad turned out that the 
height of the tide had been on an aver- 
age increased by one inch for each 
tenth of an inch that the barometer 
fell, ceeteris paribus ; but the time was 
found not to be much, if at all, affected. 
Mr, Lubbock then proceeded to examine 
the semi-menstrual declination and 
