188 Dr Traill on ihe Spec^ic Gravity of Sea~Watet\ 
mirably adapted to procure water from any required depth, 
without chance of error, than that gentleman’s marine diver ; 
which, with simplicity of construction, unites every property 
than can insure accuracy and convenience in those delicate in- 
vestigations for which it was intended by its ingenious contriver. 
I am at ,a loss what reason to assign for the increased density 
of water brought up from considerable depths ; unless, accor^ 
ding to a suggestion offered by my friend Dr Brewster, it may 
be owing to the imperfect elasticity of water, which prevents its 
particles, when compressed by the superincumbent column, from 
regaining their original condition, when the pressure is removed. 
A curious series of experiments might be made on the mecha- 
nical compression of water, by employing the bathometer of Mr 
Perkins, the inventor of the method of multiplying copper- 
plates by engravings on steel. In this machine, water inclosed 
in a brass tube, the sides of which need not exceed To^b of an 
inch in thickness, is compressed by a solid piston, sliding in a 
leather collar, and acted on by the superincumbent column 
I when sunk in the depths of the ocean. This seems one of the 
simplest means of producing an immense pressure ; and when 
conversing with Mr Perkins, I remember his stating that the 
piston did not exactly return to its original position, on bring- 
ing up the instrument. 
Liverpool, I 
December 9. 1820. j 
Art. XXXI I , View of the Monthly Mean Variation^ 
and the Mean Mp'nihly Diurnal Variation of the Needle^ with 
Tables of the State of the Atmosphere at the time of the Mag- 
netical Observations. By Colonel Beaufoy, F. R. S. In a 
Communication to Dr Brewster. 
The following Table contains the morning, noon, and even- 
ing monthly mean westerly variation of the magnetic needle. 
The columns marked Diff.” denote the increase and de- 
crease of the variation in the same nionths pf the different 
years. 
