Magnetism and Gahanism. 175 
the uniting wire, he has obtained the motion in a horizontal di- 
rection, which corresponds with the declination of the compass, 
and a motion in the magnetic meridian corresponding to the 
dip. 
Account of M, Biofs Experiments. 
On the 30th October 1820, M. Biot presented a Memoir to 
the Academy, on the Physical Laws of the ElectrorMagnetic 
Phenomena, which he had deduced from measures of the de- 
viation and the oscillation experienced by needles placed near 
the uniting wire. 
The following is the general expression which he has given 
of the action exerted at a distance upon a particle of austral or 
boreal magnetism, by a very fine uniting wire of copper, of an 
indefinite length, and put in communication with the two poles 
of a voltaic apparatus. 
From the point where the particle resides, draw a perpendi- 
cular line to the axis of the wire. The force which acts upon the 
particle is perpendicular to this line and to the axis of the wire. 
Its intensity is in the inverse ratio of the simple distance. The 
nature of its action is the same as that of a magnetic-needle, 
which is placed tangentially to the contour of the wire, so that 
a particle of austral and a particle_^of boreal magnetism would be 
drawn in opposite directions, though always in the same straight 
line determined by the preceding construction^ 
Art. XXVIII. — Account of the Piezometer^ for Measuring the 
Compressibility of Water By Jacob Perkins, Esq. 
T? HE Piezometer employed by Mr Perkins in his experiments 
on the compressibility of water, is represented in Plate III.. 
Fig. 13. The end B of a cylinder A, three inches wide and 
eighteen long, being made watertight by a plate firmly soldered 
to it a cap C, also water-tight, was made to screw on and off. 
The rod D, /^ths of an inch in diameter, and carrying a flexible 
* Abridged from ihe PUl. Trans. 1820, Part 2. p. 324. 
