185 
divide itself between the two chambers through the valve 
LV, which offers no impediment, as it opens as soon as the 
mercury sinks below it. One-fourth is now left in Z, and 
the piston must rise three-fourths of its stroke to enable its 
one-fourth to effect the lifting of UV, but the residual mer- 
cury assists in doing this. 1 may here remark that we should 
so adjust the quantity of mercury in GG' as to allow some to 
enter after every upward stroke, and some to return before 
the ring closes on the commencement of each downward 
stroke. This effected, the two great imperfections of former 
instruments are obviated, the intervalvular spaces are anni- 
hilated, by the liquid valve (the mercury) filling them up ; 
and the mechanical obstacles from adherent or heavy valv'es 
are removed by making their own weight efficient in opening 
them, and thus independent of difference of tension, which, 
in great rarefaction, becomes too impotent for the purpose. 
MATHEMATICAL AND PHYSICAL SECTION. 
December 8 th, 1859. 
Mr. Baxendell read a Paper, “ On the Rotation of 
Jupiter.” 
The appearance of several small, isolated, dark spots on 
the surface of Jupiter during the last Avinter and spring, in- 
duced the Author to make a series of observations, with the 
ultimate view of obtaining a re-determination of this planet’s 
period of rotation, and with the more immediate object of 
testing the conclusions which have been drawn from the ob- 
servations of Cassini, Sir W. llerschel, Schroeter, &c., to 
the effect that different spots have different periods of rota- 
tion ; that the velocity of rotation of the same spot will 
