152 M. Plana on the Mean Density of 



will throw some light on the climatology of the earth itself; 

 since the heat of the sun must be different, according as one 

 or other of its poles is turned towards the earth. Future 

 experiments will resolve this question. With respect to the 

 poles of the sun, I shall add here a conjecture on a fact re- 

 cently discovered by Colonel Sabine. The journal Institut 

 relates that this gentleman has found that the deviation of 

 the magnet from its mean position at the Cape of Good Hope 

 is found to be in opposite directions at the epochs of the two 

 equinoxes. Might this not be an effect of the solar magne- 

 tical polarity on the terrestrial magnetism. The fact deserves 

 to be examined, if it takes place in our hemisphere, and in 

 opposite directions. Coming again to the solar heat, I have 

 found that spots seemed less hot than the rest ; but as only 

 small groups of them were visible, no singular fact or law 

 can be stated from these observations. I shall conclude this 

 account by noticing an odd historical coincidence, namely, 

 that these observations were made in the same room where 

 it is said F. Schemer, the first who used a telescope mounted 

 equatorially, made his observations of the sun. This room 

 has been this year added to the observatory. — Proceedings 

 of the Royal Astronomical Society, November 1852. 



On the Mean Density of the Superficial Crust of the Earth. 

 By M. Plana. 



The researches of geometers have established, beyond 

 doubt, that the density of the earth increases towards the 

 centre. Assuming the densities of the successive strata to 

 increase in arithmetical progression, Laplace has investi- 

 gated the constant amount of increase for each successive 

 stratum, and has hence deduced the mean density of the ter- 

 restrial spheroid (Mec. Cel., tome v., liv. xi.) In his re- 

 searches on this subject, he supposes the density of the super- 

 ficial stratum (q) to be three times the density of the sea, 

 considered equal to unity. He remarks that this assumption 

 agrees very nearly with the density of granite. His expres- 

 sion for the density of any stratum is, 



Q = (p) (1 + e - e a), 



