508 Scientific Intelligence. 



spheres would come into contact between five and six minutes. 

 The mean density of the earth found by Professor Poynting is 

 5-49. Professor Boys result is 5-53. — Nature, Oct. 4, 1894, pp. 

 542-543. j. t. 



9. On the mean density of the earth. (Communicated). — 

 The last publication of the Philosophical Society of Washington, 

 (Bulletin xii, 369-370 8 plates 7-10) gives a determination of 

 the Mean Density of the Earth from two mountains in the 

 Hawaiian Islands. Two distinct methods were used. In the case 

 of Haleakala, astronomical latitudes were determined near the 

 sea level on the north and south side of the mountain. These 

 stations which were distant about 30 miles were connected by 

 triangulation and by this means the deflection of the plumb line 

 towards the mountain was ascertained. Contour lines for differ- 

 ences of elevation of 500 feet were established from the sea to the 

 summit which is rather more than 10,000 feet high. Using these 

 contours and a value of the mean density of the mountain 

 derived both from determinations of the force of gravity and a 

 study of the rock specimens, the attraction of the mountain was 

 calculated. This compared with the total attraction of the 

 earth gave an equation from which the earth's mean density was 

 found. 



In the case of Mauna Kea with an elevation of nearly 14,000 

 feet, contour lines could not be run, and the solution of the 

 problem depends entirely on the determination of the differences 

 of the force of gravity, combined with a value of the mountain 

 density, from about 40 specimens of rock collected on both sides 

 of the island between the lower and upper gravity stations. 



Combining the results of both Mauna Kea and Haleakala a 

 value of 5 - 35 was deduced for the mean density of the Earth. 



At the end of the Bulletin is a table containing a list of all 

 previous determinations which depend on pendulum or astronomi- 

 cal observations. The values given range from 4-67 to 6*77 and 

 give a mean of 5 '59. 



The work in the Hawaiian Islands was done by Mr. E. T>. 

 Preston of the IT. S. Coast and Geodetic Survey for the time 

 being in the employ of the Hawaiian Government Survey. 



10. The source of friction electricity. — C. Christiansen reviews 

 the work of previous observers, and makes a number of experi- 

 ments to ascertain the cause of frictional electricity. He arrives 

 at the conclusion that it is to be sought in chemical separation or 

 the relative turning of ions : and the action is analogous to that 

 which takes place in electrolysis. Every atom has a charge 

 which is positive or negative according as the atom acts as a 

 Jcation or an anion. The author divides bodies into four classes. 

 Class 1. Insulators or dielectrics. Bodies in which the ions are 

 either fixed or can oscillate around their positions of equilibrium. 

 Class 2. Metals in which the ions are either free or at least 

 exchange their charges very easily. Class 3. Electrolytes in 

 which the ions can move with more or less friction. Glass can be 



