Jory—On the Origin of the Canals of Mars. 253 
His radius we take as 18 miles (Lowell’s number), and we assume his density 
to be four times that of water. His mass is, on these data, 4074 x 10” grammes. 
Taking the constant of gravitation as 
zi Std 
——+——, dynes 
1543}x 10° 9" 
and assuming his attractive force to be concentrated at his centre, and acting upon 
a mass of one gramme placed at a distance of one centimetre from this point, 
we get 
4074 x 10” 
Saran ees 
We may assign to @ such a value that the maximum stress will act upon a circle 
220 miles in diameter, which is the width of the wider double canals. This will 
require a to be 88 miles. Twenty miles will then separate the surfaces of planet 
and satellite. The constant depending upon @ is 0°29 for the circle of maximum 
stress, very nearly ; and @ is 38 x 160923 centimetres. If the surface density of 
Mars is three times that of water, and the unit of mass be taken as the mass of 
one cubic centimetre of his surface material, we find finally, in grammes per 
square centimetre, 
4074 x 10” 3 
: ss OC 
Sites = haa e104 198; elevesanc esi" (> 
= 3828 grammes, or about 7847 pounds to the square foot. 
The following Table determines similarly the stresses at various angular 
distances from Phobos :— 
t) Stress in grms. per sq. cm. 
10° 5 : 0 - 150 
20° 4 : : 2 508 
30° 2 : ¢ 5 1167 
40° : ) ; A 1904 
50° : : 5 : 2724 
60° : : 4 ‘ 3445 
70° : 2 : C 3850 
80° : : ° ° 3380 
85° : : : ; 2462 
These are plotted in the accompanying curve (fig. 2, p. 255), the vertical 
ordinates of which gives the magnitude of the stress at different distances along 
a radial line drawn outwards from beneath Phobos. 
If we assume Phobos to be a captured asteroid and to represent one of the 
high-density class of meteorites, we may ascribe even double this stress to his 
future proximity to Mars’ surface. 
