~ 1900.) LOWELL—MARS ON GLACIAL EPOCHS. 653 
greater distance than the southern one when seen thus near its 
summer solstice ; and the irradiation being the same for both in 
seconds of arc becomes, with decreased size of disk, greater as 
measured in Martian degrees. 
14. As a preliminary to an explanation of this phenomenon, it is 
necessary to consider the general laws of planetary insolation, or, in 
other words, the amount of heat received from the sun by different 
parts of the planet at different times. 
The total quantity of heat intercepted by the planet as a whole 
in passing from any point of its orbit round to the same point again 
is a function of the eccentricity of the orbit. For heat or light, 
like gravity, diminishes inversely as the square of the distance. 
But the quantity of gravity received, if we may so express ourselves, 
is measured as follows : 
Since equal areas are swept out by the radius vector in equal 
I 
times, or 7? 70 = hat ; and since cg, = 7* 
6h. dt,g,=d0 
or 
AO Giz=27 
where G= total gravity received during a revolution. Now if the 
major axis of two orbits be the same, the period isthe same. Con- 
sequently in this case G varies inversely with 2. But 
A=y wa (i—e)? 
whence 
ah? = 1b? 
h=bY ph 
Ve 
Whence the total gravity and consequently the total heat received 
varies inversely as the minor axis of the orbit, and is therefore a 
function of the eccentricity. 
15. But the relative amount received in passing from one equi- 
nox to the other does not vary but is the same, as D’Alembert 
showed, from whichever equinox we set out. That is, the planet 
has as many units of caloric fall upon it in traveling from the ver- 
nal to the autumnal equinox as from the autumnal to the vernal one. 
Indeed, whatever point we are pleased to take for starting point, 
