Whatever may be the _mean_ temperature of Mars, we must take account of the long nights in its polar and high-temperate latitudes, lasting nearly twice as long as ours, with the resulting lowering of temperature by radiation into a constantly clear sky.

Even in Siberia, in Lat.

67-1/2 deg.N.a cold of-88 deg.F.has been attained; while over a large portion of N.Asia and America above 60 deg.
Lat.

the _mean_ January temperature is from-30 deg.F.

to-60 deg.F., and the whole subsoil is permanently frozen from a depth of 6 or 7 feet to several hundreds.