The heat had become quite oppressive.

Between the natural caloric of the Sun and the reflected caloric of the Moon, the Projectile was fast turning into a regular bake oven.

This transition from intense cold to intense heat was already about quite as much as they could bear.
"What shall we do, Barbican ?" asked Ardan, seeing that for some time no one else appeared inclined to say a word.
"Nothing, at least yet awhile, friend Ardan," replied Barbican, "I have been watching the thermometer carefully for the last few minutes, and, though we are at present at 38 deg.

centigrade, or 100 deg.

Fahrenheit, I have noticed that the mercury is slowly falling.