A thermometer of the usual kind, as may be readily supposed, would be of no use whatever in the experiment that was now about to be made. In an ordinary thermometer Mercury freezes hard when exposed to a temperature of 40 deg. below zero. But Barbican had provided himself with a _Minimum_, _self-recording_ thermometer, of a peculiar nature, invented by Wolferdin, a friend of Arago's, which could correctly register exceedingly low degrees of temperature. Before beginning the experiment, this instrument was tested by comparison with one of the usual kind, and then Barbican hesitated a few moments regarding the best means of employing it. "How shall we start this experiment?" asked the Captain. "Nothing simpler," answered Ardan, always ready to reply; "you just open your windows, and fling out your thermometer. It follows your Projectile, as a calf follows her mother. In a quarter of an hour you put out your hand--" "Put out your hand!" interrupted Barbican. "Put out your hand--" continued Ardan, quietly. "You do nothing of the kind," again interrupted Barbican; "that is, unless you prefer, instead of a hand, to pull back a frozen stump, shapeless, colorless and lifeless!" "I prefer a hand," said Ardan, surprised and interested. "Yes," continued Barbican, "the instant your hand left the Projectile, it would experience the same terrible sensations as is produced by cauterizing it with an iron bar white hot. For heat, whether rushing rapidly out of our bodies or rapidly entering them, is identically the same force and does the same amount of damage. Besides I am by no means certain that we are still followed by the objects that we flung out of the Projectile." "Why not?" asked M'Nicholl; "we saw them all outside not long ago." "But we can't see them outside now," answered Barbican; "that may be accounted for, I know, by the darkness, but it may be also by the fact of their not being there at all. In a case like this, we can't rely on uncertainties. Therefore, to make sure of not losing our thermometer, we shall fasten it with a string and easily pull it in whenever we like." This advice being adopted, the window was opened quickly, and the instrument was thrown out at once by M'Nicholl, who held it fastened by a short stout cord so that it could be pulled in immediately. The window had hardly been open for longer than a second, yet that second had been enough to admit a terrible icy chill into the interior of the Projectile. "Ten thousand ice-bergs!" cried Ardan, shivering all over; "it's cold enough to freeze a white bear!" Barbican waited quietly for half an hour; that time he considered quite long enough to enable the instrument to acquire the temperature of the interstellar space. Then he gave the signal, and it was instantly pulled in. It took him a few moments to calculate the quantity of mercury that had escaped into the little diaphragm attached to the lower part of the instrument; then he said: