Page:Travels in the Air, Glaisher, 2nd ed.djvu/19

Rh distance from the earth. This arose from an error, for I had myself used a Davy lamp and read all my instruments by its means at night; and when M. de Fonvielle left England, after his escape from Paris, with the view of attempting to re-enter the city by means of a balloon, I gave him a lamp which I had taken up several times previously in my ascents, for the triple purpose of enabling him to read the barometer, to warm his hands, and to heat coffee, with the assurance of its perfect safety.

Dr. Janssen, charged with a scientific mission to Algeria by the Government and the Academy of Sciences, for the purpose of observing the total eclipse of the Sun on December 22, left Paris December 2, 1870, at six o'clock in the morning, in the balloon Volta, of which he took the management. He carried several dismounted telescopes, packed with special care in such a manner as to prevent any accidents in the descent.

The Volta rose at first to 3,600 feet by parting with ballast, and afterwards by the action of the sun it rose by degrees to 7,200 feet, from which height it descended on the completion of its voyage.

Dr. Janssen offers some remarks on the rise and height of his balloon, and writes that, exposed to the full action of the sun's rays, the balloon itself became heated; that this heat was in turn communicated to the gas inside, which readily absorbed the higher temperature as gas absorbs, Dr. Janssen remarks, obscure heat more readily than luminous heat. Under this action the gas became equalized in temperature, and warm throughout as a solid body, leading him to believe that at 3,600 feet the action of the sun is felt chiefly through the medium of the envelope or material of the balloon, which acts as a transformer of the solar force.

But it struck Dr. Janssen with surprise that at sunrise the balloon fell, and rose again when the sun was several degrees above the horizon.

He explains these effects by the same principle: the power of the sun on rising dissipated the mist, and increased the calmness of the sky, while the envelope of the balloon radiating heat into space became rapidly cooler, and was far from receiving as much heat from the rising sun as it parted with by radiation. The result showed