lence of the phenomenon. On consulting the registers of previous observations, he discovered that similar perturbations had accompanied the aurora, even in places where it could not be seen on account of the weather, in the daytime, and in the polar regions. He recollected also that electricity is propagated in vague lights in vacuum-tubes and that these lights are deflected by the magnet; and he affirmed that auroras are electric sparks circulating in the higher parts of the atmosphere, oriented on the terrestrial magnet, and acting on the magnetic needle. This theory was attacked by Brewster, but Arago replied to his argument at length and convincingly.
The experiments which were entered upon for the purpose of measuring the force of the vapor of water were very important and very dangerous: important, because the safe working of steam-engines was dependent upon correct measurements of the force, and because all the properties of heat had to be passed in review; and dangerous, because they "imposed the task of confronting the unknown caprices of a formidable force. There were but two men to accept it and conduct it to success: Arago, who never shrank from a duty; and Dulong, already maimed by an explosion, whose previous studies had admirably prepared him for the new work." A rude manometer was extemporized, and a boiler, far less stanch than the steam-boilers of to-day, was set up, in which water was heated till the pressure was twenty-seven atmospheres. "They could not go further. At this extreme point, it leaked at all the joints, and the steam escaped through the fissures with a hissing that was of bad omen. But the observers, though aware of the danger, silent and resigned, finished without accident the measurements which they had begun." Telling M. Jamin the story, which was written out as above from his dictation, Arago said: "Only one being of our company preserved his serenity and slept quietly; it was Dulong's dog; they called him Omicron."
By the terms of the creation of the Bureau des Longitudes, the duties of the direction of the observatory and of delivering the lectures on astronomy were to be performed by the members in turn, a year at a time. Practically they fell continuously to Arago, and from 1813 to 1847 he delivered those lectures on popular astronomy which had a wonderful success, and of the life and vigor of which the tame rendering in the book of that name gives no idea. He did not write them out, but only prepared the outlines, and for the rest depended on the inspiration of the moment. They were attended by young men who went to learn, older men for the pleasure of hearing, and women, M. Jamin suggests, for the pleasure of seeing. "It was his habit, when he rose to speak, to select the least intelligent-looking face in the audience. He then never left it, but seemed to speak for it alone, and continued his demonstration, with various modes, till that face showed that its owner understood him; a fortiori, all of the auditory must have understood him as well.