264 THE POPULAR SCIENCE MONTHLY. 



lence of the phenomenon. On consulting the registers of previous 

 observations, he discovered that similar perturbations had accompa- 

 nied the aurora, even in places where it could not he seen on account 

 of the weather, in the daytime, and in the polar regions. He recol- 

 lected 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 attached by Brewster, but 

 Arago replied to his argument at length and convincingly. 



The experiments which were entered upon for the purpose of meas- 

 uring 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, be- 

 cause they " imposed the task of confronting the unknown caprices of 

 a formidable force. There were but two men to accept it and con- 

 duct it to success : Arago, who never shrank from a duty ; and Dulong, 

 already maimed by an explosion, whose previous studies had admira- 

 bly prepared him for the new work." A rude manometer was extem- 

 porized, 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 acci- 

 dent 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 cles Longitudes, the 

 duties of the direction of the observatory and of delivering the lect- 

 ures 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 

 thorn 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 andience. lie 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. 



