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Rh of gases, and the determination of the relative densities of the air and mercury, from which they were able to calculate theoretically the constant of the barometric formula. The two friends next were interested in the scheme of continuing the measurement of the arc of the meridian from the shore-line at Barcelona, where Delambre and Mechain had left it, to Majorca, and thence to Formentera. In the execution of this enterprise it became necessary to set up a beacon-light in the Island of Iviça, and to observe it from two points in Spain, forty leagues off. Arago established himself upon a rock called Desierto de las Palmas, the summit of which hardly afforded room enough for his tent and instruments. On account of the distance, the frequency of fogs, and uncertainty as to the exact direction, it was six months before ho could get sight of the beacon. As soon as the measurements were made, Biot took the first results to France, leaving his colleague to finish the work at Majorca and Formentera. Then began for Arago a series of exciting adventures. War had been declared with Spain in 1808, and the Majorcans sought to arrest him as a spy. He had barely time to disguise himself, and, gathering up the papers containing his observations and his instruments, to take refuge on the vessel which had brought him to the island. Thence he escaped, with the aid of the ship-captain, to the citadel of Belver, where he passed several months, substantially a prisoner, in making his calculations. He was at last permitted to go to Algiers, where he embarked on a vessel of the regency under the name of a Hungarian merchant, with a false passport, along with a crowd of Mussulmans and renegades, to which were added two lions and a family of monkeys which the dey was sending to his ally the Emperor of France. The vessel was taken close upon Marseilles by a Spanish corsair, "which conducted its prize to Palamos and took Arago to a country where he was only too well known and from which he had nothing good to expect." He suffered great privations till the dey was informed of the state of affairs, and bullied Spain into setting the lions, monkeys, ships, and philosopher again on the way to Marseilles. They had reached that port when a mistral suddenly arose and sent the vessel to the coast of Sardinia, and thence to Bongie, in Algeria, where the new dey, not so friendly as his predecessor, was disposed to hold Arago as a prisoner. Finally, he was allowed to return to France, running a new danger from pursuit by an English cruiser, and at length to receive a welcome from his mother, "who thanked God for having preserved his life, after having had masses said for the repose of his soul; he returned with the triple consecration of having encountered danger, done his duty, and attained a scientific success beyond his anticipations."

In 1809 the phenomena of optics engaged especial attention, and Arago entered upon the study with the ardor of his nature and his age. He engaged in the investigation of the polarization of light, which no one had as yet been able to explain, but of which he reached