At about this period some experiments were made by Wren's philosophical friends wherein he took a principal part, and to which the barometer, now in common use, is mainly due. The first instrument of the kind was invented by Torricelli, the pupil of Galileo, who used it in order to ascertain the pressure of the air on fluids, the supposed cause of which pressure was the passing by of the body of the moon. Pascal, in those earlier days when his great genius employed itself on natural philosophy, made several experiments at Rouen, in 1646, with a friend, M. Petit, using 'Torricelli's tube,' as it was called. Similar trials were afterwards made by M. Perier, his brother-in-law, among the mountains of Auvergne. They then discovered that the rising and falling of the mercury was due not to the moon, but to the differences in the specific gravity of the atmosphere. Wren's experiments led him to the same conclusion, and at a later period he and Robert Boyle continued them until they produced the barometer, though it was not used commonly as a weather-glass until a much later date. Pascal did not pursue his discovery, but was satisfied with having proved the point for which he was contending. [_THE CYCLOID._] Though Wren and Pascal never met, some communication passed between them. Pascal, who was Wren's senior by eleven years, propounded a problem, under the name of Jean de Monfert, to the mathematicians of England, adding a challenge to them to solve it by a given day. Christopher sent a solution, and in his turn propounded a problem which seems never to have been answered. Pascal is said to have considered Wren's solution very carefully, but the promised prize of twenty pistoles was withheld by some trickery. Besides this, Wren wrote four mathematical tracts on the cycloid, and sent them to Dr. J. Wallis, who was publishing a book on mathematics. He corresponded with Pascal,[65] who was writing on the cycloid by the name of _la Roulette_, the problem being 'to determine the curve made in the air by the nail of a coach wheel from the moment it rises from the ground, till the moment when the continual rolling of the wheel brings it back to the ground, after a complete turn, supposing the wheel a perfect circle and the ground perfectly level.' [_CROMWELL'S FUNERAL._]