Galileo’s fame as a teacher had now spread widely throughout Europe, and students began to flock to Padua to study under him. He had a large house, where a number of his private pupils lived with him, a garden, in which he delighted, and a workshop. Here he experimented on his next invention, that of the air thermometer. One of his friends, Castelli, wrote of this in a letter many years later, dated 1638. “I remember,” he writes, “an experiment which our Signor Galileo had shown me more than thirty-five years ago. He took a small glass bottle about the size of a hen’s egg, the neck of which was two palms long, and as narrow as a straw. Having well heated the bulb in his hand, he inserted its mouth in a vessel containing a little water, and, withdrawing the heat of his hand from the bulb, instantly the water rose in the neck more than a palm above its level in the vessel. It is thus that he constructed an instrument for measuring the degrees of heat and cold.” In 1604 the attention of all the astronomers of Europe was attracted by a new star which suddenly appeared in the constellation Serpentarius. Galileo studied it, and shortly began to lecture on the comparatively new science of astronomy. Formerly he had taught the old system of Aristotle to his classes, now, after a searching investigation, he declared his belief in the contrary conclusions of Copernicus. This study led him on and on. He became interested in the magnetic needle, and its use as a compass in navigation. Columbus’ discovery of its changing its position according to its relation to the North Pole took place on his first voyage to America, and reports of this had reached Padua. All educated men were rousing to the fact that the age was fertile with new discoveries in every branch of knowledge, and Galileo and those who were working with him gave eager heed to each month’s batch of news. Mere chance is said to have brought about the making of the first telescope. The story goes that an apprentice of Hans Lipperhey, an optician of Middleburg, in Holland, was, one day in October, 1608, playing with some spectacle lenses in his master’s shop. He noticed that by holding two of the lenses in a certain position he obtained a large and inverted view of whatever he looked at. He told Master Hans about this, and the optician fixed two lenses in a tube, and looking at the weathercock on a neighboring steeple saw that it seemed much nearer and to be upside down. He hung the tube in his shop as a curious toy, and one day the Marquis Spinola examined it and bought it to present to Prince Maurice of Nassau. Soon a number of Hans Lipperhey’s scientific neighbors were trying to make copies of his tube, and before very long reports of it were carried to Italy. The news reached Galileo while on a visit to Venice in June, 1609. This is his account of what followed, taken from a letter written to his brother-in-law Landucci, and dated August 29, 1609. [Illustration: GALILEO’S TELESCOPE]