And later on, when great nations had been formed out of the tribes that once roamed in lawless and hostile bands, the wonders of nature were still regarded with the same awe, and it was even considered impious to question their cause or study their effect.

The wonderful succession of day and night, the recurrence of the seasons, the sun and moon, the stars and the winds and the tides, and all things else, were only a part of the great mystery of life, and all equally incomprehensible, from the flaming comet that illumined the heavens with unnatural brightness to the opening of the first bud or the fall of the first snowflake; and it was never dreamed that the time would come when man would look upon these things with any feeling but amazement.

And even when the world had grown wise in many ways, and there were great cities ruled and kept by powerful kings and mighty armies, and while poets and painters were making immortal poems and pictures, and man had learned to use the winds and the tides to guide him on his journeys, still the unexplained marvels of the universe were clothed in sacred mystery, and only the priests and astrologers dared to study and proclaim their laws.

From time to time some philosopher, seeking earnestly after the truth, would assert that he had discovered some secret of nature that would lead to the better understanding and use of her laws but the world seemed so enchanted with its own ignorance that the new discoveries were either received with unbelief, or the author accused of impiety and perhaps sentenced to death.

And so long centuries passed away while man seemed to the gain knowledge of every other kind, but held the world of nature still in childish wonder, and was as much terror-stricken by the sight of a comet or the eclipse of the sun as had been his remote ancestors who dwelt in caves and went naked through the wilderness in search of food.

But there came an age at last when knowledge had so increased and was so widely diffused among people of every class, that the rulers and priests of a country could no longer prevent any new discovery from being made known.

Every city boasted of schools and universities, and in them were found not only the great scholars and philosophers, but students from every class, for ignorance was no longer considered desirable, and it was esteemed to be able to talk of history and literature, the fine arts and philosophy. These universities were frequented by visitors from all parts of the civilized world, and thus it happened that any newly discovered scientific truth or theory was at once carried to remote places, and in this manner the systems taught in one city soon became known to the others, and knowledge greatly advanced by their mutual intercourse. About the middle of the sixteenth century the universities of Italy held a high rank among institutions of learning, and within their walls could be found some of the most earnest and enlightened thinkers of the world. Many of these gave their days and nights to the study of nature, and strove with untiring zeal to grasp the secrets that had eluded the wise of other ages.

Among these restless and inquiring spirits was Galileo Galilei, a youth of Pisa, who had entered the university of his native town at the age of nineteen as a student of medicine.

Although the father of Galileo was not wealthy, and a unversity education for his son would call for considerable denial on his part, still the effort was cheerfully made, and the rapid progress of the young student immediately proved the wisdom of the step.

From his earliest childhood Galileo had shown the greatest talent for mechanical invention, his wonderful toys and little models of machinery being the admiration and delight of his companions, and as he grew older this talent developed more and more, and led to some of the most important inventions in the history of mechanics.

Two years after his entrance at the university he noticed one day, while sitting in the cathedral, a lamp swinging from the roof, and keeping as it swung a regular and uniform motion. This circumstance, which would never have attracted the notice of the careless observer, at once held the attention of the young inventor, and he watched the lamp until he became convinced, by comparing its motion with the beating of his pulse, that its vibrations, whether great or small, recurred at regular and equal intervals. He immediately saw that this discovery might lead to some useful mechanical invention, and at once set about verifying it by different experiments; the results proved the truth of his supposition, and it then occurred to him that if he were able to reckon the vibrations of a swinging body from the beat of a normal pulse, he might be able to do the reverse to ascertain the pulse of a patient by comparing it with the same vibrations. He at once constructed a simple instrument to test his theory, and the experiment proved so satisfactory that the invention at once passed into common use by all the physicians of the day.

This first pendulum—which was called a pulsilogy, from the use it was put to—consisted simply of a weight attached to a string, and a graduated scale. The string was gathered up in the hand till the vibrations of the weight coincided with the beating of the pulse, and it was then reckoned from the scale whether the rate were normal or otherwise.

Although the pendulum was invented for the sole purpose of assisting in the practice of medicine, yet the discovery of its principle by led to important results; previous to this there had been many contrivances for the measurement of time, hour-glasses, sun-dials, water-dials, burning candles, and other expedients succeeding each other in turn, but none of these had been able to measure time so accurately as the pendulum, and its use in dividing the day, and in astronomical observations soon became indispensable. Its invariable regularity was of the greatest service to the astronomers, who, by means of the pendulum-clock which was invented some years later by Huygens, a Dutch astronomer, were able to make calculations more exactly and satisfactorily than ever before, and the same instrument in time led to the knowledge of the real form of the earth. Thus the first invention of Galileo not only served the practical needs of daily life but was the means of advancing scientific observation to a plane inaccessible before.

Although Galileo had entered the university as a student of medicine this subject gradually lost all charm for him, and he devoted himself more and more exclusively to mathematics and physics. This change was at first unwelcome to his father, but as time passes and he saw that his son was irresistibly carried on by his new pursuits, he no longer opposed him, and allowed him to devote his time to the study of natural philosophy.

An essay on physics brought Galileo to the notice of one of the leading mathematicians of Italy, and through his influence the young philosopher was appointed to the lectureship of mathematics at Pisa. This new position did not prevent his pursuing his studies with undiminished vigor, and his lectures attracted immediate attention. Almost from the beginning of his university career, Galileo showed that boldness and originality of thought which distinguished him in after-life, and won the ill-will of several of the professors by his unwillingness to accept for truth many of the dogmas which they held sacred.

But Galileo had been brought up under the influence of a father who was accustomed to give full and free discussion to any subject that occupied his mind, and this training, together with his own original genius, made it impossible for the son to follow easily in the beaten paths of university life, and thus thrown back upon himself, and with only the help and sympathy of one or two of his companions, he began to find out new lines of thought, and to follow paths that had hitherto been considered unlawful.

New ways of solving old questions presented themselves freely to his inquiring mind, and were tested, and, when found satisfactory, accepted with the same readiness that was accorded the old faiths, and this could not be forgiven by the professors, who considered it the most honorable thing in the world to receive the ancient philosophies without question or disparagement, and whose greatest ambition it was to discover or wring some new meaning out of the old texts that would apply to all doubts and settle all discussion. And thus from the beginning of his career, Galileo was surrounded by the enemies of progress, and even his mechanical investigations were received with cold favor.

But this did not daunt him, and as he in his studies he subjected all the propositions of the old philosophy to the severe test of free investigation, glad when he could find no flaw in the world-old wisdom, but gladder still when he discovered an error the righting of which would lead thought into wider and purer channels. And the responsibility of his position as a teacher made him the more anxious to sift out the good from the bad, while the opportunity thus offered of influencing a younger generation made him strive with renewed earnestness after the truth.

These efforts only served to increase the hospitality that the professors had shown toward him in his student-days; but Galileo persisted in his investigations, and proved the folly of some of their most cherished beliefs, announcing the results of his experiments with a persistent dertermination and faith that won many adherents. But his enemies would not listen even when his arguments were followed by the most conclusive proofs; and on one occasion, when Galileo performed the experiment of letting two bodies of different weight, fall simulataneously from the leaning tower of Pisa, in order to prove that they would reach the ground at the same time, his angry opponents refused to believe the evidence of their own eyes, and quoted in reply the sentence from Aristotle which asserted that if two different weights were let fall from the same height the heavier one would reach the ground sooner. Such obstinancy, combined with ill-will and distrust, rendered Galileo's position at Pisa so unpleasant that, when an opportunity offered for him to take the chair of mathematics at Padua, he did not hesitate, and left Pisa after having taught there only three years.

He now began to circulate his writings more freely, one essay following another with such rapidity, and all embodying such new and startling theories, that his name soon became familiar to the scientific world, and his opinions were listened to with a respect that roused the fiercest resentment of his enemies.