IOWA ACADEMY OF SCIENCES. 2& 



PHYSICS. 



So many physical phenomena are met in daily life that it is 

 no matter of surprise that many elementary principles of 

 Physics have been understood for centuries. It is probable, 

 however, that most physicists would place the beginning of 

 scientific physics at about the time of Galileo. He has justly 

 been called the Father of Physics. It is surprising how many 

 great discoveries were made by this many-sided man. It was 

 he who discovered the law of filling bodies, the path of pro- 

 jectiles, the laws of the pendu'um, the parallelogram of forces, 

 the satellites of Jupiter, sun-spots and the rotation of the sun 

 upon its axis. He was the first to demonstrate that the air has 

 weight. He greatly improved if he did not invent the tele- 

 scope and seems to be the first who used it to observe the 

 heavenly bodies. He invented in 1593 the thermoscope consist- 

 ing of a bulb and stem, the latter partly filled with water and 

 ending in water. It was his pupil Torricelli who devised the 

 barometer and used it to measure the fluctuations in the press- 

 ure of the air. About the same time, or about the middle of the 

 seventeenth century, Von Guericke invented the air pump, and 

 a few years later Boyle discovered the important law that 

 the volume of a gas varies inversely as the pressure. 



It will be necessary to trace very briefly the progress of 

 discovery in at least three branches of Physics: Light, heat, 

 Electricity and Magnetism. 



LIGHT. 



The law of reflection of light could scarcely escape the 

 earliest observers and was known to the Greeks. But one 

 must come down to the early part of the seventeenth century 

 for any further advance in the knowledge of light. It is here 

 we meet the invention of the telescope and the microscope in 

 crude forms. It may seem strange that they were invented 

 without a knowledge of the law of refraction, which was 

 experiment illy discovered by Snell aboit 1620, and was given 

 its present form b/ Descartes in 1637. It was in 1676 that 

 Ro3mer determined approximately the velocity of light, which 

 before that time wa^ believed to be infinits, from the eclipse 

 of one of the moons of Jupiter, and in 17128 his theory was con- 

 firmed by Bradly, who made a nearer approximat'on in the 

 determination oC its velocity; for any advance in this direction 

 we mast come down to the middle of the nineteenth century. 



