126 GREAT MEN OF SCIENCE 



one hundred and nine years later, after telescopes and divided 

 circles had been again greatly improved by Fraunhofer, that 

 the parallax of a fixed star was determined for the first 

 time. 



Bradley was born in the English county of Gloucester 

 on the river Severn, and was professor of astronomy in 

 Oxford from 1721 to 1742. He owed the means for his 

 discovery to the private observatories of wealthy amateurs 

 in astronomy. He afterwards became Director of the Royal 

 Observatory at Greenwich, and successor of Halley, where he 

 carried out in particular his observations on the nutation of 

 the earth's axis, which had already been calculated by 

 Newton. 



JOSEPH BLACK {1728-1799) 

 JAMES WATT {1736-1819) 



Black was the founder of the measurement of quantities 

 of heat, or calorimetry. 



The state of heat, temperature, had been measured by 

 thermometers since the time of Galileo, at least in so far as 

 that each investigator was able to mark upon his home-made 

 thermometers points corresponding to definite temperatures. 

 The temperature of melting ice was soon afterwards brought 

 into use as a fixed point, assumed unalterable, in constructing 

 thermometers. A second fixed point was given, when Papin 

 had discovered the boiling point of water to be independent 

 of the pressure, the new fixed point being the boiling point 

 at an agreed 'normal' barometric pressure. Thenceforward, 

 properly defined thermometric scales became possible; 

 they were divided between the ice point and the boiling 

 point into 80 (Reaumur), 100 (Celsius), or 180 (Fahrenheit) 

 degrees. 



