94 THE HISTORY OF SCIENCE 



achievements, verified by observation what Harvey in 

 1628 had argued must take place. 



In this same epoch apparatus of precision developed 

 in other fields. Weight clocks had been in use as 

 time-measurers since the thirteenth century, but they 

 were, as we have seen, difficult to control and other- 

 wise unreliable. Even in the seventeenth century 

 scientists in their experiments preferred some form 

 of water- clock. In 1636 Galileo, in a letter, men- 

 tioned the feasibility of constructing a pendulum 

 clock, and in 1641 he dictated a description of the 

 projected apparatus to his son Vincenzo and to his 

 disciple Viviani. He himself was then blind, and he 

 died the following year. His instructions were never 

 carried into effect. However, in 1657 Christian Huy- 

 gens applied the pendulum to weight clocks of the 

 old stamp. In 1674 he gave directions for the manu- 

 facture of a watch, the movement of which was 

 driven by a spring. 



Galileo, to whom the advance in exact science is 

 so largely indebted, must also be credited with the 

 first apparatus for the measurement of temperatures. 

 This was invented before 1603 and consisted of a 

 glass bulb with a long stem of the thickness of a 

 straw. The bulb was first heated and the stem placed 

 in water. The point at which the water, which rose 

 in the tube, might stand was an indication of the 

 temperature. In 1631 Jean Key just inverted this 

 contrivance, filling the bulb with water. Of course 

 these thermoscopes would register the effect of vary- 

 ing pressures as well as temperatures, and they soon 

 made way for the thermometer and the barometer. 

 Before 1641 a true thermometer was constructed by 



