70 GREAT MEN OF SCIENCE 



a somewhat massive body, rotatable about an axis, to be 

 driven to and fro by the fastest-running wheel, in order to 

 set a Hmit to the rate. Clocks of this kind were already 

 known in Galileo's time, and yet Galileo measured time in 

 his experiments on bodies falling on the inclined plane by 

 the flow of water, thus using the same principle as the water 

 clocks of the ancients, since this was found to be more trust- 

 worthy. ^ Later, when Galileo had thoroughly investigated 

 the motion of pendulums, he himself recommended measur- 

 ing time by counting the oscillations of pendulums; the 

 difference of length of different pendulums allowing of easy 

 calculation. Doctors began, in consequence of Galileo's 

 proposal, to use a pendulum of thread, the length of which 

 could be varied, to determine the pulse-rate of the sick; it 

 was called 'Pulsilogia' (pulse measurer). What was needed 

 for the measurement of times of any length was a pendulum 

 which did not come to rest, and an automatic counter con- 

 nected with it. Galileo already attempted this, and it was 

 afterwards found that he had left behind suggestions for 

 such a construction, made a few months before his death 

 and after he had become blind; he dictated them to his son 

 Vincenzio. The drawing is still in existence; it was care- 

 fully preserved by Vincenzio, but not made use of. Huygens 

 was the first to actually realise the maintenance of the oscilla- 

 tions of a pendulum, and the counting of them, both in a 

 very simple manner, by putting the pendulum in place of 

 the oscillating body already mentioned and usual at the time, 

 directly coupled to the wheelwork of the clock. The great 

 and decisive difference between the oscillating body and the 

 pendulum is, that the former has no fixed natural period of 



^ Astronomy had always used, for determining time, the movement of 

 suitably chosen stars. Otherwise Tycho's observations would have been 

 valueless; he only used the clocks existing in his time as secondary stan- 

 dards. Even the clocks of to-day must be compared with the stars from 

 time to time, of course with a correspondingly increased degree of ac- 

 curacy. 



