198 M. Van Swinden on the Invention of Pendulum-Clocks 
pelled to acknowledge that no dependence could be placed upon 
them. 
The use which might be derived from the oscillations of a vi- 
brating body, first became apparent to astronomers, from the 
time that Galileo made known his theory of pendulums, partly 
by his letters, and Systema Cosmicum^ published in Italian in 
1682, but more especially by his Dialogi de Motu^ which ap- 
peared in 1639. They applied this doctrine to measure the 
time which elapsed between two observations, by means of a 
ball, suspended to a wire or metallic rod, which oscillated by its 
own gravity when impelled ; and we are truly astonished at the 
degree of accuracy of which this method became susceptible, in 
the hands of diligent observers. It was, however, subject to 
two serious inconveniences. The principal one was the neces- 
sity of assistants to count the number of oscillations of the pen- 
dulum, relieving each other at intervals, as the length of the 
observation, which sometimes lasted for twenty-four hours, re- 
quired. This made some of them intent on the possibility of 
adapting to the pendulum something which might of itself in^ 
dicate how many oscillations had taken place during the inter- 
val of observations. Hevelius affirms having succeeded in such 
a contrivance, (Machina Coelestis, i. p. 364.) ; and Wallis, in a 
letter to Huygens, {Leyden MSS.), says tnat somebody had 
added a wheel to his pendulum, which served the same purpose. 
Another defect consisted in these pendulums always returning 
to rest, after describing arcs which became continually shorter 
and shorter, so that after a certain period they required being 
put in motion agffin. Pendulums, then, in this state, could not 
be termed accurate measurers of time. In order to answer this 
end, some additional contrivance was requisite, which should, 
by its action, restore to the pendulum the loss of velocity suffer- 
ed at each vibration, and thus render its motion perpetual, whilst 
itself should in its turn be kept to a regular rate, by being 
obliged to follow the isochronous beats of the pendulum, and 
become capable of showing off with accuracy, not only the small- 
est portions of time, but in like manner those longer periods- 
which arise from the accumulation of them. 
This required a genius of a particular cast. It appeared in 
the person of our countryman Cheistian Huygens, a man of 
