120 
■ : 
Mr. Atwood’s Investigations for determining 
which are still preserved, the balance vibrated merely by the 
impulses of the wheels, without other control or regulation : 
the motion communicated to the balance by one impulse con- 
tinued till it was destroyed, partly by friction, and partly by 
a succeeding impulse in the opposite direction ; the vibrations 
must of course have been very unsteady and irregular. 
These imperfections were in a great measure remedied by 
Dr. Hooke’s ingenious invention of applying a spiral spring 
to the balance : * the action of this spring on the balance of a 
watch, is similar to that of gravity on a pendulum : each kind 
of force has the effect of correcting the irregularities of im- 
pulse and resistance, which otherwise disturb the isochronism 
of the vibrations. 
During the present century, various improvements have 
been made in the construction of watches, principally by the 
artists of this country, to whose ingenuity and skill, aided and 
encouraged by public rewards, we must attribute the excel- 
lence of the modern watches and time-keepers, so highly va- 
luable for their uses in geography, navigation, and astronomy. 
The principles on which time-keepers are constructed, con- 
sidered in a theoretical view, afford an interesting subject of 
investigation. It is always satisfactory to compare the motion 
of machines with the general laws of mechanics, whenever 
friction and other irregular forces are so far diminished as to 
allow of a reference to theory ; especially if inferences, likely 
to be of practical use, may be derived from such comparison. 
* Anno 1658.— An inscription on a balance-spring watch, presented to King 
Charles II. fixes the date of this invention to the year 1658. Dr. Derham relates, 
that he had seen the watch, on which the following inscription was engraved : “ Ro- 
bert Hooke inven. 1658. T. Tompion fecit, 1675 « Dr. Derham on Clock- 
work, p. 103. 
