20 Dr. Fordyce's Account of a new Pendulum. 
acquired the cold, it could hardly happen that any sensible dif- 
ference in the going of the clock could arise in any period ol 
twenty-four hours, whether transits of the sun or of any of 
the fixed stars were taken. 
The wire in each vibration hangs, during a certain por- 
tion of that vibration, between the two cylinders, and touches 
neither of them : during that time, the point Q must be con- 
sidered as the top of the pendulum, not the slit between the 
cylinders ; but this part of the vibration may be so very small 
a proportion of it, as not to make any sensible error ; and it is 
accompanied on the other hand by a very great advantage. 
Except in Mr. Arnold's compensation for heat in watches, in 
all the other modes a surface or surfaces necessarily slide over 
one another ; whenever this happens, if heat, by expanding one 
of the bodies, is to make its surface slide over the other, it has 
two things to accomplish, to overcome the vis insita of the mat- 
ter, and the attraction of the two surfaces to one another. 
When then there is heat enough applied just to overcome the 
vis insita, it would not be sufficient to overcome the attraction 
likewise, excepting the matter was infinitely hard and ine- 
lastic. Although the heat therefore be increased, the com- 
pensating parts at first do not move so much as to overcome 
both these resistances, afterwards the parts jerk on suddenly, 
and in many cases go beyond what they otherwise would have 
done. As none of the expanding parts are to slide upon one 
another in Mr. Arnold's compensation, and there is a time in 
every vibration, in the apparatus above described, when none 
of the expanding parts slide over any thing, this disadvantage 
is avoided. 
