An important component of these devices is the escapement.  
Chauncey Jerome was able to manufacture these devices so cheaply that the British government seized several shipments assuming that Jerome was dumping well below cost.  Mercury was added to compensate for temperature changing the length of a key component of these devices by "Honest" George Graham, who also introduced horizontal and dead-beat versions of their most important part.  The price of these devices was drastically lowered thanks to men like Eli Terry, who created the (*) Connecticut shelf variant.  One of these called the H4 finally won a Â£20,000 prize set up by a 1714 act.  An important component of these devices is the escapement.  For 10 points, name these devices, several of which were designed by John Harrison to solve the Longitude problem, and come in "grandfather" varieties.
Chauncey Jerome was able to manufacture these devices so cheaply that the British government seized several shipments assuming that Jerome was dumping well below cost.  
clock [or watch or chronometer or timepiece]