320 HISTOEY OF ASTRONOMY. 



will break the circuit whenever it passes out of the 

 mercury, and restore it again as soon as it touches the 

 mercury. 



In 1843, at the launch of the frigate Earitan at 

 Philadelphia, an attempt was made to ascertain the rate 

 of motion of the ship when running off the ways, by 

 observing the time required for certain marks on the 

 side of the ship to pass fixed points of sight. As this 

 method proved unsatisfactory, Mr. Saxton contrived a 

 machine for registering the motion with certainty. This 

 contrivance consisted of a half second's pendulum, with 

 a pin projecting from the rod. This pin acted on an 

 angle piece attached to a lever which had a small conical 

 cup at the end, containing a mixture of oil and ver- 

 milion. The cup had a small hole at its apex, and the 

 mixture was prevented from flowing out by the capillary 

 action of a small lock of cotton. A reel, containing 

 about 60 feet of white cotton tape, was so placed that 

 the tape passed under the cup as it was drawn out, and 

 had a red dot struck on it every half second by the 

 vibration of the pendulum. 



Some time afterward, when the steam frigate Princeton 

 was ready to be launched, Mr. Saxton constructed a 

 machine of the kind here described. A wire about two 

 feet long was attached to a small ring that hooked on a 

 detent, which held the pendulum when it was raised up 

 on one side. The other end of the wire was attached 

 to the bow of the ship, so that" the first motion of the 

 ship would unhook the pendulum, which, falling to its 



