Velocity of Transmission of Disturbances through Sea-water. 505 



Besides this, the time is estimated by the distance between the 

 breaks, not in one line, bnt in two lines separated by about 

 3 mm., the fork-line being about 8 cm. below the lowest of the 

 scribing lines. It was clear at the outset, therefore, that special 

 arrangements must be made if real accuracy was to be looked for. 

 This we have attained in the following manner : — 



By the construction of the myograph, and especially of the scribers, 

 it is possible to set the two scribing points and the scribing point of 

 the fork in the same vertical line. Now, by proper adjustment, this 

 line is brought to coincide with a line passing through the knife- 

 edges and the centre of gravity of the smoked surface of the rect- 

 angular glass plate when the pendnlum is at rest. The three points, 

 therefore, at any instant during the motion of the pendulum, lie on a 

 radial line passing through the centre of suspension of the pendulum. 

 If, therefore, proper radial lines are drawn on the plates, correspond- 

 ing to any epoch marked by the scriber, these lines will cut the 

 " fork " line at the point corresponding to the same epoch. In fact, 

 the object is to find the exact point on the plate which was in contact 

 with the fork at the instant that the scriber concerned began to 

 move. 



We had a scribing table made to find these points in the following 

 manner. A crossed board was prepared, about a foot longer than the 

 distance from the knife-edges of the pendulum to the bottom of the 

 glass plate. A ground axle-pin was provided, passing through the 

 board perpendicular to its plane near one end, and firmly fastened to 

 it by brass collars. A long brass rod of rectangular section was fur- 

 nished with a projection on one side at one end, and through this 

 projection was bored the hole into which the axle-pin was ground. 

 The brass rod could therefore revolve on the board about a point 

 situated at a distance of about three-quarters of an inch from one of 

 its shorter edges. The distance between the knife-edges of the pen- 

 dulum and the centre of the glass plate was carefully measured, and 

 a recess was cut in the board in such a manner that when the glass 

 plate was dropped into the recess it occupied a position with respect 

 to the pivot precisely like that occupied by it during an experiment 

 with respect to the knife-edges of the pendulum. A Y-shaped groove 

 was cut in the upper surface of the brass rod, over a length rather 

 greater than the breadth of the glass plate. A carefully made tri- 

 angular plate of brass w T as furnished with two studs on its lower face, 

 and also with an adjustable sharp scribing point, just as far perpen- 

 dicularly from the line joining the centre of the two studs as the 

 prolongation of the centre line of the Y-groove was from the centre of 

 the axle. Consequently, if the brass plate is caused to slide by means 

 of its studs up and down the groove, the scribing point marks a line 

 which if produced will pass through the centre of the axle. The 



