THE ROLLING CARRIAGE. 



95 



anced by the extension of the spiral spring, while the Rolling Carriage beai's an 

 arm, 1\ carrying a pencil which rests upon a chronograph cylinder to automat- 

 ically record this pressure, the axis of the cylinder being parallel to the track of 

 the carriage and the chronograph rotated by clock-work. The position of the 

 pencil for zero pressure on the spring is marked on the chronograph sheet, and 

 a reference line is drawn through this point, so that distances of the pencil poinl 

 from this reference line are measures of the extension of the spring, while a second 

 pencil, being placed on the opposite side of the chronograph barrel, and operated 

 by an electro-magnet in electrical connection with the mean time clock, registers 

 seconds on the chronograph barrel, and thereby every point of the pressure trace 

 made by the first pencil can be identified with the synchronous points in the 

 trace on the stationary chronograph on which is registered the velocity of the 

 whirling-table. 



Much care was bestowed upon the manufacture and calibration of the spiral 

 springs. The following is a list of the springs, giving their size, length, and 

 Aveight : 



The method of calibration adopted is as follows : 



The spring to be calibrated is fastened at one end to the brass tube of the 

 Boiling Carriage and at the other to a fixed support. A string fastened to the end 

 of the shaft passes over a light, almost frictionless pulley, and carries a bag, in 

 which the weights are placed. The extensions of the spring are registered by 

 the pencil on the chronograph barrel. Settings are made on opposite sides of a 

 mean position, first, by letting the weight fall gradually to its lowest position ; 

 and, second, by extending it beyond its normal position and allowing the tension 

 of the spring to draw it back. In both cases a series of vibrations are sent 

 through the apparatus by the jar set up on the table, by means of a large tuning- 

 fork, so as to overcome the friction of the moving parts. In a portion of the 

 calibration experiments, these vibrations were produced by an electro-magnet. 



