NATURAL PHILOSOPHY. 221 



OX THE CONSTRUCTION AND ARRANGEMENT OF TIME BALLS 

 AT THE EDINBURGH OBSERVATORY. 



The earliest signal-balls which were made, though provided with 

 ropes passing over pulleys by which they were enabled in their 

 descent to raise a series of weights in order to check in a gradual 

 manner the velocity of their fall, were yet invariably found, after a 

 short time, to pull or smash themselves to pieces. Steel springs were 

 next tried to break the force of the concussion, but were pretty sure 

 to be themselves snapped with a heavy ball, while a light one would 

 not descend quick enough on a windy day. Recourse was finally had 

 to compressed air, a spring of perfect temper, never injured by time 

 and capable of any degree of delicacy at first, and any amount of 

 violent resistance at last. To carry out this principle, a staff was 

 attached to the ball below, terminating in a piston, which in the course 

 of its descent entered an accurately turned cylinder, and compressing 

 the air therein, was gradually brought to rest. Where the cylinder 

 quite closed at the bottom, the spring of the included air might be 

 greater than required, and also have a tendancy to throw the ball 

 up the mast again, which would be somewhat troublesome to observers. 

 But by simply opening a graduated aperture below, so as to admit of 

 the air partially escaping as it is compressed, the strength of the spring 

 is diminished, and by the time that the piston has descended to the 

 lowest point, there is so little air remaining in the cylinder, and it is 

 escaping so fast, that there is no power left to make the ball rebound. 

 Thus the time-ball is made to descend without injuring the building 

 or spoiling itself; and the trigger apparatus, by which the detent that 

 holds the ball when hauled to the top of the mast is unlocked, being 

 very nicely adjusted, and observers being duly cautioned to look to 

 the instant of separation of the ball from the cross-staff the descent, 

 that is, the first part of it is as instantaneous as needs be. In the 

 next place, the trigger being pulled, not by the finger of a person at 

 the ball, but by an electro-magnet which is instantaneously set in 

 action by the contact made at the end of a wire led into the walls of the 

 observatory, and brought immediately before the transit clock itself; 

 the instant for the signal outside can be conveyed to the undeviating 

 mechanism there, with all the refinement of a chamber experiment, 

 and to the utmost extent of the observer's knowledge of the real time 

 by the stars, as obtained the previous night, and continued on by the 

 clock. 



For raising the ball, a plan has been proposed by which a weight 

 having been wound up at any previous hour of the day or night, then 

 on electrical contact being made at the observatory by the astronomer 

 at a precise moment, that weight is unlocked, immediately descends 

 and hauls up the ball. Next at five or any other number of minutes 

 a second contact being made on another wire, lets the ball down. 



