122 SEC. 3. MEASUKEMENT. 



46 6a. Jamin's Compensator. M. Lutz, Paris. 



466b. Diagram representing the Great Westminster 

 Clock. E* Dent and Co. 



This is by far the largest and most powerful clock in the world. The clock 

 frame is 15 feet 6 in. long, and 4 feet 10 in. wide. The escapement is the 

 double three-legged gravity, and the pendulum which controls it weighs 

 685 Ibs., is 14 feet 5 in. long, and vibrates once in two seconds. Its compen- 

 sation is effected by zinc and iron tubes. The dials, four in number, are 22|- 

 feet in diameter, and the bell on which it strikes, " Big Ben," weighs nearly 

 14 tons. 



463c. Diagrams representing the New Standard Clock 

 of the Royal Observatory, Greenwich. E. Dent and Co. 



One of these is a view of the back of the movement of the Greenwich 

 clock, showing the escapement, the galvanic contact springs, and the contri- 

 vances invented by Sir George Airy for altering the compensation of the 

 pendulum and for altering the rate of the clock without stopping it. The 

 other diagram shoAvs the barometric compensation. 



466d. Collection of Compensation Balances. 



E. Dent and Co. 



No. 1. An early form of balrffice. Steel connexions are fastened near the 

 root of the rims of a plain brass balance ; the expansion or contraction of these 

 being less than that of the central brass arm, the rims are by any change of 

 temperature tilted towards or away from the axis of motion. 



No. 2. An eariy form of balance. Loops formed of brass melted on to 

 steel are fastened upon each side of the axis of motion, in consequence of the 

 greater expansion or contraction of the brass, these open or close with the 

 change of temperature, and drag in or thrust out the small brass weights, to 

 which they are attached by wires. 



No. 3. An early form of balance. The riuis are of brass melted upon steel, 

 the brass being outwards ; with any change of temperature the rims open or 

 close. 



No. 4. An early form of balance. A flat steel bar has soldered to its 

 extremities underneath pieces of brass ; the ends of the steel bar carry uprights 

 bearing weights upon their summits, the brass pieces underneath having a 

 different rate of expansion to the steel, bend it either upwards or downwards, 

 and tilt the uprights carrying the weights towards or away from the axis of 

 motion. 



No. 5. A balance of similar design, but having brass melted upon the 

 steel, instead of merely being soldered to its extremities. 



No. 6. A balance of modern design, similar in its action to No. 5. 



In order to obtain perfect compensation, it is found that for an increase of 

 temperature the compensation weights must advance more rapidly towards 

 the axis of motion, than for the same decrease of temperature they would 

 recede from it. This peculiarity necessitates what is called secondary com- 

 pensation. The following balances have been introduced to obviate this 

 error : 



No. 7. Compensation pieces formed of brass melted upon steel receive such 

 curves, that with any increase of temperature the compensation weights move 

 towards the axis of motion more directly than they recede from it with any 

 decrease of temperature. (Dent's balance.) 



