340 SECTION MECHANICS. 



beneath them is a horse-shoe magnet carried by one arm of a lever, 

 the other end of which is in connection with a float icjting on the 

 lower limb of a bent barometric tube. Between the poles of the 

 horse-shoe magnet and those on the pendulum there is constant attrac- 

 tion, so that you have a force acting in the same direction as the force 

 of gravity, but variable with the density of the atmosphere. 



It is found that the clock loses as the pressure increases, so with a 

 high barometer the horse-shoe magnet is raised, the attractive force 

 increased, and the clock made to gain a corresponding amount. 



We have printed a copy of an instructive diagram prapared at the 

 Royal Observatory under the direction of the Astronomer Royal, 

 which shows in a very marked way the effect of changes of pressure 

 on the rate of the Greenwich standard clock before the magnetic 

 apparatus was applied. The red line shows the calculated rate, allow- 

 ing for the pressure, and the black line shows the actual observed rate 

 of the clock. It will be seen how nearly the one coincides with the other, 

 the greatest difference between them being less than one-tenth of a 

 second in twenty-four hours. The correction is now effected automa- 

 tically by the apparatus we have just described, and we have authority for 

 stating that it completely answers the purpose for which it was designed. 



The time measuring power of a chronometer is dependent upon its 

 balance spring, which consists generally of a slight ribbon of hardened 

 and tempered steel, coiled round and round with an upward twist 

 somewhat in the form of a cylinder. 



The elasticity of this little spring performs just the same duty for 

 the balance which gravity does for the pendulum it keeps it swinging 

 backwards and forwards with a degree of uniformity which is truly 

 surprising. 



But as a controlling power the elasticity of this little spring labours 

 under one serious disadvantage as compared with gravity : it varies 

 rapidly with any increase or decrease of temperature. 



It varies more or less according to the material of which the spring 

 is composed ; thus a gold spring suffers a greater change than a steel 

 spring, a steel spring than a palladium spring, and a palladium spring 

 than a glass spring. 



It may surprise you to hear us speak of a glass spring, but the thing 

 has been done, and is here. 



