ON TIME MEASURERS. 339 



the zinc and steel compensation. The reason is this. The mercury 

 in the jar may be considered as a solid mass presenting but a com- 

 paratively small surface to the action of the atmosphere by which 

 changes of temperature are communicated to it, whilst the steel rod 

 and suspension spring present a much larger surface in proportion to 

 their bulk, and are acted on much more quickly, so that in .a variable 

 temperature you have one part of the pendulum expanding or con- 

 tracting, as the case may be, before the other, and an unsteady rate is 

 the result. It is obvious that the greater the diameter of the jar the 

 more sluggish will be the compensation, and it has been proposed to 

 put the mercury into several small jars, but there are inconveniences 

 to this. 



Now, in a zinc and steel pendulum, those parts on which we depend 

 for the compensation are much more nearly of the same bulk, and 

 present nearly the same surface to the action of the atmosphere, so 

 that it is fair to infer they act simultaneously, or almost simultaneously. 

 In these pendulums you will observe that the bob is supported at its 

 centre, so that its expansion or contraction, let it occur when it may, 

 does not alter the theoretical length or affect the time of oscillation. 



Some persons overlook or ignore the importance of suspending the 

 pendulum bob at its centre, and so nullifying the effect of changes of 

 temperature upon it ; but we cannot but think that a little careful con- 

 sideration on their part will lead them to allow that it is not without 

 advantage. 



We have been favoured by the Astronomer Royal with a copy of 

 the rate of one of our clocks having a zinc and steel compensation. 

 This clock was going for several months in one of the small observa- 

 tories used in the Transit of Venus Expedition. It has been printed 

 for the purpose of showing the behaviour of a zinc and steel pendulum 

 under as trying circumstances as can well be conceived, the daily 

 variations of temperature being frequently fifty degrees. 



We will now pass on to the barometric connection. The extreme 

 steadiness of rate of the Greenwich Standard Clock at equal pressures, 

 led Sir Geo. Airy to attempt the corrrection by means of the 

 magnetic apparatus you may have seen below, a drawing of which we 

 have here. Two bar magnets, having their poles reversed, are fastened 

 vertically one in front and the other at the back of the pendulum bob, and 



