CHAP, iv.] APPLICATIONS OF THE LA WS OF HEAT. 375 



By this means the weight is carried a little way in towards the 

 axis of motion, which reduces the time of the swing of the balance, 

 at a sufficient rate, to compensate for its slower motion, due to the 

 loss of elasticity of the balance-spring. The reverse action takes 

 place with any fall in temperature. 



The effect of the compensation may be altered by sliding the 

 weights CC towards or away from the ends of the rims. 



The action of this balance, although sufficiently accurate for 

 ordinary purposes, still leaves a small error at wide ranges of tempera- 

 ture. For instance, supposing that you so arranged the position of 

 the weights upon the rims that the chronometer should go right at a 

 temperature of thirty-two degrees and sixty- six degrees ; if you raised 

 the temperature to one hundred degrees, 

 you would find that your chronometer 

 would lose four seconds a day ; and you 

 could not alter this error by sliding the 

 weights further along the rims to in- 

 crease their action in the heat ; because 

 by doing so, you would increase their 

 action in the cold ; and then the chro- 

 nometer would lose in that direction. 

 The best that you could do, would be 

 to divide the error and leave the chro- 

 nometer losing two seconds a day in .the FlG> 258 ._ Dellt . s com p eri sation balance, 

 heat, and two seconds a day in the cold. 



The cause of this error this secondary error, as it is called is 

 that the time of the swing of the balance varies, not as the distance 

 of its weights from its centre, but as the square of that distance. 

 Consequently it requires a greater motion of the weights inwards 

 than outwards to produce the same difference of time. The late Mr. 

 Dent, who was the first to point out the cause of this error, designed 

 the following arrangement (Fig. 258) for correcting it : 



r r is a flat compensation bar, formed of brass melted on to steel, 

 the steel being uppermost. Tjie two loops or staples, s, t, s, t, fastened 

 at each extremity are also compensation pieces, the brass being upon 

 the inside. The compensation weights, v v, are mounted upon upright 

 rods at the extremities of these loops. When there is any increase of 

 temperature the main bar, r r, bends upwards, and tilts in the staples 



