upper or lower scale, the latter of which is beneath the water in 

 the tumbler. The coincidence of position of the scales, when the 

 substance or the weight is in them is determined by the co- 

 incidence in position of the reflected image of the bead in 

 the two cases. It is only adapted for such small specimens as 

 can be placed on the scales and do not unduly strain the 

 springs. 



84. Walker's specific -gravity balance. Later form. 

 E. 48. 1889. Made by G. Lowdon. 



This is constructed on the principle of the steelyard, and 

 is suitable to solid objects of moderate size. The steelyard 

 is supported on a brass upright, on which it rests by means 

 of a knife-edge ; the long arm is graduated. The shorter arm 

 supports the knife-edge of a suspended carrier, on which may 

 be placed one or more short cylindrical brass weights ; by 

 this means the range of weighing is greatly increased. The 

 longer arm supports by means of a thread the object to be 

 weighed, which may thus be shifted along it to the position 

 of equilibrium, which gives the weight by a suitable reading of 

 the graduation, whether the object be suspended in air or in a 

 tumbler of water placed below the beam. Too great oscillation 

 of the beam is prevented by a limiting slot in an upright, in which 

 the end of the beam works. 



85. Walker's specific-gravity balance. Earliest form. 

 1887. Lent by Prof. Judd, F.R.S. 



In this the head of a geological hammer takes the place of the 

 definite brass weights, and the short arm is not graduated ; all 

 the pieces are of steel 



86. Attwood's specific-gravity balance. Made by 

 Casella. 



Lent by the Geological Laboratory, Royal College of 

 1887. Science. 



This is also on the steelyard principle, but may be suspended 

 from any convenient support, such as the ring of a retort stand. 

 The steelyard is balanced on a knife-edge working within a sus- 

 pended ring, and carries a long vertical index, which moves over 

 a fixed graduated arc below, as in an ordinary balance, to indicate 

 when the steelyard is horizontal. The specimen to be weighed 

 is hung on the longer arm, quite close to the point of support, 

 either by a hook or in a copper scale pan, suspended to the hook, 

 which has also a hook below, from which the specimen is sus- 

 pended when weighed in water contained in a tumbler placed 

 below. The weighing is effected by two sliders ; that on the 

 short arm is a heavy one, and its bevelled edge is placed on one 

 or other of the graduations which rise by ounce intervals from 



