1878.] 



using the Balance with great delicacy, Sfc. 



13 



be seen the bottom of the weight as it bangs. The weight is 4*248 

 centims. in diameter and is gilded. The mass is a spbere of an alloy 

 of lead and antimony. It was cast witb a " head " on and then 

 accurately turned. Its vertical diameter is 30'477 centims. (about 

 1 foot). The specific gravity of a specimen of the metal was found 

 to be 10422. Its weight given by a weighing machine is 340 lbs. 

 about, and this agrees very nearly with the weight calculated from the 

 specific gravity. I am obliged to accept this as the true weight pro- 

 visionally, until it is found more correctly by the large balance referred 

 to above and now being constructed. 



This mass (fig. I, M) is placed in a shallow wood cup at one end of a 

 2-inch plank, 8 inches wide and 6 feet 11 inches long, mounted on four 

 flanged brass wheels, and serving as a carriage for it (fig. I). A 

 plank about 12 feet long nailed to the floor in a direction perpendicular 

 to the beam of the balance, as shown in fig. I, pp, acts as a railway for 

 the carriage, and a firm stop at each end prevents the carriage from 

 running off the rail. The distance between the stops is rather less than 

 twice the length of the carriage, and the weights hangs down from the 

 balance exactly midway between the stops. The mass is placed on the 

 carriage so that it is exactly under the weight when the carriage is at 

 one end of its excursion against one of the stops. An empty cup 

 (c, fig. 1) of the same dimensions as that in which the mass rests is 

 placed at the other end of the carriage, and is just under the weight 

 when the carriage is against the other stop. By this arrangement no 

 correction is needed for the attraction of the carriage upon the weight 

 or counterpoise, and the effect caused by the removal of the cairiage 

 from one end of its excursion to the other is entirely due to the differ- 

 ence of attractions of the mass upon the weight and counterpoise in 

 its two positions. The position of the mass when directly under the 

 weight is called its " in position," and that when it is at the other end 

 of its excursion is called the " out position." The length of the excur- 

 sion is 5 feet 7' 3 inches. 



To draw the carriage along the rail a vertical iron shaft with a wood 

 cylinder at the lower end. pivots on the floor, and is prolonged up to 

 the level of the observer as he sits at the telescope with a handle by 

 which he can turn it. The two ends of a rope which winds round the 

 cylinder pass through pulleys on the stops, and are attached to the 

 ends of the carriage. The observer can then move the mass with 

 great ease by turning the handle, even while looking through the tele- 

 scope. 



When a series of observations is made, the general method is this. 

 The deflection (r) due to the transference of a rider from one notch to 

 the other on the beam is first observed exactly in the manner before 

 described, the mean of four or five values being taken as the true 

 value. Then the deflection (n) due to the difference of attraction of 



