1878.] 



using the Balance with great delicacy, §c. 



9 



over round to the extreme right, when A will be just over and nearly 

 touching the scale pan. By continuing the motion slightly A will be 

 gently deposited on the pan, and the side rod will move slightly down 

 leaving the weight quite free. On the scale pan are four pins, turned 

 slightly outwards, acting as guides for the small pan, and ensuring 

 that it shall always come into the same position. The wheels and 

 pinions are of such a size that two revolutions of the rod just suffice 

 to change one weight for the other. 



It will be seen that all the manipulation required from the observer 

 during a series of weighings is the simple turning of three rods, which 

 are prolonged out of the balance case to where he is stationed at the 

 telescope. By turning one of these he can change the position of the 

 rider on the beam by a known amount, and so find the value of his 

 scale. By turning a second he clamps the scale pan, and so steadies 

 the balance while the weights are changed by turning a third rod. I 

 have made this arrangement not only because it seems as simple as 

 possible to secure the end required, but also because it seemed more 

 applicable to a vacuum balance (with which I hope ultimately to test 



it). 



I take this opportunity of expressing my thanks to Mr. Thomas 

 Foster, mechanician of Owens College, for his aid in the construction 

 of the apparatus, and in the planning of many of its details. 



Method of conducting a Series of Weighings. 



After the counterpoise has been adjusted so that the beam swings 

 nearly about its horizontal position, the frame is lowered so that the 

 balance is ready for use. The pan is then clamped and the balance is 

 left to come to a nearly permanent state of flexure if possible, some- 

 times for the night or even longer. The lamp is lighted usually half- 

 an-hour or more before beginning to observe, so that its effect on the 

 balance may attain a more or less steady state. It is necessary also 

 to wait some time after coming into the room, for the opening of the 

 door will always cause a considerable and immediate deflection of the 

 beam. When a sufficient time has elapsed, the observations are com- 

 menced with a determination of the value of one scale division by 

 means of the riders. The three extremities of two successive oscilla- 

 tions are observed with one of the riders resting on the beam. These 

 are then combined as follows : — The mean of the first and third is 

 taken, and the mean again of this and the second, this constituting the 

 " resting point," that is, the position of equilibrium of the beam at the 

 middle of the time. For instance, in weighing JSTo. I (see tables at the 

 end) the three extremities of successive oscillations were 2805, 312'0, 

 and 286*0 (column 2). The resting point was taken as — 



280-5 + 286-0 + 2 x 312-0 _ 297>6O 



