498 Prof. Q. Majorana: Theoretical and 



pressure when the vacuum was established in it. Special 

 artifices have been devised in order to command from outside 

 the beam and the little rider of a centigram upon it. The 

 scales' original plates also have been taken off. Under 

 the right arm of the balance a glass tube D has been fixed 

 to the protecting box ; this tube connects the box itself with 

 the ambient in which is the mass m, as will be describe^ 

 further on. Under the left arm is a metallic protection that 

 contains a leaden ball m ! , which acts as tare of the mass m : 

 this is also a leaden ball. On the beam at its middle point 

 is a concave mirror S for the observation of the oscillations 

 with a beam of light and vertical scale. The balance and 

 the box are on the bracket table L fixed to the wall. 



Under L and on the floor of the room is the recipient U, 

 destined to receive the mercury that will surround the 

 mass m. It is made out of very strong pieces of wood 

 joined together ; it is a cylinder whose diameter and inner 

 height are about 22 cm. In the axis of the cylinder U are 

 fixed two brass tubes R, T, of which one is the prolongation 

 of the other, and these are joined by means of a hollow brass 

 sphere V, whose diameter is 79 mm. This sphere can be 

 separated in two pieces by means of a joint with screws in 

 its diametral horizontal plane. In the interior of Y and 

 concentric with it there is a second hollow brass sphere V, 

 whose diameter is 70 mm. It is connected by means of a 

 brass tube N to the glass tube I), that comes down from the 

 balance. The sphere 'V and the tube N do not come in 

 contact with any point of the sphere V and tube T. The 

 sphere V can be separated, like V, into two hemispheres; 

 so that it can contain the leaden ball m. This is suspended, 

 by means of a brass wire, to the right arm of the balance, 

 through the tubes D and N. The enlargement Z of this 

 wire allows us to control with the cathetometer the position 

 of the m ball, relatively to the recipient U. In this 

 the mercury can flow upwards ; this liquid can be removed 

 at will by aspiration. The levels that the mercury reaches 

 when U has been filled or emptied nearly completely are 

 controlled rigorously by electric contacts P, P', that can 

 be easily regulated. Moreover, a delicate system consisting 

 of a float K and its counterpoise K 7 shows, by means of a 

 mirror S', the position that the mercury takes every moment 

 in the recipient U. All the fittings are made with a precision 

 superior to 2/10 of a millimetre ; within this limit it can be 

 reckoned that the mercury has its centre of gravity coincident 

 with the centre of gravity of m, the leaden ball. This has a 

 mass of 1274 gr. ; the mercury of 104 kg. 



The balance with its accessories maintains the vacuum in 



