The pressure of the air is a convenient unit to employ in practical work, where it is called an " atmosphere "; it is made the equivalent of a pressure of one **kg**/cm'; and one ton/inch 2, employed as the unit with high pressure as in artillery, may be taken as 150 atmospheres.

Draw a vertical at D, intersecting fh, **kg**, in s and q.

With metric units, measuring P in **kg**., and C in litres, the G.D.

The kilogram (**kg**.) is represented by an iridio-platinum standard weight, of cylindrical form, by which all other metric weights, and all measures having reference to metric weight, are ascertained in the United Kingdom.

For the specific mass of the cubic decimetre of water at 4° C., under an atmospheric pressure equal to 760 mm., Guillaume and Chappuis of the Comite International des Poids et Mesures at Paris (C.I.P.M.) have obtained 0.9999707 **kg**., which has been accepted by the committee.

When the body is twisted through an angle 0 the threads make angles aC/i, be/I with the vertical, and the moment of the tensions about the vertical through G is accordingly **Kg**, where K = M gab/i.

To find a point in its line of action, take a point Q on the rod such that **KG** X GQ = R2, R having been determined experimentally by the method of i25; join G with 0 and through Q draw a line parallel to BO to cut GO in Z.

The condition that must exist in order that the balance may weigh correctly for all positions of the weight W is w = o, or tan 0 = that is, the stay **KG** must be adjusted parallel to the line joining the points A and C. From the equation for w, it is seen that the larger h is the smaller w will be.