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(a) We imagine the rigid body, to which the place specification

is referred, supplemented in such a manner that the object whose

position we require is reached by the completed rigid body.

(b) In locating the position of the object, we make use of a

number (here the length of the pole measured with the

measuring-rod) instead of designated points of reference.

(c) We speak of the height of the cloud even when the pole which

reaches the cloud has not been erected. By means of optical

observations of the cloud from different positions on the ground,

and taking into account the properties of the propagation of

light, we determine the length of the pole we should have

required in order to reach the cloud.