Carriage and gantry comprising such a carriage

[0026] Reference is made first of all to FIG. 1, which depicts a quadratic gantry 1 comprising first and second lateral girders 2, 3 substantially parallel to each other. The girders can have a length attaining 20 to 30 m, and be spaced apart by 5 to 10 m for example. The direction x of the lateral girders 2, 3 is said to be longitudinal, and the orthogonal direction y situated in the plane of the girders is said to be transverse. A third direction z, orthogonal to the other two, is also defined. The direction z is said to be vertical. However, this term is employed only for the purpose of simplification of the description, since the plane formed by the lateral girders 2, 3 may be not horizontal but inclined. A location close to the longitudinal axis situated halfway between the two lateral girders 2, 3 is said to be “internal”, in contradistinction to a location at a distance from this axis, said to be “external”. The terms “top”, “bottom”, “above”, “below”, “high”, “low” will be empl