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This instrument is a development of the peiidulum mag-iietometer f o r the ineasurenient of saturation inagnetizarion: it has been found especially useful for exploratory work on new materials: since the test samples it requires are sinal1 and of simple shape.The specimen is attached to the end of a flat nonmagnetic spring, niaintained in continuous vibration in a non-uniform magnetic jield: by observing the sesonant fiequency wffh and without the field, the magnefization can be calculated by comparison with a reference samyle.Accuracies of about 1 % can be obtained on materials with speci;ic magnetizations of 50 e.in.v. ~ wiiile the loner h i t of usefulness of the instrument is determined priinariiy by the accuracy required. The device is iobust, inexpensive to construct and, in its present forin, may be used j70m rooin teniperature up to 490" C.

Suppose we have identified three clusters of galaxies as being topological copies of the same object. How does this information constrain the possible models for the shape of our universe? It is shown here that, if our universe has flat spatial sections, these multiple images can be accommodated within any of the six classes of compact orientable three-dimensional flat space forms. Moreover, the discovery of two more triples of multiple images in the neighbourhood of the first one would allow the determination of the topology of the universe, and in most cases the determination of its size.PACS numbers: 04.20. Gz, 98.65.Cw, 98.80.Es, 98.80.Jk 1 Provided that the scale of compactification is small enough (see [2,3]).

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