It has been known for some years that parallel horizontal structures separated in a vertical plane produce patterns of equally spaced parallel lines that can be detected and mapped by dowsing detector rods, indicating that the patterns are produced by interaction between the structures and the dowsing field, whatever that is. Consequently the arrangement is called a dowsing interferometer and the lines are referred to as interference fringes. The patterns are produced equally well by electrical insulators as by electrical conductors, and the inference that the field is not electromagnetic was confirmed by experiments carried out in an electromagnetically shielded laboratory. Measurements of interferometer fringe spacings made irregularly from 1991 to 1996, analysed retrospectively, were found to be dependent on time of year, decreasing relatively suddenly by a factor of about three in November and increasing again in April. An isolated rise and fall in early March was also found. These results were confirmed by more frequent measurements in 1997 and published the following year. This paper reports a co-operative programme of interferometry between Scotland and New Zealand from 1997 to 2001. The annual pattern of the fringe spacing is clearly defined, with rapid changes between 2m and 6m in November and in April, and an isolated event in early March; but remarkably the pattern in the southern hemisphere is inverted with respect to that in the north. There are differences of a few days in the timings of the events in the north and south, and the amplitude of the March event is increasing.

This paper is a continuation of the work described in Transactions of the Royal Society of Edinburgh: Earth Sciences89 (1998) 1–9.