1. Bic&k J . 1980 Proc. R . Soc. Lond. A 371.
2. C arter B. 1973 I n Black holes (eds B. D eW itt & C. D eW itt). New Y ork: G ordon & B reach.
3. C arter B. 1979 I n General relativity (eds S. W . H aw king & W . Israel). Cam bridge U niversity Press.
4. Gibbons G. W . 1974 Gommuns math. P hys. 35 13.
5. Gibbons G. W . & P erry M. J . 1979 M ore gravitational instantons and theforces between them. (In prep aratio n .)