 For example, when the inverse-cube force is centrifugal (directed outwards), so that "μ" < 0, the curve does not even rotate once about the centre.This is represented by case 5, the first of the polar equations shown above, with "k" > 1 in this case.Samuel Earnshaw in a book published in 1826 used the term “Cotes’ spirals”, so the terminology was in use at that time.