Posthuma Fosteri: The Description Of A Ruler (1652)

This book was published posthumously in the year that Foster died. It describes a ruler with 9 scales and gives instructions on how to use the scales to solve many of the standard problems of “astronomie, navigation, and dialling”. In addition to familiar trigonometric scales, the ruler includes two new scales introduced by Foster. The first provides an easy means of laying out horizontal sundials between 30º and 60º of latitude without having to draw any auxiliary lines. The second is a true Foster innovation: with this scale and a chord scale, Foster is able to find all the elements needed to draw a sundial on an arbitrary plane. He also uses these scales to replace the usual dialing scales (also a Foster invention), laying out a sundial from equispaced points on the circumference of a circle. One page of the current edition includes a redrawn version of the ruler that the reader can use in conjunction with the text.