Many problems arise in stress analysis which are not reasonably susceptible to mathematical treatment. Recourse may then be had to experimental methods. One of the most useful of these is the photoelastic method. This method is based upon the fact that transparent materials when stressed undergo changes in their optical properties which can be measured and related to the applied stress.

Multitudes of young- men are starting to study navigation. As a rule it is only after many months of painful work that they come to see what it is all about –many of them never get to that point at all.

In none of the books on navigation have I been able to find a simple picture, defining each step in the reasoning that enables us to navigate by the stars. Hence this effort. As set out here the principles can be grasped by anyone.