I don’t want you to use rulers and compasses for free, artistic sketches and paintings. But the designs and lettering that you are doing for your Nature Book are more like commercial art. The lettering better when it is straight and not dripping down the page.
If you draw along the edge of a ruler, you should have a straight line. However, if you ruler is slanted on the paper, you will only have a straight, slanted line–that is still not appropriate for most lettering and other straight line purposes. Tlo draw a straight line for lettering, the ruler must be straight on the page.
A compass is used to draw circles. Since I have you draw clocks around your images–to help you place other marks correctly–a compass could be helpful for drawing those clocks, which are actually not part of your artisitic drawing.
When sketching something round, I do not want you to use the compass. I want your –sketches to be free of mechanical tools–like rulers and compasses