Facial Aging

A basic knowledge of the physical aging processes allows you to accurately draw people of various ages. The age regression and progression techniques, discussed in this heavily illustrated article, can be invaluable for accurately portraying or modifying the ages of portrait subjects
drawspace

Serene Scene

Sketching is an action word, and you can only learn this skill by actually sketching. To give you a feel for the sketching process, numerous illustrations and simple text take you step-by -step through the process of rendering a sketch from one of my sketches (much easier than working from an actual scene).
drawspace

Hatching Value Scales

When you can render sets of hatching lines well, you discover a very fast and simple way to achieve realistic shading in your drawings. Many different styles of hatching sets can be rendered, from lines that are very noticeable, to lines drawn so closely together that they look like a solid tone.
drawspace

shading techniques

Cartoons are always fun to draw, and a lighthearted way to strengthen your drawing skills. My sketch of this mischievous looking toddler incorporates both crosshatching and hatching shading techniques
for mor information:

Slide rule

This is not a drawing tool per se, but was used by educated people for calculations. The analogue calculator is based on the logarithms, discovered (or developed) by Leibnitz, Newton and Napir 1). This principle reduces multiplication and division to addition and substraction. Potenzation and roots are reduced to multiplication and division. But also special scales for trigonometric calculations etc. were in use. The typical slide rule of an engineer had 25 scales and was 25cm long. this allowed an accuracy of 3 - 4 digits.
The sliding window shows the square of 4 (scale D) to be 16 (scale A). The tong shows the multiplication of 1.26 by 3.17 (C) = 4 (D).

Protractor

Various forms are still in use at school. The most used one provides 180°. I still have one with 400g for the full circle (and I also have trigonometric tables for these 'neugrad' or 'new gradation' which allows more easy mathematical handling of arcs).
For military use 'artillery' versions with 6400 units for the full circle were available (at least in Switzerland).

Camera lucida

You might know the camera obscura, which works only for very luminous situations. Camera lucida (light chamber) was developed by W. H. Wollaston in 1807 to draw landscapes etc. with ease.
The main component is a 4-sided glass prism. From the oject G the light is seen by the eye A in Z, where the pencil can be used to draw the object.
When the instrument is built with mirrors rather than a prism it is called mirror lucida.