A rare 1950's large-scale double sided demonstration slide rule, by Blundell Rules Ltd of Weymouth, England. Over 5 foot in length. The slide rule or slipstick is a mechanical analog computer. The slide rule is used primarily for multiplication and division, and also for functions such as exponents, roots, logarithms and trigonometry. The slide rule was invented around 1620-1630, shortly after John Napier's publication of the concept of the logarithm. Before the advent of the electronic calculator, it was the most commonly used calculation tool in science and engineering. The use of slide rules continued to grow through the 1950s and 1960s even as computers were being gradually introduced; but around 1974 the handheld electronic scientific calculator made them largely obsolete. Bottle used to represent scale.