Richardson Ballistic

This robust artillery desktop rule was manufactured by the Geo. W. Richardson company in 1917. Fabricated with Mexican mahogany it was covered with white celluloid were scales were engraved. This rule was constructed very rigid to meet the hard military service, keep the proper alignment, and to resist atmospheric conditions changes, so the scale members were screwed on a 2 in x 18 in x 1/16 in brass metal back, with seven holes to reduce weight, that was 1.2 pounds with its leather case, and 0.75 pounds the rule itself. This rule had no cursor.

The Richardson Ballistic rule was designed to find the horizontal direction an artillery piece has to have in order to hit a non- visible target, using the information coming from an observer unit. The artillery piece, the observer, and the target form an oblique triangle on a map, that will be solved using the Law of Sines studied in a Trigonometry course. This rule has only six scales, two on the fixed parts, and two on every side of the slide. The two scales on the fixed part: A (upper) and D (lower) are the same as the regular logarithmic C-D scales of a standard slide rule. The scales on both sides of the slide are labeled B (upper) and C (lower), and these are trigonometric sine scales but in reverse order (cosecant in the forward order), and instead divided in degrees they are divided in the angle units used in artillery: mils (90 degrees : 1600 mils).

The use of cosecant instead regular sine function makes possible to solve a triangle with only one setting of the slide rule.

It is not possible to know the exact manufacturing year of the specimen in this collection, no mark or serial number is engraved on the rule, bur is by sure about 100 years old. Something that can certainly stated is the sale price: $20.00 USD (about $ 410 dollars in 2021), included in the User's Manual.