1. This rule is designed to enable a machine gun to be aimed accurately.
2. Before slide rules for the Vickers machine gun were introduced in 1924 all calculations were made using range tables. These included information on the trajectory and, in effect, the amount of scatter. Some of the information on the range tables was transferred to the slide rule and some of the information was given as a table on the back of the slide rule (Picture 3). (At least to me it is the back. In the instructions it is referred to as the front!) The columns are:
- Range in yards
- Tangent angle. The angle above horizontal required for a given range.
- Lift. The rate of change of tangent angle.
- Angle of descent. The angle at which the bullets strike the ground.
- Length of beaten zone. The scatter along the axis of firing.
- Width of beaten zone. Note how much smaller this is than the length.
- Position of lowest shot below centre of cone.
- Correction for air temperature, expressed as minutes of tangent angle for every 10°F above or below 60°F.
- Correction for barometric pressure, expressed in minutes of tangent angle for every inch of mercury above or below 30 inches.
3. Above the table, picture 3, there are the words 'Length of cord for graticule purposes = 24 " '. The cord, which has a small bead at the free end, can be seen on picture 2. This cord was used with the upper scale, picture 4, with numbers 1 to 6 highlighted. The length between the markings is 0.42". The cord is 24" long. 0.42 divided by 24 is 0.0175. Tan-1 (0.0175) = 1º (and 9"), so, by holding the bead against one eye and holding the rule with the string extended it is possible to estimate angles, in degrees between two points.
4. The lower scale on picture 4 shows the safe range to troops for a given range setting on the gun. In the example shown if the gun is aimed at a target 1900 yards away then your own troops should not advance more than 1500 yards.
5. Reversing the slide on the upper part of the rule allows wind correction to be calculated for both direction and range (Picture 5). The upper part shows how many minutes correction is need for a side wind of 20 mph. The correction can also be adjusted to allow for the direction of the wind relative to aiming direction. In the case shown at range of 1500 yards and with wind 3/4 across, the correction is 40 minutes. The corrections for wind across, half across etc, are effectively the cos of the angle. The lower part of the side shows the correction of gun elevation for range. In the example shown with the same range and the wind 1/4 against (to complement the 3/4 across) the correction is 5 minutes.
6. The rule can also be used to estimate the range to the target. Picture 6, not to scale, shows a typical situation. With the gun at the base of the triangle the object is to calculate the distance to the target at the apex. Since the triangle is not a right-angled triangle it is necessary to compensate for the fact that the base is at an oblique angle. Figure 7 shows that with an oblique angle of 40º and an oblique base of 50 yards, the true base is 32 yards. Using the true base and the angle in degrees, 3º, the range can be calculated as 600 yards.
7. The slide rules were used from 1924 until 1952 when range tables were again used until the Vickers gun was removed from service in 1968. This slide rule is a Mark III and is dated 1940. In 1942 it was replaced by the Mark IV made of aluminium.
8. For more information on Vickers machine guns visit this site: http://www.vickersmachinegun.org.uk/
9. The rule was made by Davis of Derby (UK)
My thanks to N Royall of the Machine Gunners Corps Old Comrades Association and R Fisher who maintains the web site mentioned above.
