A very unusual possibly unique early Gunnery Officers aneroid pocket barometer by Negretti & Zambra No 740, marked Ordnance Survey No 1, c1863

Pocket barometer having 1⅞” silvered and die struck dial, the periphery bearing standard meteorological terms, the barometric scale calibrated in inches of mercury with an extended range from 24” – 32” divided down to 1/20th. The lower part signed by the maker, “Negretti & Zambra, London” and serial no. “740.” Blued steel pointer, rotating bezel mounted barb index, bevelled glass.

High gilt finished finely poised movement driven from a single 1.086” nickel alloy capsule tensioned on a coil spring, decorated chassis, the spindle and cock mounted to a sub-chassis both struck “17,” compound steel braced bridge with micro height adjustment, the primary lever with inverted temperature compensation strip.

All contained within a ribbed semi barrel form case with pendant and bow, the verso engraved with early ballistic equation, OS government property mark and “No 1.”

Condition: The subject of a full service, conservation, and calibration under laboratory conditions, see performance chart from dynamic test. The movement working exceptionally well across the scale with maximum noted error of 0.15” at 24 inHg. Note: it is only been possible to estimate accuracy at 32 inHg.

The dial with minor marks and some signs of ageing. The lacquered brass case clean and bright, the ballistic equation crisp and clear to read.

Comments: This is one of the first truly pocket sized aneroid barometers made, dating from around 1863. The extended scale to 32” Hg is unusual and more generally associated with barometers detailed to mining operations.

This barometer, however, had an artillery purpose, and would have been used by a Gunnery Officer, and then one in charge of very large guns – probably fortress or seacoast artillery – which had a range, to assist in the calculation of air density. The Officer would then have passed down instructions to the gun layer, a member of the gun’s detachment, who did the final aiming of the gun.

The movement of this pocket barometer is exceptional in terms of quality and finish, clearly the subject of special attention at the time of making reflecting perhaps the importance of such a government order. The verso carries a naïve early ballistic equation, perhaps as an aide memoire to the Gunnery Officer. It should be noted that at this time huge advances were being made in the science of gunnery – particularly important was the work of Armstrong in the design of heavy artillery for land and sea with concentrically constructed rifled barrels. This in turn led to extended ranges and rapid research into the effects of range and velocity brought about by, amongst other influences, barometric pressure, denser or thicker air producing greater drag, the converse being also true.

A basic explanation of the ballistic equation is as follows:

D stands for the range of the gun, as calculated from B, b, T, and t.

The range of the gun is affected by the resistance of air to the passage of the projectile and this is related to the density of the air. More dense air will provide more resistance to the projectile, and so reduce range. A higher barometric pressure increases air density, so the range will be reduced. A higher ambient temperature reduces air density, and so increases range.

B and T are an assumed standard barometric pressure and temperature, and may vary according to the type of gun. The artillery officer would need to know what B and T are for the particular gun his unit is equipped with. For example, B might be 30 inHg, and T 55 degrees F.

b is the difference between the actual ambient pressure measured by the pocket barometer and B. If the ambient pressure is greater than B, b will be positive, if ambient pressure is less than B, b will be negative. Plugging B and b in the equation will lead to a decrease in D if b is positive (ie if the ambient pressure is greater than T), and an increase if b is negative.

Likewise t is the difference between the actual ambient temperature measured by thermometer and T. If the ambient temperature is greater than T, t will be positive, if ambient temperature is less than T, t will be negative. In this case, plugging T and t in the equation will lead to an increase in D if t is positive (ie if the ambient temperature is higher than T), and a decrease if it is negative.

Where there is less certainty is the question of what units the range D will come out as. A typical result for D would be in the region of 49,000. If this were feet, that would give a range of about 9 miles; if inches, the range would be about 1,400 yards, or a bit more than 3/4 of a mile. Research continues!

Eventually, the equation was replaced by a system of printed tables for the artillery officer to look up ranges, to increase speed and reduce errors of calculation. To use the equation, the officer needs to complete four calculations. Eg, for the temperature part 60/12, then add the result to 70. For the pressure part, 29/31. And then multiply those two results, then multiply the result by 700. And he would have to do it all over again if he noticed a change in pressure or temperature. No wonder a system of printed tables was developed. (Nowadays, of course, with nearly 200 years of development, much more complex models are used with computers to come up with the results.)

We are greatly indebted to Ann Davis BSc for her kind help with the above definition.