German-born Charles Augustus Schmalcalder, a “mathematical and philosophical instrument maker” living in Middlesex in England, patented the first prismatic sighting compass in 1812.
Compasses with sighting apparatus had been in use previously, but since the user had to sight on the target, then either look down at the dial and read the bearing, or have an assistant do the same, they were a little clumsy to use and were somewhat prone to human error. Schmalcalder designed what is really the world’s first “heads up display”. It allowed the user of a compass to sight a target and read the bearing at the same time with the same eye. This revolutionised the art and practice of surveying, meteorology and exploration and was later applied with much success to military land navigation and even later to aviation navigation.
Schmalcalder’s compass “heads up display” was achieved by the use of a flip-up prism. A folding sighting vane attached to the body of the compass was flipped up, the prism was flipped up and focused, the target was sighted using the sighting vane as the “front sight” and a groove in the prism housing as a “rear sight”. When the two “sights” aligned on the target, the bearing to the target could be read off the compass card (dial) using the prism with both the target and the bearing in view. Quick, easy and accurate.
Prismatic compasses using Schmalcalder’s design were made in several sizes, the most common being the large surveyor’s compasses which are still in use today. Less common are the pocket-sized versions designed for explorers and military officers. It is the pocket-sized version we’re looking at in this post.
With a diameter of just over 2 inches (5-ish cm), the pocket version of the compass is around the same size as the later prismatic marching compasses such as the Verner’s Pattern VIII. The compass has a brass lid which lifts off completely and which is signed: “Ross Ltd., London”. The purpose of the lid was purely to protect the glass dial cover and the delicate folding sighting vane. When removed, it was the practice to place the brass lid in a shirt or trouser pocket.
The compass card (dial) is made from brass sheet with a green paper compass rose glued to the upper side. The underside of the compass card has a magnetised pointer affixed. This pointer always points to magnetic north and since the position of the paper compass rose was calibrated perfectly with the magnetised pointer, the 360 degrees mark on the compass rose always points precisely at magnetic north.
To allow use of the compass in similar fashion to a normal non-sighting pocket compass, the north marker is a large stylised fleur de lis symbol. The green paper of the dial is interesting. In an attempt to mould-and-mildew-proof the paper, it has been treated with a strong arsenic solution, which is probably why it’s in such good condition today. Since arsenic can be absorbed readily through the skin, it’s important not to ever open one of these compasses and handle the compass card. If you see one for sale without the glass, pass on it.
The compass capsule (the housing) is also made of brass and has a brake button just below the flipped up front sighting vane. Since the compass is air-damped, it can take a moment or two to settle and give an accurate bearing. The light use of the brake button can slow the movement and reduce the time it takes for the card to settle. When the sight vane is folded for storage, the vane engages a dial lock which raises the compass card and holds it against the glass. This ensures that the fragile compass pivot stalk isn’t broken in the event that the compass is dropped.
The prism of the compass is unusual in that it remains exposed when the top cover is placed over the compass dial. It is for this reason that the compass should always be carried in a secure and relatively dustproof carry case rather than just on the end of a lanyard or tucked into a shirt pocket. As with most prismatic compasses, the prism housing can move up and down, which serves to focus the image through the prism. A small vaguely rectangular lanyard ring completes the features of the compass.
The lid and capsule of this compass would have originally been oxidised black with ammonia gas, but over the last 120 years this blacking has worn down to a nice patina with an overall brown coloured finish. There is no damage or areas of corrosion on the exterior or inside compass so it is in a remarkable condition.
I have no idea of the history or provenance of this particular compass. I acquired it from the UK so it has no specific sentimental or historic value to me beyond being a nice example of a relatively rare compass. It was made in England by Ross Ltd sometime between 1897 and 1907. Ross & Co became Ross Ltd. in 1897 and this compass doesn’t appear in the Ross Ltd. catalogues I’ve seen after 1907. This indicated to me a 10 year timeframe during which this compass may have been manufactured.
While it’s a bit more fiddly to use than the later, WWI-era Verner’s Pattern prismatic marching compasses which I favour, the pocket-sized Schmalcalder prismatic compass is a fine instrument which is still, 120 years after its manufacture, dead accurate.