The Unsolved Mystery of the Chronograph Hash Marks

Imagine Indy being chased, trying to get away in his little beat-up bi-plane, having somehow to figure out if he has enough speed to lift off without getting shot or falling off the cliff. Here, his chronograph comes to the rescue! Indy knows that he can use a simple calculation to determine his fate, using the mighty ‘Rule of three’ and his trusted old timepiece with those funky hash marks.

Three-minute as well as six-minute increments can be used to quickly calculate rates, such as distance and speed, as three minutes is exactly 1/20 of an hour and six is 1/10 of an hour. Therefore, they are much easier to work with than five-minute or 10-minute increments, which would equate to 1/12 of an hour or 1/6 of an hour respectively.

Since Indy’s speedometer is blown to bits, he needs to figure out his pace by quickly calculating the distance in yards his plane travels in three minutes, which is about equal to the speed in knots divided by 100. Let’s assume he travels one mile per minute, equaling three miles over three minutes, totaling 5,280 yards, which in turn equates to 52.8 knots. Oh dear, that won’t do it, he needs to get to 60 knots to take off. But lo and behold, Indy comes through, and he escapes into the sunset.

These types of “on the fly” calculations could be lifesaving for pilots at a time when the use of electronics was scarce, and time was of the essence. So, it would make sense to imagine that the three-minute increments had purposefully been applied to the minute chronograph counter for that exact reason.

The last theory on this subject is that the 15-minute versus the more conventional 30-minute sub-dial was intended for aviators to time their final plane check before taking off — but that just sounds a bit too simple.