A few months back I picked up an unusual William Ainsworth & Sons surveying compass that closely resembles an Ainsworth-produced Brunton Pocket Transit.  The resemblance is so close, in fact, that the lid of the compass is even marked ‘D. W. Brunton’s’.  There’s no doubt that this is an original Ainsworth instrument (as opposed to a copy or knock-off), but it certainly isn’t a pocket transit.

 From the outside it looks like a common pre-WWII pocket transit

So what do we have here? Well, it appears to be a survey compass built using a pocket transit case.  I’ve never seen or read about an instrument of this type, so I assume it’s fairly rare.

However, inside we see something quite unusual. It’s not a pocket transit but a surveyors compass!

Two key questions are, when was it made, and for whom?  Based on the case design and markings this is instrument was made after 1914, and we can be sure of that based on the ‘figure 8’ peep sight design which was patented by D. W. Brunton in 1914. But going a bit further, I’ll also wager that this instrument was built post-1927 when Ainsworth acquired the manufacturing rights to the Brunton pocket transit from D. W. Brunton’s estate.  This would account for the use of the pocket transit case and a lid that carries all the Brunton patent data on a device that is clearly not a pocket transit.  I doubt Mr.Brunton would have allowed his name and patent information to be used on something that was not of his design.

William Ainsworth & Sons was a well known instrument maker and produced high end instruments for the mining, geological exploration and surveying industries. The idea that they would consider using a pocket transit case to produce a small survey compass is not at all far-fetched. In fact, it makes perfect sense. The cases were already in production, the pocket transit was a well known and highly regarded design and it would have been easy to simplify the housing design to contain just a compass needle. In theory it would also have been cheaper to build since the bubble levels, clinometer arm, scales and external clinometer lever are all left out of the design.

This compass leaves out the standard pocket transit bubble levels, clinometer arm and scales. It also uses a type of bar compass needle commonly found on surveyors compasses but never seen before on an Ainsworth pocket transit from this era

But was this an attempt to bring a simpler and cheaper pocket instrument to market or was this made to answer a product requirement submitted by a customer? We may never know, and there aren’t enough clues we can glean from the instrument itself to make a good guess. This particular instrument is painted in a dark green, very similar to Army olive drab. The paint job is well executed and was probably done at the factory. It’s also stamped with the serial number ‘1042’, but the serial number placement, text size and style are not what we’re used to seeing on other Ainsworth instruments. This indicated it may be part of a special production run for a large customer – perhaps the US military or another federal agency such as the USGS or the US Forest Service.

Another interesting design feature relates to how the compass is mounted to a tripod. Pocket transits are secured to tripods using a ‘U’ shaped bracket the slides into grooves machined into the side of the transit case. This compass design takes a more common approach, and one used by most surveyor compass manufacturers. That is, the mounting thimble or bracket screws into the base of the instrument. Pocket transits can’t use this arrangement because of the clinometer lever that extends through the bottom of the case. But since this compass doesn’t have a clinometer Ainsworth was free to use the more conventional mounting method.

 The compass mounting thimble screws directly into the base of the compass. A common arrangement with surveyors compasses, but not seen with pocket transits

This view shows the mounting socket at the base of the compass

So there we have it. An interesting instrument that represents the adaptation of the popular pocket transit design for another purpose.

If any of my readers have any additional information on this instrument I’d love to hear from you. Please just leave your comments on the blog for all to see. Thanks!

– Brian