'In appearance this level does not look like an ordinary level but resembles a small amateur's cine camera. It is particularly suitable in building and engineering work for setting our [sic.] foundations, gradients, etc., and, although it can be used for running short lines of levels from point to point, it is not so suitable as an ordinary level for carrying forward long lines of levels. It has the advantage of being very simple and quick to use and it is very cheap considering the degree of accuracy obtainable with it. It has also the advantage of not requiring a skilled surveyor to manipulate it.'¹

This is how the instrument is introduced to us in the fifth edition of Clark's Plane and Geodetic Surveying that spends three whole pages on the instrument.²

The Cowley level was invented by Australian Keith Cowley in 1944.³ As can be seen in figure 1 and figure 2 the above description was rather accurate. The instrument, tripod (see figure 5) and levelling staff (see figure 6) were all designed along very simple lines, reducing the number of parts in all these three components to a bare minimum. As above work credited Hilger & Watts for the drawings (see figure 16 and figure 17), it was most likely this firm that manufactured the instrument for Cowley.

The instrument is not even fixated to the tripod, but simply slides over a smooth pin to stay in place (see figure 12). On the rear of the instrument a warning was pasted not to carry the instrument around while mounted on the tripod for the evident risk of loosing it (see figure 2). The pin also releases the pendulum once the instrument was placed on the tripod.

The 'optics' are very simple as well. There are no lenses, just glass covered apertures, two mirrors at the left hand side of the instrument and tree mirrors at the right hand side (see L.H. and R.H. mirrors in diagram in figure 17 and picture in figure 18).

Although not the first automatic levelling instrument (for a discussion on that see the 1960s Zeiss Opton Ni 2 page), it possibly was the first popular one, although that was more thanks to its price than its performance. The Cowley level was invented roughly half a decade before the famous (and much more accurate) Zeiss Option Ni2.

Not much accuracy can be expected from a simple level like this, there are even no screws to adjust the instrument when needed. The manufacturer stated that it would be about "1/4 in. at 100 ft." (approximately 20mm/100m).⁴ The staff is metric, divided in half a centimetre intervals, and can be read by estimation at a millimetre level (see figure 9). As was more common in the late 19^th and early 20^th century it is not a self reading staff, but has to be read by the assistant if used for levelling. Most likely Cowley levels were however used as a modern rotating laser by setting the cursor at a certain level after which the levels of points around the instrument were verified.

The instrument was donated to my collection in 2016, is in remarkable fine state and very complete. It came complete with its original leather case (with key!), tripod, a 1.5 metres wooden levelling staff, and a 2 foot cursor (see figure 7). Cowley levels are also known to have been sold with cursors of a foot and a half with a different pattern (see figure 8).

The staff and cursor are 'made in England'. Being divided in metres instead of feet, makes clear that the instrument was also produced for the continental market. This specific example was sold by Gadella, an Utrecht based Dutch firm known for selling Watts surveying instruments.⁵ They still sold Cowley levels in 1971, in which year a complete Cowley levelling set would have sold for fl.340,- (just over €610,- in 2013).⁶

In June 2016 I was hinted that a variable Fall or Rise 'slope' attachment (see figure 3) for the Cowley level was available on an on-line auction. This simple attachment allows the Cowley level to be used to set out slopes with gradients up to about 1:10 (see figure 21). It came complete with its original case (see figure 20) and manual (see figure 24 and figure 25).

Internally the slope attachment consists of a pair of glass wedges that are mounted in plastic collars with toothed rims. A knob with a tooth wheel turns both collars simultaneously in opposite directions. This system was invented by Boskovic and Bosshardt and has also been implemented in tacheometers as the 1952 Wild RDH. When set at infinity the two wedges counteract each other exactly, while a rotation of both collars over 90 degrees results in a maximum slope double the value of each wedge.

The knob of the variable Fall or Rise 'slope' attachment is graduated from 10 to 50 and has a '0' mark (see figure 22). A blocking pin inside the knob prevents the knob to be turned more than approximately 340 degrees, leaving the zero mark at the side of the 50 mark. This zero mark was either for assembly or to indicate infinity (even though it is slightly off for that). The figures on the scale indicate the ratio of the slope, which is thus running from 1:10 (10cm per meter) to 1:50 (2cm per meter).

The whole attachment is mounted to the Cowley level using an elastic band. Four pins at the rear of the attachment prevents it from turning freely around the optical axis (see figure 23). It depends on the mounting direction whether the slope will be rising or falling (see figure 21 and figure 22).