The Dragoyle Evaporimeter

Evaporimeters determined the rate of air evaporation from a wet surface to the atmosphere, which is another way of measuring air humidity. Not only were evaporimeters used to study conditions of the atmosphere, but they also became useful for early forms of 'climate control'. The "Dragoyle" Air Tester in the Whipple's collection measures evaporation from a wet cloth that is kept damp from water contained within the object. The decreasing level of water over a period of hours or days was used to calculate the rate of air evaporation.

Babbage's Florentine thermometer
Image 1 Wh.5790. The "Dragoyle" Air Tester, a speacialty evaporimeter. Image © the Whipple Museum.
Babbage's Florentine thermometer
Image 2 Wh.5790. The booklet that accompanied the "Dragoyle" Air Tester. Image © the Whipple Museum.

Monitoring air comfort in the work environment

The Dragoyle, which is designed to look like a curious dragon-like creature, measured the "air-comfort" of workplaces such as factories, workshops, and offices (Image 1). The principles behind this evaporimeter were first developed by the Scottish mathematician and physicist Sir John Leslie (1766-1832). By 1925 the instrument was packaged and sold as the Dragoyle in America. 'Air comfort' was determined by measuring the combined effects of temperature, humidity, and air-circulation.

The Dragoyle is made from a sealed glass piece consisting of a large bulb, which features as the head of the Dragoyle, with a long tube that resembles the creature's tongue. A millimeter of coloured liquid rests in the 'tongue' tube.

As seen in Image 1), the large bulb of glass is covered with a piece of fabric. The ends of the fabric are suspended like a wick in an internal reservoir of water and the wet cloth creates temperature and pressure differences as the water evaporates. The bead of liquid inside the Dragoyle's 'tongue' moves to equalize the pressure between the two sections of the glass vessel, causing the creature's head to rock back and forth. These movements or 'strokes' are counted by the minute and indicate how humidity and air circulation might be adjusted for a more comfortable work environment.

Depending of the rate of 'strokes' per minute, temperature or draughts could be better regulated. By the early twentieth century, it was recognized that metabolic processes within the worker's body significantly affected closed or confined spaces such as the factory and that better indoor conditions improved the efficiency of the worker. The ideal conditions for the sedentary worker differed from those standing or involved in more physical activity, as described in the accompanying instruction booklet.

Allison Ksiazkiewicz

Allison Ksiazkiewicz, 'The Dragoyle Evaporimeter', Explore Whipple Collections, Whipple Museum of the History of Science, University of Cambridge, [, accessed 25 November 2017]

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