The passions of Robert Louis Stevenson, the author, did not include the weather. But he came of scientific stock. His grandfather, Robert Stevenson, was his generation's best-known light-house engineer, his greatest oeuvre being the Bell Rock light near the Firth of Tay in Scotland: the weather reports still emanating from that lonely pinnacle are a lasting epitaph to his achievement.
However, it was Robert Louis's father, Thomas, who left his mark on meteorology. Thomas Stevenson was born in 1818 and, having qualified as an engineer, became one of several brothers who carried on the family lighthouse-building business. His initial fascination, understandably, was with the development of waves, and in a memoir entitled Records of a Family of Engineers, his son describes his enthusiasm for the subject:
"He would pass hours on the beach brooding over the waves, counting them and noting their least deflection, and the way in which they broke. On Tweed side, or by Lyne or Manor, we have spent together whole afternoons.
"The river was to me a pretty and various spectacle; I could not see, or could not be made to see, it otherwise. But to my father it was a chequer-board of lively forces which he traced from pool to shallow with minute appreciation and enduring interest, until I had worn him out with my invincible triviality and delight."
Thomas Stevenson is first mentioned in the annals of meteorology in 1842, when he reported to the Scottish Meteorological Society on the effect of strong winds on the accuracy of rain gauges. But it was in the measurement of temperature that he quite literally made his name.
If air temperature is to be measured in such a way that values measured at different places are directly comparable, a uniformity of method is essential. It is necessary that the thermometers be protected from the direct rays of the sun, that the air be allowed to circulate freely around them, and that they be exposed at a standard height above the ground.
Thomas Stevenson provided the answer in 1864. The white wooden box with louvred sides which he designed was inexpensive and effective. It is made of wood, or nowadays sometimes of plastic, because these materials absorb heat slowly and are therefore not directly affected by radiant heat from the sun.
It is painted white to reflect the sunlight, rather than absorb its heat. And the louvred sides allow air to circulate freely inside the chamber, while at the same time the thermometers are shielded from the direct effects of wind and rain.
The Stevenson screen can still be seen at any weather station in the world.