I'm wondering when Stevenson screens were first invented and widely used because I know that pre-19th century thermometers were often not screened at all. This had the effect of overestimating maxima and undersestimating minima. And as you were saying, the distortion was often enhanced when there was a lot of snow on the ground. Apparently, some thermometers were simply placed on fences! My question is if pre-19th century data was so unreliable, doesn't this raise the question of whether the "Little Ice Age" was exaggerated by these distorted readings. There were quite a lot of readings below -20C even in Southern England in the 17th and 18th centuries. If we now say that these are unreliable, then what grounds do we have for saying there was really such a thing as the LIA? Apart from the Thames freezing...etc?