From “The Energy Exchange of Ruminants” a PhD thesis paper by John Patrick Joyce for the University of Glasgow, 1964, page 28.
“Photograph of outdoor experimental site demonstrating the collection of sheep by when wearing a face mask.”
“A large pit 9 ft x 4 ft x 1 ft 6 inches was dug just in front of the skeleton framework and was lined with wood. This pit acted as a guard for two large Douglas bags when they were being filled by the animal and prevented them from blowing away. Placing the Douglas bags below ground level also ensured they did not act as a wind shelter for sheep, especially when the bags were filled with exhaled air.”