Captain Petrie's Magic Garden Rake



This month's lesson concerns another one of Australia's outstanding heroes of WW1 who, like Captain Tom White of the Australian Flying Corps, is now all but forgotten.

Captain H. A. Petrie was the commander of the Australian Half Flight in Mesopotamia (Iraq) in 1915, and, with the other pilots, was flying reconnaissance for the British Army which was gearing up for an attack on Ctesiphon (20 miles from Baghdad). Next stop would have been Baghdad, but General Townshend's forces were outnumbered and he was forced to retreat to Kut (kut-al-Amara). Frederick Morley Cutlack, in The Australian Flying Corps in the Western and Eastern Theatres of War 1914-1918 gives us an insight into the ingenuity of Petrie in solving pressing problems.

"The main duty of the air reconnaissance was to supplement existing maps. Owing to the shifting course of the Tigris, the army maps of Mesopotamia were inadequate for tactical purposes, and mapping from the air with primitive appliances was a long and laborious task. Photographs were taken with an ordinary reflex camera, by pilot or observer holding the camera over the side of the machine or pointing its lens through a hole in the floor. Although the photographs showed the formation of trenches and redoubts, they did not locate these with sufficient accuracy for mapping purposes. A night march by compass hearing on the enemy's flank was to he a feature of the attack on Ctesiphon; precision, therefore, was essential...

An implement for accurate mapping was devised by Captain Petrie. It was like a small garden rake. A short handle at the side enabled its longer axis to be brought on a level with the eye; the pegs were carefully spaced: and when the implement was held to the forehead,  the pegs showed the degrees of distance from the center. With this crude instrument White mapped and verified the location of the enemy's line, particularly fixing the position known as "V.P." (vital point) — a flank redoubt which was to be used as a pivot of manoeuvre in the coming attack. Much of the country in the vicinity of Ctesiphon is scarred with remains of high-level canals of ancient Assyrian and Persian days. The ruined palace of the Persian King Chosroes was a conspicuous landmark in the center of the Turkish position. The mean of two bearings upon each of several prominent points was taken with the garden rake device from over known points within the British line. Back bearings were then taken in a similar way when the machine was over "V.P." and the ruined palace. The mean of all these bearings was plotted after return to camp on a map marked with a circle of degrees. On the day of battle this map proved accurate."

What a pity that the ingenuity of Australians today is hampered by all kinds of government red tape, and laws designed to protect monopoly interests. Oops, this isn't a political commentary. is it?