Thanks to the good fortune of a local collector, our acquisition committee recently voted to acquire a rare navigational instrument for the national collection. The deviascope is a practical tool for demonstrating the magnetic forces which cause compass deviation on ships and for instructing students on how to compensate for these forces.
Magnetic deviation describes an array of compass errors caused by a ship’s own residual magnetism. That magnetism resides in the iron and steel parts of the ship. In wooden sailing ships iron parts like nails, bolts, spindles, anchors, stanchions and chains became magnetized by the Earth’s magnetic field. This caused the compass needle to deviate from magnetic north, with the error varying according to proximity to the pole, among other things. Iron or steel-hulled ships had an additional form of magnetism: permanent or hard magnetism caused by the pounding and riveting of the metal during construction. Each ship had “a unique magnetic signature” but that “could change at sea under the pounding of waves or the shaking of the hull by engines, paddle wheels, or screw propellers.” Also, the magnetism of iron hull plates changed polarity after crossing the equator. Finally, “heeling error” occurs when an iron or steel ship rolls changing its magnetic field in relation to the compass and causing “wild oscillations” of the compass card. 
The complex nature of magnetic deviation made it a difficult but essential subject to teach to mariners. In the 1880s, Captain George Beall, Principal Examiner of Masters and Mates to the Board of Trade in the UK, recognized the need for a tool to help mariners to understand the behaviour of the magnetic compass. In 1886 he introduced his deviascope which was immediately embraced by the marine training community. Instructors and other experts produced manuals and textbooks to go with the deviascope – we have examples from 1943 and 1970 though they apparently go back as far as 1886 – and instrument makers created their own versions of the device to meet the demand from teaching establishments.
This particular deviascope was made in London by Henry Hughes & Son and dates from about 1912. The current owner, Mr. Murray Shaw, acquired it from the Montreal firm of Harrison and Company in 1978. Harrison was a Montreal firm that made and sold scientific, marine and surveying instruments from the 19th century until it closed its doors in 1978. Mr. Shaw is a collector of instruments and lived in Montreal in the late 1970s. Knowing that Harrison’s was closing, he paid them a visit and asked if they had any objects that they might be willing to part with for a reasonable price. They took him into the basement and showed him the deviascope. It was wrapped in newspapers from 1912 suggesting it had languished there for some time.
Who knows what they would have done with it if Mr. Shaw had not come along when he did. Thanks to him it has been well preserved for future generations of Canadians.
Babaian, Sharon. Setting Course A History of Marine Navigation in Canada. Ottawa: Canada Science and Technology Museum, 2006.
Brown, Charles H. Deviation and the Deviascope Including the Practice and Theory of Adjustment. Glasgow: Brown, Son & Ferguson, Ltd, 1943.
Gurney, Alan. Compass A Story of Exploration and Innovation. New York: W.W. Norton & Company, 2004.
Grant, G.A.A. and J. Klinkert. The Ship’s Compass. London: Routledge and Kegan Paul Ltd., 1970
Kemp, Peter, ed. The Oxford Companion to Ships and the Sea. Oxford: Oxford University Press, 1979.
 Gurney, 216
 Gurney, 221, 278. See also Charles H. Brown, Deviation and the Deviascope Including the Practice and Theory of Adjustment (Glasgow: Brown, Son & Ferguson, Ltd, 1943) 53-92; G.A.A. Grant and J. Klinkert, The Ship’s Compass (London: Routledge and Kegan Paul Ltd., 1970) 119-192 and Kemp, 382-3.