|Sid G. Hedges (Editor). The Universal Book of Hobbies. London: Odhams Press Limited, 1935.|
"Motor Caravaning" had become a popular pastime in England in the 1930's. According to the source for this photo, the advice was to choose a caravan's weight depending on the power of your car's engine, the rule of thumb being one hundredweight per horsepower. Thus, a 12 h.p. car could pull a 12 cwt. caravan. There were even caravans available for 7 h.p. engines for pulling behind an Austin Seven or even a motorcycle combination! Under British road laws of the time, maximum speed whilst towing a caravan was 30 mph with a 2-wheel rig, and 20 with a 4-wheel one. It must have been really irritating when out on your Velocette to get stuck behind such a combination on the twisty British roads of the day.
For those who want to know more, I've uploaded the full article here.