Comparison of the imperial and US customary measurement systems adopted a system based on the 231-cubic-inch wine gallon for all fluid purposes. This became the US fluid gallon. Both the imperial and US fluid gallon are divided into 4 quarts, 8 pints or 32 gills. However, whereas the US gill is divided into 4 US fluid ounces, the imperial gill is divided into 5 imperial fluid ounces. So whilst the imperial gallon, quart, pint and gill are about 20% larger than their US fluid measure counterparts, the fluid ounce is about 4% smaller. Note that one avoirdupois ounce of water has an approximate volume of one imperial fluid ounce
Gills Bay more common, this area having one of the largest British populations, however the common seal is also present. Wild dogs are often spotted around the Gills area which are believed to have come across on the ferry from Orkney. Porpoises can be seen all year round in all parts of the firth and at Gills Bay as they tend to favour shallower water. Usually seen singly or in small numbers although in the autumn many large groups may be seen in Gills Bay where they congregate. Basking sharks are rare but are being seen more often around May to August.