How much is a bushel of coal?
Before measures and weights had been unified by "The Weights and Measures Act 1835" the consumption of coal is not easy to calculate. Often a peck or a bushel is used (4 peck = 1 bushel), but a peck and a bushel are actually a measure for volume. We could use the density of coal [21] to calculate the weight of the coal in a peck or a bushel. Fortunately I could find a discussion about the weight of coal in a bushel [30]. It was stated that 1 bushel of Newcastle coal would vary between 80 and 84 lbs., whereas a heaped-up bushel of Welsh coal could weight as much as 101 lbs. The above text also states that ¾ of a hundredweight is exactly a bushel. The Weights and Measures Act 1835 formally established the present imperial hundredweight cwt of 112 lb, which defines a bushel as 84 lbs. As long as no specific weight is given by a historic source, all calculations on my website assume that a bushel of coal weighs 84 lbs. = 38.1kg.
As the heating value of bituminous coal is in a range of 8.06 kWh/kg and 8.39 kWh [22]; I am using a mean heating value of 8.24kWh per kg, which gives a heating value of 314kWh per bushel coal.
The range in the heating value already introduces an uncertainty of ±4%. Even more challenging is the inaccuracy when an author speaks of 1 bushel per hour. This might be well between 7/8 and 9/8 of a bushel, as most citations use only fractions of quarters. When comparing different results, especially from different sources a total error of ± 20% has to be expected.
The custom of measuring coals by a bushel is rarely practised, except for those coals which are transported by sea, and which are, in consequence, chargeable with a duty. The great exports of coals by sea are from Newcastle in Northumberland, and Swansea in South Wales, and Workington in Cumberland. It is usually implied that coals are brought from one or other of those places, wherin they are reckoned by bushels ; and both, the Newcastle and Swansea coals are commonly of a better quality than the inland coals of Staffordshire, Yorkshire, or Lancashire, which are reckoned by the hundred weight (= 112 lbs.) and the ton (= 2240 lbs.)
The mode of measuring coals in a bushel is subject to considerable uncertainty, because they are directed to be heaped up in a conical form, above the top edge of the bushel, but the height of the heap is only determined by the eye. It requires a reference to several acts of parliament to find what the dimensions of the legal coal bushel ought to be. By an act of parliament, 16th and 17th year of King Charles II. it is enacted, that all coals brought into the river Thames, and sold by the chaldron, shall be at the rate of 36 bushels heaped up.
According to an act of the 12th of Queen Anne, the coal bushel must be round, with an even bottom, and be 19½ inches, from outside to outside, and must contain one Winchester bushel, and one quart of water. Now, the Winchester bushel (also called the malt bushel) according to an act 13th William III. 1713, is a circular measure, 18½ inc. diam., and 8 inc. deep; this contains 2150.42 cubic inches; and a quart = 67.2 cubic inc. ; hence the contents of the coal bushel should be 2217.62 cubic inch.; and a cylindrical measure, to contain this quantity, would be 18.8 inc. diam., and 8 inches deep. Again, another act, 47th George III. (1807) directs that the coals shall be heaped up above the bushel, in the form of a cone, at least 6 inches high, and of the same size at the base, as the outside of the measure, viz. 19½ inc. diam.; this cone will contain 597.3 cubic inches.
The legal coal bushel may, therefore, be stated to contain 2815 cubic inches, or 1.63 cubic feet. Three bushels of coals are put into a sack, and 12 such sacks (=36 bushels) make a chaldron. This is the mode of reckoning coals by retail in London, but in the wholesale trade, when 5 chaldrons are measured, an ingrain, or extra allowance of 1/20 is given, making 189 bushels in 5 chaldrons, which is called the pool measure.
The weight of coals varies considerably in the different sorts, but the chaldron (=36 bushels) is usually reckoned to weigh 27 hundred weight ( x 112)=: 3024 lbs., which is at the rate of ¾ of a cwt. (or 84 lbs.) per bushel : this number has been adopted in the present work, because it is the actual weight of the best qualities of the Newcastle coals, though inferior sorts are not above 78 or 80 lbs. per bushel.
John Faray, p. 337