Proof-spirit meant originally a spirit sufficiently strong to take light, and which, if poured on gunpowder and lighted, would cause the powder to explode. This was the ‘Holland-proof,’ By a statute of 1816 it was defined as of specific gravity 12/13 that of water at 51° F., which is = 0·923; but at the present standard temperature of 60° = 15·5° C. it is 0·920. With Sykes’s hydrometer, used in England: _Under Proof_ (U.P.)—each degree means 1 per cent. of water, the rest being proof spirit. _Over Proof_ (O.P.)—each degree means 1 per cent. of water required to be added to bring the spirit down to proof. The scale of Tralles’s alcoholometer only differs from Gay-Lussac’s by water being taken at 39·2° and pure alcohol as of sp. gr. 7939 at 60°. Alcohol Sykes° Baumé° Sp. Gr. Vol. per cent. (Excise) (French) 1·000 0 100 U.P. 10 0·920 57·05 Proof 22 0·848 85·2 49·6 O.P. 36 Fr. Rect. Spirit Troix-six. 0·838 88·8 55·8 „ 38·2 Rect. Spirit, Brit. Pharm. 0·821 93·75 64·3 „ 41·8 Strongest Rect. Spirit. 0·794 100 —— 48 Absolute Alcohol. The approximate relation of the five usual scales for proof and French rectified spirit are as follows: Proof 0·920 Fr. Rect. Sp. 0·848 Gay-Lussac 58° 86° Tralles 56·3° 85° Cartier 21·6° 34° Baumé 22·6° 36° Trois-six Sykes Proof 49·6° _Compound Industrial Units_ Units of Power, of Electricity, of Caloric, &c., are abstract compound units based on units of time, weight, length, &c., combined for industrial convenience, e.g. a certain weight moved a certain distance in a certain time. The H.P., engine horse-power, is 550 lb. raised one foot in one second. In France this becomes 75·9 kilos, raised one metre in one second, = about 3/4 of the kilo-watt unit of electric power. ------------------------------------------------------------------------ CHAPTER XVI THE ELLS The Ells are the Cubits of the modern West. They are of two kinds: the Foot-Ells, of which the Persian cubit and the Beládi cubit, divisible into 2 feet, were the types, and the Span-Ells, of 3, 4, 5 or 6 spans. 1. THE FOOT-ELLS In France the Aune was 4 Roman feet. In the Italian states the Braccio was usually 2 local feet, but sometimes an Eastern cubit. In the German and Norse states the Eln was 2 local feet. In Spain the Covado, of 2 Burgos feet, was the Beládi cubit. 2. THE SPAN-ELLS