50 SEC. 3. MEASUREMENT. 



220. Cast-iron Vessel, containing Friction Disk, to 



revolve under mercury. Used in 1 849 to determine the mecha- 

 nical equivalent of heat by the, friction of cast-iron against cast- 

 iron. The equivalent arrived at was 775 ft. Ib. 



221. Electro-magnet consisting of a broad plate" of half- 

 inch iron, .having a bundle of copper wires coiled round it. Em- 

 ployed in the first determination of the mechanical equivalent of 

 heat. 



222. Apparatus for determining the temperature of water 

 at its maximum density. 



Used in the experiments on atomic volume and specific gravity by Play fair 

 and Joule (Memoirs of the Chemical Society, vol. iii., 1846). It consists of 

 two tall vessels, connected together by a stop-cock at the bottom, and a trough 

 at the top. A minute difference of the temperature of the water in one of the 

 vessels from that of the maximum density, determines a flow through the 

 trough to the vessel still nearer the temperature of maximum density. 

 The temperature of water at maximum density was thus shown to be 39 1 . 



223. Paddle Apparatus, by means of which Dr. Joule 

 determined the dynamical equivalent of heat. Described in Philo- 

 sophical Transactions for 1850. page 65. Sir William Thomson. 



II. MEASUREMENT OF LENGTH. 

 A, STANDARD SCALES. 



223a. Model of an Ancient Egyptian Standard Cubit, 



dated in the reign of King H^rus, 9th Pharaoh of the 18th dynasty 

 (1657 B.C.). Mrs. Chisholm. 



The ancient standard measure, of which this is a copy, was found in the 

 ruins of Memphis, and is now in the Royal Museum at Turin. It is a Royal 

 cubit of seven palms : or 28 digits. The total length of this end standard 

 measure is 523 5 millimetres or 20 6 inches, and agrees very nearly with that 

 of several other ancient Pharaonic cubits still existing, as well as with the 

 length of the Royal cubit as deduced by Sir Isaac Newton from the dimensions 

 of the Great Pyramid, the mean length being 525 millimetres. The original 

 natural cubit, or cubit of a man, of 6 palms is also marked upon this measure, 

 being equal to 463 millimetres or 18-24 inches, and also the ancient Egyptian 

 foot of 16 digits, or f of the natural cubit, and equal to 12' 16 inches, or 1-013 

 English foot. The great span of 14 digits and the small span of 12 digits are 

 also marked. 



227. Standard Scale , in porcelain, showing the relations of 

 modern British and ancient Great Pyramid inches. 



Prof. Piazzi Smyth. 



This scale was prepared to order by M. Casella, of London. It exhibits 

 side by side 25 modern British inches and the same number of ancient Great 

 Pyramid inches, similarly subdivided. 



