42 SEC. 3. MEASUREMENT. 



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. 



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



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

 nical equivalent of heat from 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 Playfair 

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



II. MEASUREMENT OF LENGTH. 



A. STANDARD SCALES. 



224. Brass Standard Scale (Bird's). Date about 1750. 



Royal Society. 



225. Brass Standard Scale (Sir G. Shuckburgh's). 



Royal Society. 



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. 



The divisions of both sets of inches coincide at the left hand exactly, 

 but from thence the gradual growth of the difference of O'OOl of an inch per 

 inch in favour of the Great Pyramid scale may be traced, until at the 25th 

 inch the difference amounts to 0'025 of the British inch. At that point, 

 however, it is to be noted that 25 Great Pyramid inches are just one 10 

 millionth of the earth's semi -axis of rotation, or the nearest earth commensur- 

 able and scientific unit ever yet proposed as a standard of length. 



