was kept up until the cable was again in working order.  Of the two lines used in this case, one extended from the sea at the end of the island near Hurst Castle, through the length of the island, and entered the sea again at Rye; while the line on the mainland ran from Hurst Castle, where it was connected with the sea, through Southampton to Portsmouth, where it again entered the sea.  The distance between the two terminals at Hurst Castle was about one mile, while that between the terminals at Portsmouth and Rye amounted to six miles.—­Science.

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PYROMETERS.

The accurate measurement of very high temperatures is a matter of great importance, especially with regard to metallurgical operations; but it is also one of great difficulty.  Until recent years the only methods suggested were to measure the expansion of a given fluid or gas, as in the air pyrometer; or to measure the contraction of a cone of hard, burnt clay, as in the Wedgwood pyrometer.  Neither of these systems was at all reliable or satisfactory.  Lately, however, other principles have been introduced with considerable success, and the matter is of so much interest, not only to the practical manufacturer but also to the physicist, that a sketch of the chief systems now in use will probably be acceptable.  He will thus be enabled to select the instrument best suited for the particular purpose he may have in view.

The first real improvement in this direction, as in so many others, is due to the genius of Sir William Siemens.  His first attempt was a calorimetric pyrometer, in which a mass of copper at the temperature required to be known is thrown into the water of a calorimeter, and the heat it has absorbed thus determined.  This method, however, is not very reliable, and was superseded by his well-known electric pyrometer.  This rests on the principle that the electric resistance of metal conductors increases with the temperature.  In the case of platinum, the metal chosen for the purpose, this increase up to 1,500 deg.C. is very nearly in the exact proportion of the rise of temperature.  The principle is applied in the following manner:  A cylinder of fireclay slides in a metal tube, and has two platinum wires one one-hundredth of an inch in diameter wound round it in separate grooves.  Their ends are connected at the top to two conductors, which pass down inside the tube and end in a fireclay plug at the bottom.  The other ends of the wires are connected with a small platinum coil, which is kept at a constant resistance.  A third conductor starting from the top of the tube passes down through it, and comes out at the face of the metal plug.  The tube is inserted in the medium whose temperature is to be found, and the electric resistance of the coil is measured by a differential voltameter.  From this it is easy to deduce the temperature to which the platinum has been raised.  This pyrometer is probably the most widely used at the present time.