In the Transactions of the Royal Society of Edinburgh,' vol. xiii. will be found an account of an ingenious pyrometer by Mr. Adie, for measuring the expansion of different kinds of stone. We have not space to describe the instrument, but we may mention one curious effect, namely, that there was an increase in length in white Sicilian marble, every time its temperature was raised.
We may also refer to M. Pouillet's method of measuring directly the linear expansion of solids applicable to very high temperatures. It consists of a solid plate of metal on which is placed a radius turning on a centre, and traversing a graduated arc, the divisions of which are read off by a microscope. The radius carries a telescope of short focus, fixed at right angles to its direction, while a similar telescope is fixed to the plate itself, allowing the radius to traverse under it. The bar which is the subject of experiment is placed in a copper trough, furnished with parallel plates of glass, through which its ends can be seen. Now if one extremity of the bar be kept opposite the fixed telescope, while the moveable telescope is directed to the other extremity at the commeucement of tae observation, any expansion of the bar caused by raising its temperature may be estimated by the arc through which the radius must be turned in order to bring the moveable telescope to bear on the other extremity in its new the distance of the radius from the bar being accurately known. For
very high temperatures the bar may be placed in a furnace, and when raised to the required temperature, apertures may be opened in furnace walls, so as to give a view of the ends of the bar, and allow expansion to be measured as before. By means of this apparatus an expansion in the bar of millimetres or about inch can be appreciated.
We may also refer to an ingenious contrivance adopted by General Colby in measuring the base line of Loch Foyle, for the Ordnance Survey of Ireland. It consisted of a compound bar of iron and brass so arranged that their different powers of expansion and contraction should preserve exactly the same distance between two points at the extremities of the bars, instead of allowing for the change according to the observed temperature. The two bars, one of iron the other of brass, each 10 feet long, were placed parallel to each other and riveted together in the centre. It was ascertained by experiment that the iron and brass bars expanded and contracted by varying temperatures in the proportion of three to five. Across each extremity therefore of these combined bars was fixed a tongue of iron, with a minute dot of platinum so placed, that under every degree of expan sion and contraction of the rods the dots at each end always remain at the constant distance of 10 feet.