"_Q e_ is a lever whose fulcrum is _e_, the counterpoise _f_ nearly supporting it; _s_ is an opaque plate of mica, with a small aperture at _p_, through which the light passes, having before been refracted by a cylindrical lens into a long ray, the portion only of which opposite the aperture _p_ impinges on the paper; _d_ is a wire supported by a float on the surface of the mercury; _G H_ is the barometer; _p_, the vertical cylinder charged with photographic paper; _r_, the photographic trace; _I_, the timepiece, carrying round the cylinder by the projecting arm _t_. It is evident that the respective distances of the float and the aperture _p_ from the fulcrum may be regulated so that the rise and fall of the float may be multiplied to any extent required." When _only_ the lower surface of the mercury in a syphon barometer is read, as in the instrument just described, a correction for temperature is strictly due to the height of the quicksilver in the _short_ tube; but this in so short a column will rarely be sensible. CHAPTER IV. MOUNTAIN BAROMETERS. =37. The Syphon Tube Mountain Barometer, on Gay Lussac's principle=, constructed as described at page 31, and fixed in a metallic tubular frame, forms a simple and light travelling instrument. The graduations are made upon the frame, and it is suspended for reading by a ring at the top, from beneath an iron tripod stand, which is usually supplied with it. Considerable care is requisite in adjusting the verniers, so as to keep the instrument steady and vertical. A drawback to the convenience of this barometer is the movement of the mercury in the short limb, which is generally not confined, and hence has every facility for becoming quickly oxidised in travelling. To remedy this, Messrs. Negretti and Zambra so construct the Mountain Syphon Barometer that by a simple half turn of a screw the mercury can be confined for portability, while the lower limb can be taken out for cleaning whenever found requisite.