=63. Thermometers of extreme Sensitiveness.=--Thermometers for delicate experiments are no novelty. Thermometers have been made with very delicate bulbs to contain a very small quantity of mercury. Such instruments have also been made with spiral or coiled tubular bulbs, but the thickness of glass required to keep these coils or spirals in shape, and in fact to prevent their falling to pieces, served to nullify the effect sought to be produced, viz. instantaneous action; and where a small thin bulb was employed, the indicating column was generally so fine that it was positively invisible except by the aid of a powerful lens. Messrs. Negretti and Zambra have now introduced a new form of thermometer, which combines sensitiveness and quickness of action, together with a good visible column. The bulb of this thermometer is of the gridiron form. Care has been taken in constructing the bulb, so that the objections attending spirals and other forms have been overcome; for whilst the reservoir or bulb is made of glass so thin that it is only by a spirit lamp and not a glass blower's blowpipe that it can be formed, yet it is still so rigid (owing to its peculiar configuration) that no variations in its indications can be detected, whether it be held in a horizontal, vertical, or oblique position, nor will any error be detected if it be stood on its own bulb. They have made thermometers with bulbs or reservoirs formed of about nine inches of excessively thin cylindrical glass, whose outer diameter is not more than a twentieth of an inch; so that, owing to the large surface presented, the indications are positively instantaneous. This form of thermometer was constructed expressly to meet the requirements of scientific balloon ascents, to enable thermometrical readings to be taken at the precise elevation. It was contemplated to procure a metallic thermometer, but on the production of this perfect instrument the idea was abandoned. 64. VARIETIES OF THERMOMETERS. Fig. 37 is an illustration of boxwood scale thermometers for general use and common purposes. Fig. 38, Negretti and Zambra's Travelling Thermometer; it is fixed in a plated metal (silver or otherwise) case, similar to a pencil-case, and has the scale divided upon its stem. Fig. 39, Thermometer mounted on a slab of glass, upon which the scale is etched, the back being either oak, mahogany, or ebony. Fig. 40, Portable Thermometer, in a bronzed brass or German silver revolving case. Fig. 41, Pocket Thermometer, on ivory or metallic scale, in morocco or papier-mache case. [Illustration: Fig. 37.] [Illustration: Fig. 38.] [Illustration: Fig. 39.] [Illustration: Fig. 40.] [Illustration: Fig. 41.] Fig. 42, an Ornamental Drawing-room Thermometer, on ebony or ivory stand, with glass shade.