In meteorological observations, it is necessary to attend to the greatest rise and fall of the thermometer ; and therefore attempts have been made to Make them mark the greatest degree of heat and cold, in the absence of the ob server. We will notice one, intended to show the greatest degree Of heat. AB, fig. 6, is a glass tube, with a cylindrical bulb, 13, at the lower end, and capillary at the other, over which there is a fixed glass ball, C. The bulb, and part of the tube, are filled with mercury, the top of which shows the degrees of heat. The upper port of the tube, above the met- cury, is filled with spirit of wine ; the ball, C, is likewise tilled with the same liquor, almost to the top of the capillary tube. When the mercury rises, the spirit of wine is also raised into the ball, C, which is so made that the liquor cannot return into the tube when the mercury sinks; of course, the height of the spirit in the ball, added to that in the tube, will give the greatest degree of heat. To make a new observation, the instrument must be in clined till the liquor the ball cover the end of the capillary tube.
In 1782, Mr. Six proposed another self= registering thermometer. It is properly a spirit of wine thermometer, though mercury is also employed for supporting an index : a b (fig. 7) is a thin tube of glass sixteen inches long, and five-six teenths of an inch calibre : c d e, and fg h, are smaller tubes, about one-twentieth of an inch calibre. These three tubes are filled with highly rectified spirit of wine, except the space between d and g, which is filled with mercury. As the spirit of wine contracts or expands in the middle tube, the mercury falls or rises in the outside tubes. An index, such as that represented in fig. 8, is placed on the surface, within each of these tubes, so light as to float upon it : k is a small glass tube, three-fourths of an inch long, hermetically sealed at each end, and inclosing a piece of steel wire nearly of its own length. At. each end, / m, of this small tube, a short tube of black glass is fixed, of such a diameter as to pass freely up and down within either of the outside tubes of the thermometer, c e or f h. From the upper end Of the
index is drawn a spring of glass to the fineness of a hair, and about five-sevenths of an inch long ; which being placed a little oblique, presses lightly against the inner surface of the tube, and prevents the index from descending when the mercury desQends. These indexes be ing inserted one into each of the out side tubes, it is easy to understand how they point out the greatest heat or cold that has happened in the observer's ab sence. When the spirit of wine in the middle tube expands, it presses down the mercury in the tube, and conse quently raises it in the tube, e o ; conse quently, the index on the left hand tube is left behind, and marks the greatest cold, and the index in the right hand tube rises, and marks the greatest heat.
The common contrivance fora self-re gistering thermometer, now sold in most of the Loudon shops, consists simply of two thermoMeters, one mercurial, and the other of alcohol, (fig. 9) having their, stems horizontal; the former has for its index a small bit of magnetical steel and the latter a minute thread of glass, having its two ends formed into small knobs, by fusion in the flame of a can dle.
The magnetical bit of wire lies in the vacant space of the mercurial thermome ter, and is pushed forward by the mercu ry whenever the temperature rises, and pushes that fluid against it ; but when the temperature falls, and the fluid re tires, this index is left behind, and con sequently shows the maximum. The other index, or bit of glass, lies in the tube of the spirit thermometer immersed in the alcohol : and when the spirit re tires, by depression of temperature, the index is carried along with it, in appa rent contact with its interior surface ; but, on increase of temperature, the spi rit goes forward and leaves the index, which therefore shows the minimum of temperature since it was set. As these indexes merely lie in the tubes, their re sistance to motion is altogether inconsi derable. The steel index is brought to the mercury by applying a magnet on the outside of the tube, and the other is du ly placed at the end of the column of alcohol, by inclining the whole instru ment.