III. SPECIAL THERMOMETERS. 691 



the electric current from a powerful Ruhrnkorff coil by the -aid of which the 

 vacuum can be ascertained to exist to within one-tenth of an inch of 

 pressure. 



2827k. Thermometers of extreme sensitiveness. 



Negretti Sf Zambra. 



Negretti & Zambra's Instantaneous Thermometer, with gridiron, spiral, or 

 coiled form of bulb, and divided upon the stem, as shown in the International 

 Exhibition of 1862, and used by Mr. Glaisher in his balloon ascents to obtain 

 very rapid thermometric readings. 



2826g. Standard Thermometer, graduated on the stem, in 

 maroon case, with Kew certificate. L. Casella. 



2826h. Standard Maximum Thermometer, graduated on 

 the stem (Kew certificate). L. Casella. 



28261. Sensitive Minimum Thermometer, with forked 

 bulb (Kesv certificate). L. Casella. 



2826j. Thermometer, 2 meters long, set in wood and zinc, 

 divided into J, for investigations of terrestrial heat. 



Warmbrunn, Quilitz, $ Co., Berlin. 



2826k. Two Thermometers, maximum, in case. 



Will, ffaak, Neuhaus, Thuringen. 



28261. Two Thermometers. Maximum on brass scale. 



Will. Haak, Neuhaus, Thuringen. 



2826m. Continuous Self-registering Thermometer. 



W. Harrison Cripps, F.R.C.S. 



The object of the instrument is to obtain a continuous registration of heat. 

 The instrument is in two portions : 1st, the thermometer for indicating the 

 temperature ; 2udly, the clock-work for registering the hours and minutes. 

 The thermometer consists of six coils of glass tubing wound concentrically 

 round an axis in such a manner as to form a spiral glass wheel 4 inches in 

 diameter. The last coil is moved slightly away from the others, so that it 

 shall form the circumference of a circle 5 inches in diameter. To each end 

 of the axis a fine needle-pointed-pivot is attached. These pivots rest in 

 minute depressions between two parallel metal uprights. By this arrange- 

 ment the glass wheel can rotate freely between the uprights. The spirit in 

 the thermometer fills the spiral portions of the tube, and also 3 or 4 inches of 

 the last coil (the one forming the circle). The spirit then comes into contact 

 with a column of mercury 4 inches in length. Beyond the mercury are a few 

 drops of spirit to moisten the glass. The remaining portion of the tube is 

 hermetically sealed, enclosing a small quantity of air. On the spirit expand- 

 ing with heat, the column of mercury is driven forwards. This immediately 

 alters the centre of gravity, and the wheel revolves in a direction contrary to 

 that of the moving mercury. When the spirit contracts on cooling the en- 

 closed air acting as an elastic spring keeps the mercury in contact with it, 

 and the wheel regains its original position. By this arrangement the two 

 forces, heat and gravity, acting in contrary directions, generate a steady 

 rotatory motion. 



The method by which this movement is made serviceable is by a grooved 

 wheel 2 inches in diameter fixed to one of the pivots, and therefore revolving 



Xx 2 



