| Fig. 45. | Fig. 46. | Fig. 47. | Fig. 48. | Fig. 49. | Fig. 50. | ||||||
65. Superheated Steam Thermometer.—The great advantage gained by the use of superheated steam in marine and other steam-engines being now generally admitted by engineers, reliable thermometers, reading to 600° at least, are of the utmost importance. To meet this want, Messrs. Negretti and Zambra have constructed for the purpose a substantial form of thermometer, on their patent porcelain scales, in strong and convenient metal mountings, with perforated protection to the bulb. The scales cannot be deteriorated by steam, heat, oil, or dirt; and an occasional wiping will be all that is necessary to keep the divisions and figures clean and visible for any length of time; while careful calibration of the thermometer tubes ensures the most accurate indications attainable. These thermometers are illustrated by figs. 49 & 50. A similar, but cheaper, construction is given to thermometers to be used with hot air, or hot water, apparatus.
66. Thermometer for Sugar Boiling is protected by a metallic frame; and is usually from three to four feet long, the graduations being confined to a space of about twelve inches at the upper part of the instrument, allowing the bulb and greater part of the tube to be immersed in the boiling sugar. The graduations extend to 270° or further. An index is sometimes attached to the scale, which may be set to any degree of heat required to be maintained.
67. EARTH THERMOMETER.
The Earth Thermometer is for ascertaining the temperature of the soil at various depths. It is protected by a brass frame, pointed and strengthened at the end to facilitate insertion into the ground, as in fig. 51.
Fig. 51.
Utility of a Knowledge of the Temperature of the Soil.—The temperature of the soil is an important element in the consideration of climate, as it concerns the vegetable kingdom.
Dr. Daubeny, in his Lectures on Climate, gives the following statement with respect to some temperatures which have been observed just beneath the earth’s surface, in different parts of the globe:—
| Country. | Temperature. | Authority. |
| Tropics, often | 162-184° | Humboldt. |
| Egypt | 133-144 | Edwards & Colin. |
| Orinoco | In white sand, 140 | Humboldt. |
| Chili | 113-118, among dry grass | Boussingault. |
| Cape of Good Hope | 150, under the soil of a bulb garden | Herschell. |
| Bermuda | 142, thermometer barely covered in earth | Emmet. |
| China | Water of the fields, 113; adjacent sand, much higher; blackened sides of the boat at midday, 142-150 | Meyer. |
| France | 118-122, and in one instance 127 | Arago. |