[Ill.u.s.tration: Fig. 37.]

[Ill.u.s.tration: Fig. 38.]

[Ill.u.s.tration: Fig. 39.]

[Ill.u.s.tration: Fig. 40.]

[Ill.u.s.tration: Fig. 41.]

Fig. 42, an Ornamental Drawing-room Thermometer, on ebony or ivory stand, with gla.s.s shade.

Fig. 43, representation of highly carved or engine-turned design for thermometer mounts, in ivory or wood, for the drawing-room. Some have the addition of a sundial or compa.s.s at the top; they may also be formed for a watch-stand.

Fig. 44, =Bath Thermometer=, having a float to admit of its being kept in the water.

[Ill.u.s.tration: Fig. 42.]

[Ill.u.s.tration: Fig. 43.]

[Ill.u.s.tration: Fig. 44.]

Fig. 45, Thermometer with ivory scale in gla.s.s cylinder, mounted on oak bracket with metal top, for out-door use; as at a window.

Fig. 46, Thermometer for the window, on patent porcelain or gla.s.s scale, with oak bracket and convenient bra.s.s supports, for placing the instrument at any angle.

Fig. 47, =Chemical Thermometer=, on boxwood scale, jointed near the bulb on a bra.s.s hinge, ranging from 300 to 600.

Fig. 48, =Chemical Thermometer=, for acids, graduated on its own stem, suitable for insertion in the tubulure of retorts; they are also made insulated in gla.s.s cylinder to protect the graduated stem; ranging from 0 to 600.

[Ill.u.s.tration: Fig. 45.]

[Ill.u.s.tration: Fig. 46.]

[Ill.u.s.tration: Fig. 47.]

[Ill.u.s.tration: Fig. 48.]

[Ill.u.s.tration: Fig. 49.]

[Ill.u.s.tration: 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 ill.u.s.trated 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 s.p.a.ce 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 bra.s.s frame, pointed and strengthened at the end to facilitate insertion into the ground, as in fig. 51.

[Ill.u.s.tration: 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 gra.s.s

Boussingault.

Cape of Good Hope

150, under the soil of a bulb

Hersch.e.l.l.

garden

Bermuda

142, thermometer barely covered

Emmet.

in earth

China

Water of the fields, 113;

Meyer.

adjacent sand, much higher;

blackened sides of the boat

at midday, 142-150

France

118-122, and in one instance 127

Arago.

"The importance of this to vegetation may be estimated by the following considerations:--

"It is known that every plant requires a certain amount of heat, varying in the case of each species, for the renewal of its growth, at the commencement of the season.

"Now when this degree of heat has spurred into activity those parts that are above ground, and caused them to elaborate the sap, it is necessary that the subterranean portions should at the same time be excited by the heat of the ground to absorb the materials which are to supply the plant with nourishment. Unless the latter function is provided for, the aerial portions of the plant will languish from want of food to a.s.similate.

Indeed, it is even advisable that the roots should take the start of the leaves, in order to have in readiness a store of food for the latter to draw upon." In another place the professor remarks:--"It has been calculated by Mr. Raikes, from experiments made at Chat Moss, that the temperature of the soil when drained averages 10 more than it does when undrained; and this is not surprising, when we find that 1 lb. of water evaporated from 1,000 lbs. of soil will depress the whole by 10, owing to the latent heat which it absorbs in its conversion into vapour."

68. MARINE THERMOMETER.

This instrument is a special construction to meet the requirements of navigation. It consists of a carefully constructed thermometer divided on its stem to degrees, which are sufficiently large to admit of subdivision into tenths of degrees by estimation, and ranging from 0 to 130. The scale is porcelain, having the degrees etched upon it, and burnt-in a permanent black. The instrument is made to slide into a j.a.panned metallic case, for handy use and protection. It is therefore adapted for almost any ordinary purpose; and cannot be injuriously affected by any chemical action arising from air or sea-water. A set of these thermometers consists of six, carefully packed in a neat box; two having j.a.panned metallic cases (fig. 52), the others being designed for use without the case, or to replace a breakage.

[Ill.u.s.tration: Fig. 52.]

This thermometer is employed in the Royal Navy, and for the observations made at sea for the Board of Trade.

The thermometer is now considered a necessary instrument on board ship.

Not only is it of invaluable utility in connection with the barometer as a guide to the weather, but its indications are of service in showing the presence of a warm or cold current in the sea; many of the great oceanic currents being characterised by the warmth or coldness of their waters. In seas visited by icebergs, the habitual use of the thermometer would indicate their proximity, as the water is rendered colder for some distance around by the thawing of huge ma.s.ses of ice. The water over a shoal in the sea is generally colder than the surface-water of the surrounding ocean; which may result from the cold water being brought to the surface by the current of water encountering the shoal. With this fact navigators are well acquainted; and therefore a fall in the sea-water thermometer may forebode that shallow water is at hand. It has been ascertained that fish inhabit regions of the oceans and seas having the peculiar temperature suitable to their habits. The better and firmer sort of fish are found where cold waters exist. Those taken in warmer belts or streams of water, even in the same lat.i.tude, are far inferior in condition, and less approved by the palate. The fish of the Mediterranean, a warm sea, are generally poor and scarce. Fish taken in the cold waters between the American sh.o.r.e and the Gulf Stream are much esteemed; while in and on the other side of the stream they are said to be tasteless, and of no flavour. Between the coasts of China and the warm waters of the j.a.panese current, the seas abound with excellent fish; but in the warm waters of the current and beyond, they are never seen in such shoals.

In fact, it is clearly ascertained that fishes are adapted to climates, like birds and beasts. It has been even affirmed, after careful investigation, that herrings, which abound in the British Seas, and form a most important branch of our fisheries, can only be found in a temperature varying from 54 to 58. Hence the thermometer, if brought into use by the fishermen, would guide them to the spots where they may with the best chance cast their nets on dark nights, when other indications are not perceptible.

This thermometer in its metallic case is perfectly suited for dipping overboard, or placing in a bucket of water just taken from the sea, to ascertain its temperature.

CHAPTER VII.

SELF-REGISTERING THERMOMETERS.