=69. Importance of Self-Registering Thermometers.=--Heat being apparently the most effective agent in producing meteorological phenomena, the determination of the highest temperature of the day, and the lowest during the night, is a prime essential to enable an estimate of the climate of any place to be formed. To observe these extremes by means of the ordinary thermometer would be impracticable, from the constant watchfulness which would be necessary. Hence, the utility and importance of self-recording thermometers are evident. A thermometer constructed to _register_ the highest temperature is usually called a _maximum thermometer_; one to show the lowest temperature is termed a _minimum thermometer_; and if made to record both extremes of temperature, it is designated a _maximum-and-minimum thermometer_. We will, for the sake of method, describe the instruments in use in this order.

It would carry us beyond our scope to explain in detail the methods of dealing with temperature observations; but we may remark that half the sum of the maximum and minimum temperature of each day of twenty-four hours, is not what meteorologists designate the _mean daily temperature_, although it very frequently approximates to it. The mean temperature of the day is understood to be the average of twenty-four consecutive hourly readings of a thermometer; and meteorology now supplies formulae whereby this result can be deduced from two or three observations only in a day.

But we would observe that the actual mean temperature of any place has not such an important influence upon life, either animal or vegetable, as the abruptness and magnitude of the variations of temperature. Climate, therefore, should be estimated more by the range of the thermometer than by the average of its indications. The Registrar General's returns prove that with a wide range of the thermometer, the mortality greatly increases; and it is now becoming apparent to meteorologists that the daily range of the thermometer marks the effects of temperature on the health of men, and the success of crops, better than any other meteorological fact of which we take cognizance. Now that self-registering thermometers are constructed with mercury, the most appropriate of all thermometric substances, not only for maxima, but likewise for minima temperatures, the determination of the diurnal range of temperature is rendered more certain, and observations at different places are more strictly comparable.

MAXIMA THERMOMETERS.

=70. Rutherford's Maximum Thermometer.=--The maximum thermometer, invented by Dr. John Rutherford, differs from an ordinary thermometer in having a small cylinder of steel, porcelain, or aluminium, moving freely in the tube beyond the mercury, so as to form an index. The stem of the thermometer is fixed horizontally on the frame, which must be suspended in the same position, as represented in fig. 53. The instrument is set by holding it bulb downward, so as to allow the index to fall by its own gravity into contact with the mercury. Increase of heat produces expansion of the mercury, which consequently pushes forward the index. When the temperature decreases, the mercury recedes from the index, leaving it so that the extremity which was in contact with the mercury indicates upon the scale the highest temperature since the instrument was last set.

[Ill.u.s.tration: Fig. 53.]

As it is easily constructed and is comparatively cheap, it is still employed for ordinary purposes. Its disadvantages are, firstly, its liability of soon getting out of order by the index becoming embedded in the mercury, or fixed by oxidation, thus rendering it altogether useless; secondly, the ease with which the index can be displaced by the wind moving the instrument, or other accidental disturbance, so as to cause it to give erroneous indications occasionally; and thirdly, its consequent total unfitness for use at sea.

In the part of the tube beyond the mercury, a small quant.i.ty of air is enclosed for the purpose of preventing the metal flowing freely in the tube. This necessitates the construction of a larger bulb, which renders the thermometer less sensitive. Moreover, as it frequently happens that some mercury pa.s.ses the index, particles of air insinuate themselves in the metal, and cause separations in the column, which very often can be removed only by a maker. To facilitate this re-adjustment, a small chamber is left at the end of the tube, and the mercury being expanded into it by heat until the index and air bubbles are forced into it, if possible, upon the cooling down again, by a little management, the mercury will contract, leaving the air and index behind. Yet sometimes the index cannot be moved in the least from its place of fixture, so that the instrument must be virtually reconstructed.

=71. Phillip's Maximum Thermometer.=--A maximum thermometer, better perhaps in its action than Rutherford's, has been suggested by Professor John Phillips, of Oxford. A small portion of air is introduced into an ordinary thermometer, so as to cut off about half an inch of the mercurial thread near its end in the tube. This forms a maximum thermometer, when the stem is arranged horizontally. The isolated portion is pushed forward by expansion, and is left in this position when the mercury contracts.

The end remote from the bulb shows on the scale the maximum temperature.

When made with a capillary tube so fine that the attraction arising from capillarity overcomes the force of gravity, and prevents the mercury falling to the end of the tube when the instrument is inverted, it forms a very serviceable thermometer, quite portable and suitable for use on board ship. In such a tube a smart shake from a swing of the hand is required to bring the detached portion back to the column, so as to set the instrument for future observation; no ordinary motion will move it. When the thermometer has not this peculiarity, the mercury will flow to the end, if held bulb downward; and in this state it is not at all a satisfactory instrument, as the air is likely to be displaced, and a great deal of tact is requisite to again get it to divide the column suitably. It has been found in practice that the air bubble at different temperatures a.s.sumes different lengths, and if very small it disappears in a few years by oxidation and by diffusion with the mercury, so that the instrument becomes defective and uncertain in action,--results which led to the construction of the self-registering mercurial maximum thermometer, invented and patented by Messrs. Negretti and Zambra. It has been before the public about twelve years; we may therefore, now, safely speak of its merits.

=72. Negretti and Zambra's Patent Maximum Thermometer= consists of a gla.s.s tube containing mercury fitted on an engraved scale, as shown in fig. 54.

The part of the thermometer tube above the mercury is entirely free from air; and at the point A in the bend above the bulb, is inserted and fixed with the blow-pipe a small piece of solid gla.s.s, or enamel, which acts as a valve, allowing mercury to pa.s.s on one side of it when heat is applied, but not allowing it to return when the thermometer cools. When mercury has been once made to pa.s.s the contraction, which nothing but the expansive force of heat can effect, and has risen in the tube, the upper end of the column registers the maximum temperature. To return the mercury to the bulb, we must apply a force equal to that which raised it in the tube; the force employed is gravity, a.s.sisted when necessary by a little agitation of the instrument.

[Ill.u.s.tration: Fig. 54.]

The degrees are generally divided on the stems of these thermometers, but their frames of course bear a scale as well. The makers have various styles of framing in wood, metal, porcelain, and even gla.s.s. Each material is eligible according to requirements. Porcelain scales, having the marks _etched_ upon them by acid and permanently blackened and baked in,--by a process for which the inventors have a separate patent,--will be found very serviceable, as they do not corrode or tarnish by exposure to any kind of weather; while any amount of dust and dirt can readily be cleaned off.

The chief recommendation of this thermometer is its simplicity of construction, enabling it to be used with confidence and safety. Of no other maximum thermometer can it be said that it is impossible to derange or put it out of order; hence, as regards durability, it surpa.s.ses all others. Nothing short of actual breakage can cause it to fail. Hence it is the most easily portable of all self-registering thermometers, an advantage which renders it suitable for travellers, and for transmission abroad. In the year 1852, the British Meteorological Society reported this thermometer to be "the best which has yet been constructed for maximum temperature, and particularly for sun observations." Since then eleven years have elapsed, and it is still without a rival.

_Directions for use._ In using this thermometer for meteorological observations, it should be suspended by means of two bra.s.s plates B, C, attached for that purpose, in such manner that it hangs raised up a little at C, and so placed that it is in the shade, with the air pa.s.sing freely to it from all sides; then, on an increase of heat, the mercury will pa.s.s up the tube as in an ordinary thermometer, and continue doing so as long as the heat increases. On a decrease of heat, the contraction of mercury will take place _below_ the _bend_ in the tube, leaving the whole column of mercury in the tube, thus registering the highest temperature, and showing such till the instrument is disturbed.

To prepare the instrument for future observations, remove and hold it perpendicularly, with the bulb downward, and then shake it. The mercury will then descend in the tube, and indicate the temperature of the air at that time; and, when again suspended, is prepared for future observation.

After the temperature has attained a maximum, there will be, with a decrease of heat, a slight contraction of mercury in the tube--as well as of that in the bulb--and hence doubts have arisen as to the accuracy of the registration; but calculation shows, and critical trial has proved, that the greatest daily range of temperature will not produce an error large enough to be appreciable on the scale.

A very great advantage of this thermometer is that the mercury may be allowed to flow to the end of the tube without the maximum temperature attained during an experiment being lost. It can be employed with the bulb uppermost. All that is necessary for reading the maximum temperature is to slope the instrument so that the mercury flows gently towards the bulb. It will then stop at the contraction so as to show the maximum temperature on the scale. Afterwards the mercury is driven into the bulb by agitating the instrument while held in the hand. Hence the instrument is invaluable as a registering thermometer on board ship, as its indications are in no way affected by the motions and tremors of the vessel.

For physiological experiments, such as taking the temperature of the mouth in fever, this thermometer is the only one that can be used with certainty, as it can be held in any position, without losing the maximum temperature attained.

MINIMA THERMOMETERS.

=73. Rutherford's Alcohol Minimum Thermometer=, fig. 55, consists of a gla.s.s tube, the bulb and part of the bore of which is filled with perfectly pure spirits of wine, in which moves freely a black gla.s.s index.

A slight elevation of the thermometer, bulb uppermost, will cause the gla.s.s index to flow to the surface of the liquid, where it will remain, unless violently shaken. On a _decrease_ of temperature the alcohol recedes, taking with it the gla.s.s index; on an _increase_ of temperature the alcohol alone ascends in the tube, leaving the end of the index _farthest_ from the bulb indicating the minimum temperature.

[Ill.u.s.tration: Fig. 55.]

_Directions for using, &c._--Having caused the gla.s.s index to flow to the end of the column of spirit, by slightly tilting the thermometer, bulb uppermost, suspend the instrument (in the shade with the air pa.s.sing freely to it on all sides) by the two bra.s.s plates attached for that purpose,--in such manner that the bulb is about half an inch lower than the upper, or the end of the thermometer farthest from the bulb; then, on a decrease of temperature, the spirits of wine will descend, carrying with it the gla.s.s index; on an increase of temperature, however, the spirits of wine will ascend in the tube, leaving that end of the small gla.s.s index farthest from the bulb indicating the minimum temperature. To reset the instrument, simply raise the bulb end of the thermometer a little, as before observed, and the index will again descend to the end of the column, ready for future observation.

_Precautions._--1. By no means jerk or shake an alcohol minimum thermometer _when resetting_ it, for by so doing it is liable to disarrange the instrument, either by causing the index to leave the spirit, or by separating a portion of the spirit from the main column.

2. As alcohol thermometers have a tendency to read lower by age, owing to the volatile nature of the fluid allowing particles in the form of vapour to rise and lodge in the tube, it becomes necessary to compare them occasionally with a mercurial thermometer whose index error is known; and if the difference be more than a few tenths of a degree, examine well the upper part of the tube to see if any alcohol is hanging in the bore thereof; if so, the detached portion of it can be joined to the main column by swinging the thermometer with a pendulous motion, _bulb downwards_.

3. The spirit column is sometimes much separated by jolting in travelling.

If the instrument is in such a condition when received, it should be held by the right hand, bulb downward, and the frame tapped smartly, but cautiously, against the palm of the left hand. The broken thread of spirit will soon begin to join, and by continuing the operation a sufficient time all the bubbles will disappear, and the thermometer become as good as ever.

=74. Horticultural Minimum Thermometer.=--This instrument, represented in fig. 56, is a special construction of Rutherford's minimum thermometer to meet the requirements of horticulturists. It is desirable, if not essential, that gardeners should have the means of ascertaining to what temperature stoves and greenhouses descend on cold nights, especially in winter. This thermometer is mounted on a strong cast zinc frame, with the divisions and figures of the scale raised.

[Ill.u.s.tration: Fig. 56.]

The sunk surface of the frame is painted dark; the figures and division a bright colour, so that observations can be made without a close inspection of the instrument.

The directions for using are the same as those given in the preceding section. It may be used as an ordinary thermometer, by simply hanging it from the top loop, in which position, the coloured liquid will always indicate the present temperature.

It was a source of annoyance with the ordinary boxwood and flat metal scales, that after a time, exposure to a damp warm atmosphere favoured the growth of confervae upon them, and obliterated the divisions; the plan of raising the figures and divisions of the scale has been found to prevent the destruction of the instrument in this way.

=75. Baudin's Alcohol Minimum Thermometer.=--This instrument resembles Rutherford's thermometer in appearance; its indications are given by the expansion and contraction of alcohol, and its minimum temperature is likewise registered by a gla.s.s index being pulled back and left behind by the alcohol, as in Rutherford's instrument. There is, however, a great improvement in Baudin's instrument; for whilst Rutherford's thermometer can only register in a horizontal position, Baudin's can be used either horizontally or vertically, as necessity may require. This important change is effected in the following manner:--Instead of the index in the thermometer being loose and free to run up and down according to the position in which the instrument is held, as in Rutherford's, the index in the new instrument is made to fit the bore of the tube as nearly tight as possible, so much so that in holding the thermometer even upside down, or shaking it, the index will not shift from its position; but, inasmuch as a minimum thermometer with an immoveable index could not be set when required for observation, and would consequently be useless, the inventor has introduced behind the index a piece of solid gla.s.s, about one-and-a-half inch in length, which moves freely in the alcohol. The addition of the weight of this piece of gla.s.s on the top of the index, when turned upside down, forces the index down to the edge of the alcohol; and it is there left, as in the case of the ordinary Rutherford's thermometer. It is, therefore, by turning the thermometer upside down, and letting the moveable piece of gla.s.s fall on the index, that the index is driven to the end of the alcohol; after this operation the thermometer is hung up either horizontally or vertically, and will then be ready for use.

The index, although immoveable _per se_, is by the alcohol drawn back, as in the ordinary minimum, and its indications are read off on the scale from the top of the index.

=76. Mercurial Minima Thermometers desirable.=--Alcohol does not expand equally for equal increments of heat, consequently errors are likely to exist in the scale indications unless the graduations are very accurately--not necessarily equally--made. On this account, as well as from the volatility of alcohol, and the intervention of gaseous part.i.tions in the tube, a good and thoroughly reliable minimum thermometer was for a long time a desideratum. It was desirable to obtain a thermometer which should register the lowest temperature by mercury, the fluid in general use for meteorological thermometers. Several instruments have recently been invented to meet this requirement, which are suitable and satisfactory for land purposes, but one well adapted for use on board ship is still very much wanted.

For very low temperatures, alcohol thermometers will always be required; as mercury freezes at -40 F, and contracts very irregularly much before this point, while alcohol has never yet been frozen.

=77. Negretti and Zambra's Patent Mercurial Minimum Thermometer=, represented by fig. 57, has a cylindrical bulb of large size, which, at first sight, might induce the idea that the instrument would not be sufficiently sensitive; but as length is given to the cylinder instead of increasing its diameter, it will be found as sensitive as a globular bulb of the same diameter, and much more so than an ordinary alcohol thermometer.

[Ill.u.s.tration: Fig. 57.]

The reason for having the bulb large is to allow the internal diameter of the thermometer tube to be larger than that generally used for thermometrical purposes, so that a steel index, pointed at both ends, may move freely within when required.

The tube is blown, filled and regulated in the usual way, 60 of temperature being about half-way up the tube. A small cylindrical bulb is then formed at the upper end of the tube, and then is introduced a steel needle pointed at both ends, that in contact with the mercury being abrupt, the other more prolonged. The open extremity of the tube is now drawn out into a fine capillary tube, and the bulb of the instrument warmed so as to cause the mercury to fill the tube completely. When the mercury reaches the capillary tube, the flame of a blow-pipe is applied; the gla.s.s is dexterously melted, the superfluous part taken away, and the tube left hermetically closed. During this operation, the steel index has been embedded in the heated mercury. As the instrument cools, if held upright, the mercury will recede and expose the needle, which will then follow the descending column simply by its own gravity. In this condition the thermometer resembles Rutherford's maximum, being a tube of mercury with a steel index floating on its surface; but it possesses these important advantages: it is quite free from air, so that the mercury can move with perfect freedom; and the index is pointed at both ends, to allow the mercury to pa.s.s, instead of being ground flat to prevent it.

_To use the Thermometer_, it is suspended perpendicularly (figure 57) with the steel index resting on the surface of the mercurial column. As the mercury in the cylinder contracts, that in the tube descends, and the index, of its own gravity, follows it; on the contrary, as the mercury expands and rises in the tube, it pa.s.ses the index on one side, and in rising, exerts a lateral pressure on the needle, and jams it to one side of the tube, where it remains firmly fixed, leaving the upper point of the needle indicating the minimum temperature. In this thermometer, the reading is always from the upper point of the needle, and not from the mercury itself.

_To extricate the Needle_ from the mercury, a magnet is used, when, if the needle is embedded only a few degrees, it can readily be withdrawn without altering the position of the instrument. Should the magnet not be sufficient for the purpose, we simply turn the thermometer on its support from the upright position, slightly elevating the bulb (fig. 58 (=2=)).

The mercury and index will then flow into the small reservoir. Should the index not freely leave the tube with the mercury, a.s.sist it with a magnet, and when the mercury and index are in the upper bulb (figure =2=), apply a magnet outside, which will attract and hold fast the index; and whilst thus holding it, again bring the thermometer to the upright position, when the mercury will immediately fall back into the tube, leaving the index attached to the magnet (figure =4=), with which it is guided down to the surface of the mercury, ready for another observation.

[Ill.u.s.tration: Fig. 58.]

Care must be taken not to withdraw the magnet until the index is in contact with mercury; for, if released before touching, it might plunge too deeply, and give a false indication. The rule for re-setting it will be to bring the needle-point in contact with the mercury, and then withdraw the magnet, having previously ascertained that no particles of mercury are attached to the index.

It may sometimes, though rarely, happen, that from the time a minimum temperature is registered by the index, and by the time an observation is made, the mercury may have risen so high in the tube as to completely pa.s.s the index, as shown (figure =3=). Should it so happen, the s.p.a.ce which the index occupies will readily be observed, as it will be pressed to one side of the tube, causing a different appearance in that part, although the point of the needle may not be seen. If such be the case, apply a magnet to the spot where you see the index is fixed: this will hold the needle firmly. Then, by slightly tilting the thermometer bulb uppermost, the mercury will flow into the top bulb, leaving the index attached to the magnet, and quite uncovered. Having taken the reading, draw the needle into the top bulb, and hold it there whilst you adjust the thermometer by again bringing it to the upright position.