A Treatise on Meteorological Instruments Part 19

Self-registering thermometers should be protected by a similar screen. It has been found that thermometric observations made at sea are not valuable for scientific purposes unless the instruments have been duly protected by such a screen.

=150. Anemoscope=, or Portable Wind Vane for travellers, with compa.s.s, bar needle, &c., shows the direct course of the wind to half a point of the compa.s.s.

[Ill.u.s.tration: Fig. 95.]

=151. Evaporating Dish, or Gauge= (fig. 95), for showing the amount of evaporation from the earth's surface. This gauge consists of a bra.s.s vessel, the area or evaporating surface of which is accurately determined; and also a gla.s.s cylindrical measure, graduated into inches, tenths, and hundredths of inches. In use, the evaporating gauge is nearly filled with water, the quant.i.ty having been previously measured by means of the gla.s.s cylinder; it is then placed out of doors, freely exposed to the action of the atmosphere; after exposure, the water is again measured, and the difference between the first and second measurement shows the amount of evaporation that has taken place. If rain has fallen during the exposure of the gauge, the quant.i.ty collected by it must be deducted from the measured quant.i.ty; the amount is shown by the quant.i.ty of rain collected in the rain gauge. The wire cage round the gauge is to prevent animals, birds, &c., from drinking the water.

=152. Dr. Babington's Atmidometer=, or instrument for measuring the evaporation from water, _ice or snow_, consists of an oblong hollow bulb of gla.s.s or copper, beneath which and communicating with it by a contracted neck is a second globular bulb, duly weighted with mercury or shot. The upper bulb is surmounted by a small gla.s.s or metal stem, having a scale graduated to grains and half-grains; on the top of which is fixed horizontally a shallow metal pan. The bulbs are immersed in a vessel of water having a circular hole in the cover through which the stem rises.

Distilled water is then gradually poured into the pan above, until the zero of the stem sinks to a level with the cover of the vessel. Thus adjusted, as the water in the pan evaporates, the stem ascends, and the amount of evaporation is indicated in grains. This instrument affords a means of measuring evaporation from _ice or snow_. An adjustment for temperature is necessary.

=153. Cloud Reflector.=--At the International Exhibition 1862, Mr. J. T.

G.o.ddard exhibited a cloud mirror, for ascertaining the direction in which the clouds are moving.

The mirror is laid on a horizontal support near a window, and fastened so that the point marked north may coincide with the south point of the horizon,--the several points will consequently be reversed. The edge of a conspicuous cloud is brought to the centre of the mirror, and the observer keeps perfectly still until it pa.s.ses off at the margin, where the true point of the horizon _from which_ the clouds are coming can be read off.

=154. Suns.h.i.+ne Recorder.=--Mr. G.o.ddard also exhibited an instrument which he calls by this name. It works by letting the sun's rays pa.s.s through a narrow slit, and fall on photographic paper wound round a barrel moved by clock-work; the paper being changed daily, and the photographic impression developed and fixed in the usual manner.[19]

155. SET OF PORTABLE INSTRUMENTS.

In a small box, 8 in. by 8 in. by 4 in., a complete set of meteorological instruments have been packed. The lid of the box, by an ingenious arrangement, is made to take off and hang up; on it are permanently fixed for observation, a maximum and minimum, and a pair of dry and wet bulb thermometers. The interior of the box contains a maximum thermometer in vacuo for solar radiation, and a minimum for terrestrial purposes, one of Negretti and Zambra's small pocket aneroid barometers, pedometer for measuring distances, pocket compa.s.s, clinometer, and lastly a rain gauge.

This latter instrument consists of an accurately turned bra.s.s ring having an india rubber body fastened to it to receive the rain, which is measured off by a small graduated gla.s.s, also contained in the box. Gentlemen travelling will find this compact observatory all that can be desired for meteorological observations.

156. IMPLEMENTS.

The practical meteorologist will find the following articles very useful, if not necessary. They scarcely require description; an enumeration will therefore suffice:--_Weather Diagrams_, or prepared printed and ruled forms, whereon to exhibit graphically the readings of the various instruments to render their indications useful in foretelling weather, &c.;--_Meteorological Registers_, or Record Books, for recording all observations, and the deductions;--_Cloud Pictures_, by which the clouds can be readily referred to their particular cla.s.sification, very necessary to the inexperienced and learners;--Cyclone Gla.s.ses, or Horns, outline Maps with Wind-markers, are also useful, especially in forecasting weather.

157. HYDROMETER.

A simple kind of hydrometer is very much used at sea, as "a sea-water test;" and as the observations are usually recorded in a meteorological register or the s.h.i.+p's log-book, it may not be altogether out of place to give a description of it here.

[Ill.u.s.tration: Fig. 96.]

[Ill.u.s.tration: Fig. 97.]

It is constructed of gla.s.s. If made of bra.s.s, the corrosive action of salt-water soon renders the instrument erroneous in its indications. The shapes usually given to the instruments are shown in figs. 96 and 97. A globular bulb is blown, and partly filled with mercury or small shot, to make the instrument float steadily in a vertical position. From the neck of the bulb the gla.s.s is expanded into an oval or a cylindrical shape, to give the instrument sufficient volume for flotation; finally, it is tapered off to a narrow upright stem which encloses an ivory scale, and is closed at the top. The divisions on the scale read downward, so as to measure the length of the stem which stands above the surface of any liquid in which the hydrometer is floated. The denser the fluid, the higher will the instrument rise; the rarer, the lower it will sink.

The indications depend upon the hydrostatic principle, that floating bodies displace a quant.i.ty of the fluid which sustains them equal to their own weight. According, therefore, as the specific gravities of fluids differ from each other, so will vary the quant.i.ties of the fluids displaced by the same body when floated successively in each.

The specific gravity of distilled water, at the temperature of 62 _F_, being taken as unity, the depth to which the instrument sinks when gently immersed in such water is the zero of the scale. The graduations extend from 0 to 40; the latter being the mark which will be level with the surface when the instrument is placed in water, the specific gravity of which is 1040. In recording observations, the last two figures only--being the figures on the scale--are written down. Sea-water usually ranges from 1020 to 1036.

A small tin, copper, or gla.s.s cylinder is useful for containing the water to be tested. It should be wider than the hydrometer, and always filled to the brim. If fitted to a stand, which is supported by gimbals, it will be very convenient. Water in a bucket, basin, or other wide vessel, acquires motion at sea, and the eye cannot be brought low enough (on account of the edges) to read off the scale accurately.

Errors of observation may occur with the hydrometer, if it be put into water without being clean, or without being carefully wiped. The instrument is extremely accurate if correctly used. It should be kept free from contact with the sides of the vessel; and all dust, smears, or greasiness, should be scrupulously avoided, by carefully wiping it with a clean cloth before and after use.

Whenever the temperature of the water tested differs from 62, a correction to the reading is necessary, for the expansion or contraction of the gla.s.s, as well as the water itself, in order to reduce all observations to one generally adopted standard.

Negretti and Zambra's hydrometer, with thermometer in the stem, shows the density and temperature in one instrument.

For the following Tables we are indebted to the kindness of Admiral FitzRoy:--

TABLE for reducing observations made with a BRa.s.s HYDROMETER, a.s.suming the linear expansion of bra.s.s to be 0000009555 for 1 F. The correction is additive for all temperatures above 62, and subtractive for temperatures below 62.

+----------------------------------------------------------------------+ _t_ Correction. _t_ Correction. _t_ Correction. _t_ Correction. ----+-----------++----+-----------++----+-----------++----+----------- 32 -00014 48 -00010 64 +00002 80 +00020 33 0014 49 0009 65 0003 81 0021 34 0014 50 0009 66 0004 82 0023 35 0014 51 -00008 67 0005 83 0024 36 0014 52 0008 68 +00006 84 0026 37 0014 53 0007 69 0007 85 +00027 38 -00014 54 0006 70 0008 86 0029 39 0013 55 0006 71 0009 87 0030 40 0013 56 -00005 72 0010 88 0032 41 0013 57 0004 73 0011 89 0033 42 0013 58 0003 74 +00013 90 +00035 43 0012 59 0003 75 0014 91 0036 44 -00012 60 0002 76 0015 92 0038 45 0011 61 -00001 77 0016 93 0040 46 0011 62 00000 78 0018 94 0041 47 -00010 63 +00001 79 +00019 95 +00043 +----------------------------------------------------------------------+

TABLE for reducing observations made with a GLa.s.s HYDROMETER, a.s.suming the linear expansion of gla.s.s to be 000000463 for 1 F. The correction is additive for temperatures above 62, and subtractive for temperatures below 62.

+----------------------------------------------------------------------+ _t_ Correction. _t_ Correction. _t_ Correction. _t_ Correction. ----+-----------++----+-----------++----+-----------++----+----------- 32 -00019 48 -00012 64 +00002 80 +00023 33 0019 49 0011 65 0003 81 0024 34 0018 50 0011 66 0004 82 0026 35 0018 51 -00010 67 0005 83 0027 36 0018 52 0009 68 +00007 84 0029 37 0017 53 0008 69 0008 85 +00031 38 -00017 54 0008 70 0009 86 0032 39 0017 55 0007 71 0010 87 0034 40 0016 56 -00006 72 0012 88 0036 41 0016 57 0005 73 0013 89 0037 42 0015 58 0004 74 +00014 90 +00039 43 0015 59 0003 75 0016 91 0041 44 -00014 60 0002 76 0017 92 0042 45 0014 61 -00001 77 0018 93 0044 46 0013 62 00000 78 0020 94 0046 47 -00013 63 +00001 79 +00021 95 +00048 +----------------------------------------------------------------------+

158. NEWMAN'S SELF-REGISTERING TIDE-GAUGE.

At places where the phenomena of the tides are of much maritime importance, a continuous series of observations upon the rise and fall, and times of change, is essentially necessary as a basis for the construction of good tide tables; and as such observations should also be accompanied with the registration of atmospheric phenomena, we have no hesitation in inserting a description of an accurate self-registering tide-gauge.

The tide-gauge, as shown in the ill.u.s.tration, consists of a cylinder, _A_, which is made to revolve on its axis once in twenty-four hours by the action of the clock, _B_. A chain, to which is attached the float, _D_, pa.s.ses over the wheel, _C_, and on the axis of this wheel, _C_ (in about the middle of it) is a small toothed wheel, placed so as to be in contact with a larger toothed wheel carrying a cylinder, _E_, over which pa.s.ses another smaller chain. This chain, pa.s.sing along the upper surface of the cylinder, _A_, and round a second cylinder, _F_, at its further end, is acted on by a spring so as to be kept in a constant state of tension. In the middle of this chain a small tube is fixed for carrying a pencil, which, being gently pressed down by means of a small weight on the top of it, performs the duty of marking on paper placed round the cylinder the progress of the rise or fall of the tide as the cylinder revolves, and as it is drawn by the chain forward or backward by the rise or fall of the float. The paper is prepared with lines equidistant from each other, to correspond with the hours of the clock, crossed by others showing the number of feet of rise and fall.

[Ill.u.s.tration]

The cylinder while in action revolves from left to right to a spectator facing the clock, and the pencil is carried horizontally along the top of the cylinder; and the large wheel being made to revolve by the rise and fall of the float, turns the wheel with the small cylinder, _E_, attached to it. If the tide is _falling_, the small chain is wound round the cylinder, _E_, and the pencil is drawn towards the large wheel; but if the tide is _rising_, the small chain is wound on to the cylinder, _F_, by means of the spring contained in it, which constantly keeps it in a state of tension. Thus, by means of the rise and fall of the tide, a lateral progress is given to the pencil, while the cylinder is made to revolve on its axis by the clock, so that a line is traced on the paper showing the exact state of the tide continuously, without further attention than is necessary to change the paper once every day, and to keep the pencil carefully pointed; or a metallic pencil may be used, which will require little, if any, attention.

A good self-registering tide-gauge is a valuable and important acquisition wherever tidal observations are required, and the only perfectly efficient instrument of this kind is that invented by the late Mr. John Newman, of Regent Street, London. It is now in action in several parts of the world, silently and _faithfully_ performing its duty, requiring no other kind of attention than that of a few minutes daily, and thus admitting the employment of the person on any other service whose duty it would otherwise have been to have registered the tide. It has done much by its faithful records in contributing to the construction of good tide tables for many places; for those unavoidable defects dependent on merely watching the surface on a divided scale are set aside by it, all erroneous conclusions excluded, and a true delineation of Nature's own making is preserved by it for the theorist.

ADDENDA.

1. French barometers are graduated to millimetres. An English inch is equal to 2539954 millimetres. Hence, 30 inches on the English barometer scales correspond to 762 millimetres on the French barometer scales.

Conversions from one scale to another can be effected by the following formulae:--

(1) Inches = millimetres divided by 2539954 (2) Millimetres = inches multiplied by 2539954

Of course, a table of equivalent values should be drawn up and employed, when a large number of observations are to be converted from one scale to the other.

2. In Germany, barometers are sometimes graduated with old French inches and lines,--the vernier generally indicating the tenth of a line.

OLD FRENCH LINEAL MEASURE.

English Inches.

1 douzieme, or point = 00074 12 points = 1 ligne = 00888 12 lignes = 1 pouce = 1065765 12 pouces = 1 pied = 127892 1 pied = 3247 millimetres.