acted on by currents through the wires, G, H. The point attached to its
armature traces a plain spiral line on the revolving cylinder, except at
the instant when the current is established or interrupted. And the
phenomenon to be timed is in some way made to accomplish the making and
breaking of this circuit. This may be perhaps better understood by an
example. It has sometimes happened that the boxes employed in the
pneumatic dispatch stick fast in the tubes, and resist all efforts to
dislodge them by manœuvres with the compressed or rarefied air, or other
means. In such a case it becomes necessary to ascertain with tolerable
accuracy the position of the obstruction, so that the tube may be cut at
the right place and the obstacle removed. The known velocity of sound
has been ingeniously used for this purpose; the electric chronograph
being made the means of ascertaining, to a small fraction of a second,
the time required for the report of a pistol to be propagated through
the air of the tube and reflected back from the obstruction. An elastic
membrane is spread over the open extremity of the pneumatic tube; this
membrane has in the centre a little disc of platinum, in electric
connection with one of the wires, G, H. The other wire passes to a
galvanic cell or battery, and the return wire from the battery is
connected with a platinum point, the distance of which from the disc can
be adjusted with nicety by means of a screw, so that the circuit may be
complete only when the platinum point and the platinum disc are in
contact. The screw is adjusted so as to bring the point as near as
possible to the disc without actual contact. The chronograph cylinder
having been set in motion, a small pistol is fired into the pneumatic
tube through a side opening. Sound-waves of alternate compression and
rarefaction pass along the tube, and are reflected backwards and
forwards many times in succession between the obstacle and the membrane.
By these the membrane is alternately forced out and drawn in, making and
breaking the electric contact accordingly, and thus causing the point of
the electro-magnet to describe in the cylinder an indented line, the
intervals of which indicate the time the sound requires to traverse the
tube to the obstruction; and thus the position of the latter may be
known with sufficient accuracy.
A very interesting application of the electric chronograph is to the
measurement of the velocities of projectiles. The science of gunnery has
acquired an exactness unknown before electricity was made to carry
messages from the cannon-ball in its swiftest flight, and to write the
record of its own course. Instruments for thus measuring the velocities
of projectiles have been contrived by several electricians, among whom
Wheatstone appears to have been the first. The principle in most of
these chronographs is precisely the same as that on which the apparatus
represented in Fig. 316 is constructed. The action of the projectile
which is electrically indicated is the severing of a slender wire, which
is stretched from side to side of a wooden frame, so that it passes
continuously backwards and forwards in parallel lines. Thus a kind of
screen is formed, through which the missile must pass, and in its
passage must rupture the wire. If the wire conveys a current of
electricity, this current is therefore interrupted at the moment the
ball passes. Sometimes the immediate effect of the breaking of the wire
is mechanical, as in Wheatstone’s arrangement, where the wire is
stretched by a weight over a series of pulleys, and attached to a
contact-maker, which completes the circuit when it is set free by the
rupture of the wire. A similar arrangement in the screens has been
proposed by Mr. Siemens for the establishment of circuits in connection
with charged Leyden jars, the sparks of the discharges being made to
take place at the surface of a revolving cylinder of polished steel,
where the place is shown by the spot they leave on the metal. M.
Pouillet’s chronoscope dispenses with the revolving cylinder, and
measures the _duration_ of the current established by the projectile at
one part of its course and cut off at another, by the arc through which
the needle of a galvanometer is impelled.
The instrument which has been most employed in this country by
artillerists is that invented by Professor Bashforth. Its indications
are extremely accurate, for readings may be taken to the two-thousandth
part of a second. From ten to fifteen screens are placed in the path of
the projectile at distances asunder which may vary from 15 ft. to 150
ft., but which, of course, are carefully measured. Each screen is formed
of a wire carrying an independent current, which also circulates in the
coils of an electro-magnet. The ten electro-magnets have styles attached
to their armatures in such a manner that when all the currents are
passing, ten parallel spiral lines are traced on the surface of a
vertical revolving cylinder, about 12 in. long and 4 in. diameter. The
cylinder is covered with a sheet of highly-glazed paper coated with
lampblack; and when the sheet is removed, the lampblack may be fixed on
the paper if desired, and the record made permanent. The equal intervals
of time are marked on the same cylinder by another electro-magnet
connected with a pendulum beating half-seconds. The axis of the
revolving cylinder, &c., carries a heavy flywheel, which is set in
motion by the hand at the rate of about three revolutions in two
seconds, and the rest of the mechanism is thrown into gear just before
the signal to fire is given. The arrangement of the electric connections
at the screens is such that at the moment of the rupture of the wire,
the circuit is broken for an instant only. At this moment, the iron of
the corresponding electro-magnet ceasing to attract its armature, the
latter is drawn away by a spring; but the re-establishment of the
current immediately brings it back, and the style continues to trace the
same spiral line as before. The passage of the ball through the screen
is therefore marked by a little notch in the spiral line, thus, ——ᐱ——,
and the point where the deviation begins indicates the time at which the
ball passed. In order to read off this time, a straight-edge is applied
to the cylinder parallel to its axis; and by means of a scale sliding
upon the straight-edge the distances between the notches on the several
spirals are compared with those between the pendulum marks.
To record the instant at which a projectile passes determined points in
a cannon’s bore, lateral plugs are screwed in, each having, just
projecting into the bore, a small steel ball, which, pressed outwards by
the passing projectile, causes a cutter to divide the primary wire of a
Ruhmkorff coil, whereupon the spark that passes in the secondary circuit
leaves its record on a uniformly moving disc. Each plug has its own
battery, coil, and disc.
A special feature of recording instruments may be exemplified by certain
applications of the principle to the investigation of physiological
actions. A skilled physician is often able to detect in the pulse of his
patient certain characteristics besides the mere rate, which are highly
significant as regards the condition of the circulatory system. The
range of these indications has been greatly extended by an instrument
invented by MM. Chauveau and Marey, by which the pulse is made to write
down a graphic representation of its action. The patient’s arm having
been placed on a suitable support, a little stud covered with soft
leather is lightly pressed against the artery by a spring. The stud is
in contact with the shorter end of a very light lever, the other
extremity of which is furnished with a point, which registers its
movements on a cylinder of blackened metal, made to rotate and advance
longitudinally by clockwork; or the record is taken on strips of flat
smoked glass. As the motion is much magnified by the lever, every
variation in the pressure of the blood in the artery during the beat of
the pulse is distinctly and faithfully indicated. From the line so
traced the physician may obtain infallible data for judging of the
condition of the heart, the action of its valves, &c. It is marvellous
to observe the manner in which the curves of the _sphygmograph_, as the
instrument is termed, change their form when certain drugs are
administered: the change in some cases occurs immediately, so that the
eye can detect by the inspection of the sphygmographic curve almost the
instant at which the drug was introduced into the system, and the nature
of its action on the heart.
[Illustration:
FIG. 317.—_Negretti’s Deep-Sea Thermometer._
]
Another instrument which is doing good service in the hands of medical
investigators is the _spirograph_, in which the rise and fall of the
chest in breathing are similarly traced by the motions of a lever. In
this instrument a small pad, which presses on the chest, communicates
its movements to an elastic membrane, which, like the skin of a
drumhead, covers one end of a cylindrical box maintained in a fixed
position relatively to the person of the patient. The air in this box is
in communication, by means of a flexible tube, with the interior of
another similarly closed box; the elastic membrane of the latter acts
against the short end of a lever, which is made to register its
movements as in the sphygmograph, for the compression of the air caused
by the rise of the chest is conveyed to the second box through the
flexible tube. The curves furnished by this instrument also give
valuable indications, and exhibit marked changes under any influence in
the least degree affecting the respiratory system.
The value of a self-registering instrument for solving problems, the
intricacy of which is increased by the multiplicity and rapidity of the
actions to be observed, cannot be better illustrated than by the success
with which Professor Marey has thus studied some complicated actions of
locomotion, as related in his extremely interesting work entitled, “La
Machine Animale,” a translation of which has appeared in “The
International Series.” The action of the horse in the various paces,
walking, trotting, galloping, &c., has been an endless subject of
discussion, with no other data than the shoe-marks left in soft ground,
and the general appearance of the animal’s movements to an observer. But
M. Marey—by means of elastic bags containing air, communicating the
pressure through flexible tubes, so as to move little levers, which
write their traces on a revolving cylinder impelled by clockwork, and
carried by the rider,—has completely and finally settled all the points
in dispute. It is now definitely known how the horse’s feet are placed
on the ground in each of his paces, and the actual and relative time
that each foot remains down. The instruments are also made to register
the vertical movements of the animal, so that a complete record of its
motion can be obtained.
[Illustration:
FIG. 318.—_Negretti’s Deep-Sea Thermometer, general arrangement._
]
It was long a difficulty to obtain data as to the temperature of the sea
at great depths below the surface. It is obvious that the ordinary
maximum and minimum registering thermometers would not give the
temperature at any particular depth to which they might be submerged,
but merely the temperature of the warmest or coldest stratum of water
through which they passed in their descent or ascent. No plan which has
been devised to obviate these difficulties appears to have been attended
with success, until a quite recent invention of Messrs. Negretti and
Zambra supplied the desideratum, and furnished a convenient instrument
for trustworthy determination of the temperature of the ocean at any
required depth. The same firm long ago constructed thermometers for
deep-sea soundings, with bulbs protected from the pressure of the water
by an outer covering of thick glass surrounding the delicate bulb of the
thermometer, between which and the outer casing a space was left,
partially filled with mercury, so that heat might readily pass to or
from the inner bulb without the latter being exposed to the
superincumbent pressure. The new recording deep-sea thermometer differs,
however, from all other registering thermometers by containing mercury
only, without alcohol, or springs, or other removable indices, and,
consequently, it is free from liability to derangement. The following is
the description of the instrument:
In the first place it must be observed that the bulb of the thermometer
is protected so as to resist the pressure of the ocean, which varies
according to depth, that of 3,000 fathoms being something like 3 tons
pressure on the square inch. The new instrument is in shape like a
syphon with parallel legs, all in one piece, and having a continuous
communication, as shown in Fig. 317. The scale of the thermometer is
pivoted on a centre, and being attached in a vertical position to a
simple apparatus (which will be presently described), is lowered to any
depth that may be desired. In its descent the thermometer acts as an
ordinary instrument, the mercury rising or falling according to the
temperature of the stratum through which it passes; but so soon as the
descent ceases, and a reverse motion is given to the line, so as to pull
the thermometer towards the surface, the instrument turns once on its
centre, first bulb uppermost, and afterwards bulb downwards. This causes
the mercury, which was in the left-hand column, first to pass into the
dilated syphon-bend at the top, and thence into the right-hand tube,
where it remains, indicating on a graduated scale the exact temperature
at the time the thermometer was turned over. The cut shows the position
of the mercury _after_ the instrument has been turned on its centre. A
is the bulb; B the outer coating or protecting cylinder; C is the space
of rarefied air, which is reduced if the outer casing be compressed; D
is a small glass plug, as in Negretti and Zambra’s maximum thermometer,
which at the moment of turning cuts off the mercury in the tube from
that in the bulb, thereby ensuring that none but the former can be
transferred into the indicating column; E is an enlargement made in the
bend, so as to enable the mercury to pass quickly from one tube to
another in revolving; and F is the indicating tube, or thermometer
proper. When the thermometer is put in motion, as soon as the tube has
acquired a slightly oblique position, the mercury breaks off at the
point D, runs into the curved and enlarged portion, E, and eventually
falls into the tube, F, as the instrument resumes its original vertical
position.
[Illustration:
FIG. 319.—_The Atmospheric Recording Instrument._
]
The contrivance for turning the thermometer over at the bottom of the
sea may be described as a vertical propeller, to which the instrument is
pivoted. So long as the instrument is descending the propeller is lifted
out of gear and revolves free; but as soon as the ascent commences, the
action is reversed, the propeller falls into gear with a pinion
connected with the thermometer, and by these means the thermometer is
turned over, and after one turn it remains locked and immovable. The
engraving, Fig. 318, shows the general arrangement, T being the
thermometer, S a metal screw connected with the frame of the thermometer
by a wheel-and-pinion movement at W; S^† is the stop for arresting the
movement of the thermometer when it has made one complete turn.
The atmospheric recording thermometer (Fig. 319) differs from the
deep-sea thermometer by not having the double or protected bulb, as it
is not required to resist pressures. In this form of the instrument, the
thermometer is turned over by a simple clock movement, which can be set
to any hour that may be desired. It is fixed on the clock, and when the
hand arrives at the hour determined upon, and to which the clock has
been set as an alarum clock is set, a spring is released, and the
thermometer turns over as before described. A wet and dry-bulb
hygrometer is also arranged on the same plan. For observatories, or
where it is important to obtain hourly or half-hourly records of the
temperature, twelve or more thermometers are placed on a frame, and
these are turned over by clockwork one after the other at every hour or
half-hour as required.
The reader can hardly fail to perceive that powerful aid to the
investigation of the laws of nature must be afforded by such instruments
as we have described. And we have but taken an example here and there of
the scientific uses of the recording principle, selecting those that are
most readily understood, or that are connected with matters coming home
to the business and bosom of every one. The science of meteorology does
not deal with subjects which furnish merely amusing speculation for the
hour. Forecasts of storms and cyclones would often save many lives and
much valuable property; and our dependence upon meteorological
conditions cannot be more forcibly illustrated than by reference to the
disastrous floods which this year (1875) desolated some districts of
France. Meteorology has received a great impulse from the introduction
of recording instruments; and the vast number of results which are now
hourly recorded must lead to the certain development of the science, and
its reduction to exact laws. For even the winds obey laws—laws as
definite as those which control the motions of the planets; and could we
but take into account _the whole_ of the circumstances upon which the
movements and other conditions of the atmosphere depend, we should be
able to forecast the weather with the same certainty as—thanks to the
great and simple law of gravitation—we predict eclipses or other
astronomical phenomena. Already, by aid of the telegraph, it is often
possible to send a day’s warning of approaching storms to localities
lying in their probable track. The Signal Service, which is a Department
of the United States War Office, has a corps of meteorological observers
spread over the length and breadth of the States, who send every eight
hours, to a Central Office in Washington, a report of the force and
direction of the wind, height of the barometer, &c. The officer at
Washington sends back by telegraph to the public press a synopsis of
each day’s weather, and points out what weather will probably follow;
but if any city or port be threatened with a storm, special telegrams
are sent. Thus, a warning of the approach of a great storm, which
entered the American continent at San Francisco on the 22nd Feb., 1871,
was sent to Cheyenne, Omaha, and Chicago, twenty-four hours before the
storm reached these cities, which it was foreseen lay in its track.
Although the hurricane did much damage at some of these places, it would
probably have been far more destructive had not the inhabitants been
prepared for its approach.
An elegant form of _barograph_ or recording barometer has been brought
out, which is small, but sufficiently accurate for all ordinary
purposes. It is founded on the _aneroid_, which, as everybody knows, is
an instrument for indicating atmospheric pressure by the changes of form
it produces in a thin circular metallic box, partially exhausted of air.
The ordinary form of the aneroid is very sensitive and portable
(sometimes it is made only the size of a small watch), and bears an
index needle moving over a graduated disc; which arrangement is in the
barograph aneroid, replaced by a long lever, carrying an ink tracing
point in contact with the face of a cylinder that is caused by clockwork
to make one revolution per week. On the cylinder is spread a printed
paper diagram, divided by lines for each day of the week and each hour
of the day, and on this the tracing point marks a continuous curve,
showing all the fluctuations of the barometric pressure. The diagram is
removed at the end of the week, and a fresh form adjusted to the
cylinder. The impressed papers thus form a permanent and continuous
record, from which the height of the barometer, at any given moment, may
be read off.
_THE PHONOGRAPH._
Everything yet contrived in the way of recording instruments is eclipsed
in wonder and interest by one which is among the latest marvels of the
age. It is a recording instrument, and more than a recording instrument,
for it can reproduce to the senses the very phenomena it records; and
these same phenomena are the most familiar in their effects, and, at the
same time, so subtle and delicate, that the impressions they convey are
not generally thought of otherwise than in connection with our finest
intellectual and emotional perceptions. We are alluding to the
_phonograph_, which can register for us music and song, and articulate
human speech in all their tones and modulations, and, like an aërial
spirit, address them to the ear again, as often as we wish, and thus
Inform the cell of hearing, dark and blind;
Intricate labyrinth, more dread for thought
To enter than oracular cave,
Strict passage through which sighs are brought,
And whispers for the heart, their slave;
And shrieks, that revel in abuse
Of shivering flesh; and warbled air,
Whose piercing sweetness can unloose
The chains of frenzy, or entice a smile
Into the ambush of despair;
Hosannas pealing down the long-drawn aisle,
And requiems answered by the pulse that beats
Devoutly, in life’s last retreats!
In order that the reader may understand the action of the phonograph, it
is necessary that he should know something of the science of sound. Then
we must remember that this word is commonly used to express sometimes
those sensations of which the ear is the organ, and at other times the
external cause of those sensations. It is with the former meaning that
we use such expressions as “a sweet sound”; with the latter, such
phrases as “sound travels.” It will not be necessary to speak of the
physiology of the organ of hearing; but attention should be directed to
the different kinds of audible perceptions we can distinguish, let us
suppose, when listening to a song: First, there is the pitch, or the
notes in the musical scale, which, by their particular sequence,
constitute the air or melody. Second, there is the degree of loudness or
lowness of the notes. Third, the enunciation, or those differences by
which we distinguish, for example, the vowels _a_, _e_, _i_, _o_, _u_,
one from another. Fourth, the quality of the voice by which we can
distinguish between two vocalists singing the same vowel on the same
note with equal loudness. Observe that these four kinds of sound
perceptions are independent one of another. The last kind of difference
may also be well illustrated by the instance of musical instruments of
different kinds sounding the same note, in which case the difference of
the quality or _timbre_ is readily recognized. We have now to show the
nature of the mechanical movements outside of us which act on the ear
and reach our perception as sounds, giving the distinguishable
impressions that have been enumerated; and for the present we shall
consider the case of such musical sounds as those just referred to.
That the source of a sustained sound is an elastic body in a state of
vibration is a fact of which, in most cases, we are easily made aware by
the evidence of sight and touch; as a bell, a violoncello string, a
pianoforte wire, or a tuning-fork. On p. 656 is described a simple
method by which a tuning-fork may be made to write down its own
vibrations, and the more exact plan of recording them on the surface of
blackened paper on a revolving and advancing cylinder has also been
referred to. By the intervention of appropriate apparatus, a similar
record may be obtained from all sounding bodies. From observations of
this kind, and others in which totally different methods are used for
counting the number of vibrations per vibrations made in a given time,
it is known that the _pitch_ of the sound or note depends on the
rapidity of the vibrations—the pitch rises with the number of them per
second, and the relationship between the notes of a musical scale
depends entirely on these numbers. Thus, when the vibrations for the
eight notes of an octave are counted, the numbers always have this
_proportion_, beginning from the lowest note-–24, 27, 30, 32, 36, 40,
45, 48. Thus of the two notes—
[Music]