[p.64] ...The object of the
Legislature, in directing a new survey of
Ireland to be made, appears from the Report of the Select Committee
appointed in 1824, "to consider the best mode of apportioning more
equally the local burthens collected in Ireland." The object was to
obtain a survey sufficiently accurate to enable the valuators. acting
under the superintendence of a separate department of the Government,
to follow the surveyors, and to apportion correctly the proper amount
of the local burthens. These burthens had previously been apportioned
[p.65] by
Grand Jury assessments. The assessments had, in some
districts of Ireland, been made by the civil division of plough-lands;
in others by the division of town-lands; the divisions, in either case,
contributing in proportion to their assumed areas, which bore no
defined proportion to their actual contents. The result was great and
much-complained-of inequality in levying the assessment, which it was a
primary object of the survey to remove by accurately defining the
divisions of the country. The Committee reported that it was expedient
to give much greater despatch to this work than had occurred in the
Trigonometrical Survey of England. They recommended that every
facility, in the way of improved instruments, should be given to the
Ordnance officers by whom the survey was to be conducted; and concluded
with the hope, that the great national work which was projected
“will be carried on with energy as well as with skill, and
that it will, when completed, be creditable to the nation, and to the
scientific acquirements of the age.”
This survey was to task to the full
Drummond's powers of invention. When the trial came he was well
prepared for it. From the moment when he joined the service he had
striven with all his powers to qualify himself for advancing its
interests. He, had thrown his heart into it, and eagerly cultivated all
those branches of knowledge which bore on its necessities. He betook
himself with renewed energy to the study of the higher mathematics. He
became devoted to chemistry. For years he used to rise at four or five
o'clock in the morning, light his own lamp and fire, and, taking a cup
of coffee, study without interruption till eight or nine, when official
duties claimed his attention. The days thus early begun were utilised
[p.66] throughout.
He had a special subject of study for every interval
of business, so that no portion of his time passed unimproved.
He had attended the chemistry classes in
the University of Edinburgh. In London he resumed the study under
Professors Brande and Faraday, whose morning lectures at the Royal
Institution he for some time sedulously attended. His friend Dr Prout,
an enthusiastic chemist, is supposed to have given him a bias towards
this study. Be that as it may, Drummond prosecuted it with zeal,
always on the alert in this field, as in every other, to make the
knowledge he acquired available to the service.
General Larcom states that the use of
lime for the Drummond light had
its origin in a suggestion received at the Royal Institution.
“The incandescence of lime having been spoken of in one of
the lectures,1 the idea
struck him that it could
be employed with
advantage as a substitute for Argand lamps in the reflectors used in
the Survey, for rendering visible the distant stations, because, in
addition to greater intensity, it afforded the advantage of
concentrating the light as nearly as possible into the focal point of
the parabolic mirror, by which the whole light would be available for
reflection in a pencil of parallel rays, whereas in the Argand lamp
only the small portion of rays near the focus was so reflected. On this
subject his first chemical experiments were performed. Captain Dawson
recollects Drummond mentioning the idea when returning from the
lecture, and that on the way he purchased a blow-pipe, charcoal,
&c. That evening he set to work with these simple means, and
resolved that he would hence-forth devote to his new pursuit the hour
or two immediately [p.67]
after dinner, when, he said, he could do
nothing else, remarking how much Dr Prout had done during the intervals
of active professional occupations."
We have Drummond's own more full account
in a paper “On the
Means of Facilitating the Observation of Distant Objects in Geodetical
Operations” published in the “Philosophical
Transactions” of 1826. It is prefaced by a brief sketch of
the history of the use of lights in survey operations.
“In the beginning of the
Survey, General Roy, on several
occasions, but especially in carrying his triangles across the Channel
to the French coast, made use of Bengal and white lights prepared at
the Royal Arsenal; for these, parabolic reflectors, similar to those
with which our lighthouses arc suplied, and illuminated by Argand
burners, were afterwards substituted as more convenient, but they have
been gradually discontinued, the advantages derived from them proving
inadequate, from their want of power, to the trouble and expense
incident to their employment. In the trigonometrical operations of
1821, carried on by Colonel Colby and Captain Kater, conjointly with
MM. Arago and Mathieu, for connecting the meridians of Greenwich and
Paris, an apparatus of a very different kind was employed for the
first time—a large plano-convex lens, 0.76 metre square,
being
substituted for a parabolic reflector, and the illuminating body an
Argand lamp with four concentric wicks. The lens was composed of a
series of concentric rings, reduced in thickness, and cemented together
at the edges. This apparatus resulted from an inquiry into the state of
the French lighthouses, and was prepared under the direction of MM.
Fresnel and Arago. Its construction and advantages are explained in a
‘Memoire sur un Nouveau Systême
d'élairage,’ by M. Fresnel.
The light which it gave is stated to possess 3¼ times the
intensity
of that given by the reflector. It was employed, during the operations
alluded to, at Fairlight Down and Folkestone Hill, on the English
coast; at Cape Bancez and Montlambert, on the French coast; the
greatest distance at which it was observed being 48 miles, and [p.68]
its appearance, I have understood from Colonel Colby, was very
brilliant.
“But valuable as this
apparatus may be when employed in a lighthouse, the purpose for which
it was indeed invented and constructed, the properties of the simple
parabolic reflector appeared to give it a preference for the service of
the Trigonometrical Survey, provided a more powerful light could be
substituted in its focus instead of the common Argand lamp.
“With this object in view, I
at first endeavoured to make use of the more brilliant pyrotechnical
preparations; then phosphorus burning in oxygen, with a contrivance to
carry off the fumes of phosphoric acid, were tried; but the first
attempts with these substances promising but little success, they were
abandoned. The flames, besides being difficult and troublesome to
regulate, were large and unsteady, little adapted to the nature of a
reflecting figure, which should obviously, when used to the utmost
advantage, be lighted by a luminous sphere, the size being regulated by
the spread required to be given to the light. This form of the focal
light, it was manifest, neither could be obtained nor preserved when
combustion was the source of light; and it was chiefly this
consideration which then led me to attempt applying to the purpose in
view the brilliant light emanating from several of the earths when
exposed to a high temperature; and at length I had the satisfaction of
having an apparatus completed, by which a light so intense was
produced, that when placed in the focus of a reflector, the eye could
with difficulty support its splendour even at the distance of forty
feet, the contour of the reflector being lost in the brilliancy of the
radiation.
“To obtain the requisite
temperature, I had recourse to the known effect of a stream of oxygen
directed through the flame of alcohol2 as a
source of heat free from danger, easily procured and regulated, and of
great intensity.
“To ascertain the relative
intensities of the different incandescent substances that might be
employed, they were referred, by the method of shadows, to an Argand
lamp of a common standard, the light from the brightest part of the
flame being [p.69]
transmitted through apertures equal in diameter to
the small sphere of the different substances submitted to experiment.
“The result of several trials
made at the commencement gave for
| Lime, |
... ... ... ...
... |
37 times, |
| Zirconia, |
... ... ... ...
... |
31 times |
| Magnesia, |
... ... ... ...
... |
16 times |
the
intensity of an Argand
burner. The oxide of zinc was also tried, but besides wasting away
rapidly, it proved inferior even to magnesia.
“Of these substances, and also
of their compounds with one another, lime appearing to possess a
decided superiority, my subsequent experiments were confined to it
alone; and by a more perfect adjustment of the apparatus, by bringing
the maximum heat, which is confined within narrow limits, exactly to
the surface of the ball, and by using smaller balls than those employed
in the early experiments, a very material increase of light has been
obtained. The mean of ten experiments, made lately with every
precaution, gives for the light emitted by lime, when exposed to this
intense heat, eighty-three times the intensity of the brightest part of
the flame of an Argand burner of the best construction and supplied
with the finest oil. The lime from chalk, and such as is known at the
London wharves by the name of flame lime, appears to be more brilliant
than any that has been tried.”
It thus appears that before he had
thought of any of the earths as a means of light, he had been
experimenting with various pryotechnical preparations and chemical
stuffs as lights to be used with the reflector, and had been perplexed
by the shape and unsteady character of the flames; in fact, bad his
attention particularly fixed on the very difficulty from which the use
of an earth offered escape. Experiment, in short, had brought him to
the point at which he was fully prepared to picture the lime ball in
the focus of the reflector the [p.70]
moment he should think of
it—or have it suggested to him—as a source of light.
The
use of the Argands by Colby and Kater in 1821, may have first
directed his attention to this subject. I think it is certain that he
was engaged in his experiment in the winter of 1823, and probable that
the fact was one of the reasons why Colby selected him to be his
right-hand man in his preparations for the Irish survey. The chief was
anxious that this survey should he more perfect than any yet executed,
and particularly desirous to obtain improved means of over-coming the
difficulties of observation which, it was anticipated, would be
encountered in Ireland, at once owing to the climate and the size of
the triangles. These means Drummond seemed to be likely to furnish; a
light he had ready to hand, and was working to fit it for use on the
survey; the other thing most wanted was a heliostat—a means
of continuously reflecting the sun's rays from one point to another.
And this Drummond supplied in the course of 1824 and spring of 1825.
In
the paper already cited, he gives a brief sketch of the history of
the use of the sun's reflection as an aid in survey
operations:—
“The
reflection
of the sun from a plane mirror, as affording a point of observation
that might be seen at remote distances, was suggested and employed by
Professor Gauss in 1822, while engaged with a trigonometrical
measurement in Hanover; and the result of the first trials made at
Inselberg and Hohenhagen rendered it highly probable that it might be
applied with much advantage to this purpose.
“The principle was adopted in this country when Colonel Colby
and Captain Kater were engaged, in 1822, in verifying General Roy's
triangulation connecting the meridians of Paris and Greenwich. At their
concluding station on Shooter’s [p.71]
Hill seven or eight
days had elapsed, during which Hanger Hill Tower, though only ten miles
distant, having remained completely obscured by the dense smoke of
London, tin plates were attached to the signal post, so as to reflect
the sun towards the station at stated times on a certain day.
“At the hours for which they had been calculated these plates
became visible, and the observations were in consequence immediately
and easily completed. In the subsequent operations of 1823, recourse
was again had on two important occasions to the same method, and with
equal success. I allude to Leith Hill, near Dorking, in Surrey, and
Wrotham Hill, in Kent, stations which it was of the utmost consequence
to observe from Berkhampstead Tower, near Hertford. Our efforts to
effect these observations having for some time been rendered unavailing
by the thick mist so frequently overhanging the bed of the Thames, a
series of bright tin plates was put up on both stations: Each set,
consisting of six or eight plates, was attached to a smooth flat board.
placed vertically by the plumb line, and turning on a pivot; the
respective inclinations of the plates with the face of the board being
determined, so that they might have the positions required for
reflecting in succession the sun's rays towards Berkhampstead Tower,
when the surface of the board was turned at right angles to the line of
direction. Although this method admitted but of rude execution, it
fully answered the purpose for which it was employed; the plates became
visible in succession at the appointed hours, the duration of each
varying with the inequality of its surface, but being generally from
ten to fifteen minutes; they were seen nearly at the same hours for
some days before and after that for which they were calculated.
“The distance to Leith Hill is forty-five miles, and the
observations were in this way completed without the hill itself having
been visible during the whole of our stay, which was nearly three
weeks.”
The utility of employing the sun's
reflection as a point of observation being established by the result
these experiments, the problem to which Drummond [p.72] addressed
himself was to invent an instrument, simple in its construction and
easy of management, which might be used on all occasions and at any
station. Colby's plates were a temporary expedient, suited only for the
particular place and time for which the angles of the plates were
calculated. The same pole and set of plates answered only for a single
station and a short time, owing to the rapid motion of the sun, or
rather of the earth in its orbit, quickly throwing the reflections wide
of the station of observation.
Drummond's solution of the problem was
as simple as it was happy and ingenious. His heliostat consisted of a
mirror connected with two telescopes; the one, forming the axis of the
instrument, for looking towards the station of observation, the other
for looking at the sun. The former telescope being turned on the
station, and brought, along with the mirror, to a. position of
horizontality by means of screws and spirit-levels, the mirror was so
connected with the latter telescope, that, as the telescope was turned
to the sun, the mirror, moving with it, went into position to reflect
the sun's rays to the station of observation. It was a problem of
geometrical construction prettily solved. He had done many as difficult
in the class of Professor Leslie; but this one was of his own
proposing, and its solution proved of the highest practical importance.
He made improvements on the instrument afterwards, which we shall
notice in the proper placw. The heliostat and the apparatus for
exhibiting the light in survey operations, are fully described in the
paper in the “Philosophical Transactions.”
By these two inventions Drummond armed the Survey officers with the
most powerful means of overcoming the difficulties of observation by
day and night. The [p.73]
light was not long till it made a sensation
beyond the small circle of the corps; in the scientific world its
splendour and utility were at once acknowledged. “It is with
a melancholy pleasure,” says Sir John Herschel,3
“that I recall the impression produced by the view of this
magnificent spectacle as exhibited (previous to its trial in the field)
in the vast armoury in the Tower, an apartment 300 feet long, placed at
Mr Drummond's disposal for the occasion. . . . The common Argand burner
and parabolic reflector of a British lighthouse were first exhibited,
the room being darkened, and with considerable effect. Fresnel's superb
lamp was next disclosed, at whose superior effect the other seemed to
dwindle, and showed in a manner quite subordinate. But when the gas
began to play, the lime being now brought to its full ignition and the
screen suddenly removed, a glare shone forth, overpowering, and, as it
were, annihilating both its predecessors, which appeared by its side,
the one as a feeble gleam, which it required attention to see, the
other like a mere plate of heated metal. A shout of triumph and
admiration burst from all present. Prisms to analyse the rays,
photometric contrivances to measure their intensity, and screens to
cast shadows, were speedily in requisition, and the scene was one of
extraordinary excitement.” It must have been a proud moment
for the inventor when he witnessed this enthusiasm in the elite of
London scientific society. But there was a prouder in reserve for
him—the triumph of his light and heliostat over obstacles to
observation,
which for months had impeded the progress of the Survey.
The first step in the Irish Survey was a “general [p.74]
reconnaissance” of Ireland made by Colby and Drummond in the
autumn of
1824. They traversed the country from north to south, fixed upon the
mode of conducting the survey, and selected the stations for the great
triangulation, as well as the most fitting place for measuring a base.
Enough has been seen of Colby's manner of making a reconnaissance of a
country, to enable the reader to judge what exposure and fatigue were
suffered in this expedition.
In the autumn of 1825 the
triangulation commenced on the Divis mountain, near Belfast. Officers
were sent into Cumberland and the Isle of Man to recover and erect
marks on the old stations there as points of observation from Divis for
the connection of the Irish and English Surveys. While they were on
this duty, Drummond, with his heliostat, light apparatus, and a
complete observatory of meteorological and other philosophical
instruments, was encamped on Divis preparing the station for the great
theodolite. On the 23rd of August a conspicuous object was placed on
the summit of Slieve Snaght, the highest hill of Innishowen, about 2100
feet above the sea. This was an important point in the triangulation
which connects the north of Ireland with the western islands of
Scotland, and it was necessary that it should be observed from Divis.
The party of observers on Divis were in camp as early as the date of
marking Slieve Snaght. They continued there till the 26th October
without being able to make the necessary observation. The mountain
continued enveloped in a haze so impenetrable as to render unavailing
every effort made for the purpose. All this time it seems not to have
occurred to put either the heliostat or light in use. But now, late as
it was in the Season. Colonel Colby determined that Drummond [p.75]
should ascend the mountain, and attempt to overcome the formidable
obstacle to the completion of the observations by the aid of his
apparatus. He did so, and with what success we may see in the narrative
of General Larcom:— “Mr. Drummond took the lamp and
a small party of
men to Slieve Snaght, find by calculation succeeded so well in
directing the axis of the reflector to the instrument on Divis, that
the light was seen, and its first appearance will long be remembered by
those who witnessed it. The night was dark and cloudless, the mountain
and the camp were covered with snow, and a cold wind made the duty of
observing no enviable task. The light was to be exhibited at a given
hour, and to guide the observer, one of the lamps formerly used, an
Argand in a lighthouse reflector, was placed on the tower of
Randalstown Church, which happened to be nearly in the line at fifteen
miles. The time approached, and passed, and the observer had quitted
the telescope, when the sentry cried, ‘The light!’
and the light indeed
burst into view, a steady blaze of surpassing splendour, which
completely effaced the much nearer guiding beacon. It is needless to
add that the observations were satisfactorily completed, the labours of
a protracted season closed triumphantly for Drummond, and the Survey
remained possessed of a new and useful power.”
This year's
operations did grievous mischief to Mr Drummond's health. A residence
on the top of Slieve Snaght at such a season of the year might have
tried one more robust. He was borne up for the time by the excitement
attending the first practical test of his inventions, only to suffer
the more severely afterwards. With what joy he hailed the success of
his operations on the lonely mountain we fortunately may see from a
[p.76] letter
written in his hut on the night of November 11, 1825. In
the moment of success his first thought was to communicate the pleasure
of it to his beloved mother:—
“SLIEVE
SNAGHT,
“Friday night,
Nov. 11, 1825.
“M
Y
DEAR MOTHER,—What
has become of Tom? and why does he not write? are questions which you
may of late have not unfrequently asked, and, I dare say, without
anyone being able to give a very satisfactory answer. Why, then, I am
perched upon the top of Slieve Snaght (the Snowy Mountain), 2100 feet
high in the centre of Innishowen, the wildest district in Ireland.
Since the 23d of August, when a pole was placed on this hill, we have
endeavoured to observe it from Divis, near Belfast, on which our tent
was placed, but in vain. Constituting an important point in the
triangulation of Ireland, our sojourning on the hilltops has been
prolonged to an unusually late period, in the daily hope that it would
have been visible. Disappointment, however, was our lot, and the
weather becoming broken and tempestuous, the colonel determined upon
breaking up the camp and retiring to winter quarters. Just at that
moment a letter was received from one of our officers encamped on Knock
Layd, a hill about 40 miles distant, giving a splendid description of
the solar reflection which I had exhibited to him, and which had been
seen through a very hazy atmosphere, and seen for a time with the naked
eye; and one of our officers tells me that the country people, whom
curiosity had attracted to the spot on hearing the distance at which it
was placed, actually raised a shout of exultation at its brilliant
appearance. This being known at Divis, it became a question whether
Slieve Snaght should be attempted at this season; and after due
deliberation, it being decided that it should. I made a forced march
upon this place, and, leaving Belfast on Tuesday forenoon, slept on
this mountain on Thursday night, the 27th October, our tents erected,
and hut constructed, and all the apparatus of the lamp ready for work.
For the first week our life was a struggle with tempest—our
[p.77]
tents blown down, our instruments narrowly escaping, and ourselves
nearly exhausted At length, by great exertions, we got two huts
erected, one for the seven men who are with me, the other for
me—a
lonely and humble dwelling, it is true, and now that the snow has
fallen, so completely covered up that it is not very easily
distinguished; nevertheless affording good shelter, warm and
comfortable, and, at the present moment,
with a good peat fire.
The weather at length improved, and Wednesday the 9th instant brought
our exertions to a successful termination. The colonel, after making
the necessary arrangements, took his departure for London on the very
day I arrived here, leaving Murphy and Henderson to keep a constant
look-out for the lights. Their assiduity has been unremitting, and
their fatigue by incessant watching not a little. This day brought me a
letter from Murphy, which begins thus: ‘Your light has been
most
brilliant to-night for three hours and twenty minutes, as was your
solar reflection to-day. I began by giving you the pleasing
intelligence in a condensed form, but now I must most heartily
congratulate you, my dear friend, on the complete success which has
thus crowned your very ingenious and laborious exertions for the good
of the service. I trust they may eventually prove as beneficial to
yourself. I really feel sincere pleasure in making you this
communication. I will now give you some details. I first had notice of
your appearance from Elliot, who called out that he saw the light, and
in fact, though five times more remote, you were much brighter and
larger than the Randal's tower reflector.’ I have given you a
long
extract, because I think it will interest and please you. I have only
to inform you now that the distance in n straight line between the two
places is about 67 miles. I had a letter from the colonel to-day in
London, very anxious to know the result of our labours. To-morrow I
commence my retreat; on Monday I shall be in Derry, where I shall have
to remain a day. . . . . From Derry I proceed to Belfast, where I shall
be detained two or three days, and then I make direct for Edinburgh. At
Belfast I entreat you to let me hear from you, and I am anxious to hear
how Eliza bore the journey from Callendar, and how
[p.78] the house is.
My last intelligence is her own letter, which I received about the 19th
ultimo, on the evening succeeding a gale of wind, which overthrow two
of our marquees, and set fire to our cooking-house. I have written you,
my dear mother, a long and gossipping letter, and it being now three
o'clock in the morning, it is fit that I should stop. To John and Eliza
my kindest love, and to Eliza my best thanks for her kind letter. It
may amuse my aunt to read this letter to her, and tell her that I add
my best regards.—And now, my dear mother, believe me your
affectionate
son,
“T.
DRUMMOND.”
“The
triumphant success which attended the lamp and heliostat at the close
of 1825, was purchased at the cost of a severe illness. A mountain
camp, at an altitude of two thousand feet in the winter of these
climates, is, under any circumstances, a severe trial; but Drummond and
his little party were peculiarly exposed. Few in number—being
merely
detached from the general camp at Divis—they were ill able to
buffet
with the storms of these wild regions; and the tents were so frequently
blown down, that after the first few days they abandoned them, and
constructed huts of rough stones, filling the interstices with turf.
Such, without the additional luxury of a marquee lining, was the study
and laboratory on which depended the success of the new instruments.
Here were to be forfeited the delicate manipulations their adjustment
required. Here was to be manufactured the oxygen destined for the
portable gasometer; and, cowering over the fire, or wrapped in a pilot
coat, was Drummond day and night at work. A frame and system attenuated
by fatigue and excitement were ill able to bear up against such
exposure. He struggled to the last; but no sooner had his efforts been
crowned with success than he sank, and a severe illness compelled him
to return to Edinburgh, to the care of his family and
friends.”
Footnotes
1. Memoir, p.5.
2. Annals of Philosophy, vol. ii. p. 99.
3. Letter to Mrs Drummond.
Extract from Memoir of Thomas Drummond,
by
John Ferguson M'Lennan (1867), part of Chapter VII, pages
64-78.
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