Complete Works of Robert Louis Stevenson (Illustrated) (876 page)

Dully against the lantern;

Yet he stirs not, lifts not his head

From the desk where he reads,

Lifts not his eyes to see

The chill blind circle of night

Watching him through the panes.This is his country’s guardian,

The outmost sentry of peace.

This is the man,

Who gives up all that is lovely in living

For the means to live.

Poetry cunningly gilds

The life of the Light-Keeper,

Held on high in the blackness

In the burning kernel of night.

The seaman sees and blesses him;

The Poet, deep in a sonnet,

Numbers his inky fingers

Fitly to praise him:

Only we behold him,

Sitting, patient and stolid,

Martyr to a salary.

1870.

ON A NEW FORM OF INTERMITTENT LIGHT FOR LIGHTHOUSES

The necessity for marked characteristics in coast illumination increases with the number of lights. The late Mr. Robert Stevenson, my grandfather, contributed two distinctions, which he called respectively the
intermittent
and the
flashing
light. It is only to the former of these that I have to refer in the present paper. The intermittent light was first introduced at Tarbetness in 1830, and is already in use at eight stations on the coasts of the United Kingdom. As constructed originally, it was an arrangement by which a fixed light was alternately eclipsed and revealed. These recurrent occultations and revelations produce an effect totally different from that of the revolving light, which comes gradually into its full strength, and as gradually fades away. The changes in the intermittent, on the other hand, are immediate; a certain duration of darkness is followed at once and without the least gradation by a certain period of light. The arrangement employed by my grandfather to effect this object consisted of two opaque cylindric shades or extinguishers, one of which descended from the roof, while the other ascended from below to meet it, at a fixed interval. The light was thus entirely intercepted.

At a later period, at the harbour light of Troon, Mr. Wilson, C.E., produced an intermittent light by the use of gas, which leaves little to be desired, and which is still in 221 use at Troon harbour. By a simple mechanical contrivance, the gas jet was suddenly lowered to the point of extinction, and, after a set period, as suddenly raised again. The chief superiority of this form of intermittent light is economy in the consumption of the gas. In the original design, of course, the oil continues uselessly to illuminate the interior of the screens during the period of occultation.

Mr. Wilson’s arrangement has been lately resuscitated by Mr. Wigham of Dublin, in connection with his new gas-burner.

Gas, however, is inapplicable to many situations; and it has occurred to me that the desired result might be effected with strict economy with oil lights, in the following manner: —

Fig. 1.

In Fig. 1, AAA represents in plan an ordinary Fresnel’s dioptric fixed light apparatus, and BB’ a hemispherical mirror (either metallic or dioptric on my father’s principle) which is made to revolve with uniform speed about the burner. This mirror, it is obvious, intercepts the rays of one hemisphere, and, returning them through the flame (less loss by absorption, etc.), spreads them equally over the other. In this way 180° of light pass regularly the eye of the seaman; and are followed at once by 180° of darkness. As the hemispherical mirror begins to open, the observer receives the full light, since the whole lit 222 hemisphere is illuminated with strict equality; and as it closes again, he passes into darkness.

Other characteristics can be produced by different modifications of the above. In Fig. 2 the original hemispherical mirror is shown broken up into three different sectors, BB´, CC´, and DD´; so that with the same velocity of revolution the periods of light and darkness will be produced in quicker succession. In this figure (Fig. 2) the three sectors have been shown as subtending equal angles, but if one of them were increased in size and the other two diminished (as in Fig. 3), we should have one long steady illumination and two short flashes at each revolution. Again, the number of sectors may be increased; and by varying both their number and their relative size, a number of additional characteristics are attainable.

Fig. 2.

Colour may also be introduced as a means of distinction. Coloured glass may be set in the alternate spaces; but it is necessary to remark that these coloured sectors will be inferior in power to those which remain white. This objection is, however, obviated to a large extent (especially where the dioptric spherical mirror is used) by such an arrangement as is shown in Fig. 4; where the two sectors, WW, are left unassisted, while the two with the red 223 screens are reinforced respectively by the two sectors of mirror, MM.

Fig. 3.

Fig. 4.

Another mode of holophotally producing the intermittent light has been suggested by my father, and is shown in Fig. 5. It consists of alternate and opposite sectors of dioptric spherical mirror, MM, and of Fresnel’s fixed light apparatus, AA. By the revolution of this composite frame about the burner, the same immediate alternation of light and darkness is produced, the first when the front of the 224 fixed panel, and the second when the back of the mirror, is presented to the eye of the sailor.

Fig. 5.

One advantage of the method that I propose is this, that while we are able to produce a plain intermittent light; an intermittent light of variable period, ranging from a brief flash to a steady illumination of half the revolution; and finally, a light combining the immediate occultation of the intermittent with combination and change of colour, we can yet preserve comparative lightness in the revolving parts, and consequent economy in the driving machinery. It must, however, be noticed, that none of these last methods are applicable to cases where more than one radiant is employed: for these cases, either my grandfather’s or Mr. Wilson’s contrivance must be resorted to.

1871.

 Read before the Royal Scottish Society of Arts on 27th March 1871, and awarded the Society’s Silver Medal.

ON THE THERMAL INFLUENCE OF FORESTS

The opportunity of an experiment on a comparatively large scale, and under conditions of comparative isolation, can occur but rarely in such a science as Meteorology. Hence Mr. Milne Home’s proposal for the plantation of Malta seemed to offer an exceptional opportunity for progress. Many of the conditions are favourable to the simplicity of the result; and it seemed natural that, if a searching and systematic series of observations were to be immediately set afoot, and continued during the course of the plantation and the growth of the wood, some light would be thrown on the still doubtful question of the climatic influence of forests.

Mr. Milne Home expects, as I gather, a threefold result: — 1st, an increased and better regulated supply of available water; 2nd, an increased rainfall; and, 3rd, a more equable climate, with more temperate summer heat and winter cold. As to the first of these expectations, I suppose there can be no doubt that it is justified by facts; but it may not be unnecessary to guard against any confusion of the first with the second. Not only does the presence of growing timber increase and regulate the supply of running and spring water independently of any change in the amount of rainfall, but as Boussingault found at Marmato, denudation of forest is sufficient to decrease that supply, even when the rainfall has increased instead 226 of diminished in amount. The second and third effects stand apart, therefore, from any question as to the utility of Mr. Milne Home’s important proposal; they are both, perhaps, worthy of discussion at the present time, but I wish to confine myself in the present paper to the examination of the third alone.

A wood, then, may be regarded either as a
superficies
or as a
solid
; that is, either as a part of the earth’s surface slightly elevated above the rest, or as a diffused and heterogeneous body displacing a certain portion of free and mobile atmosphere. It is primarily in the first character that it attracts our attention, as a radiating and absorbing surface, exposed to the sun and the currents of the air; such that, if we imagine a plateau of meadow-land or bare earth raised to the mean level of the forest’s exposed leaf-surface, we shall have an agent entirely similar in kind, although perhaps widely differing in the amount of action. Now, by comparing a tract of wood with such a plateau as we have just supposed, we shall arrive at a clear idea of the specialities of the former. In the first place, then, the mass of foliage may be expected to increase the radiating power of each tree. The upper leaves radiate freely towards the stars and the cold inter-stellar spaces, while the lower ones radiate to those above and receive less heat in return; consequently, during the absence of the sun, each tree cools gradually downward from top to bottom. Hence we must take into account not merely the area of leaf-surface actually exposed to the sky, but, to a greater or less extent, the surface of every leaf in the whole tree or the whole wood. This is evidently a point in which the action of the forest may be expected to differ from that of the meadow or naked earth; for though, of course, inferior strata tend to a certain extent to follow somewhat the same course as the mass of inferior leaves, they do so to a less degree — conduction, and the conduction of a very slow conductor, being substituted for radiation.