The Victorian Omnium-Gatherum
March 24. R, Stephenson, Esq., M.P., in the chair.
The following papers were read:—' On the Meteorological and Physical Observations taken on the 15th of March, the day of the Solar Eclipse,' by J. Glaisher, Esq.
Mr. Glaisher writes as follows:—In anticipation of the eclipse, the Council forwarded a list of suggestions to the members of the Society, concerning the observations it would be desirable to record in connexion with its influence upon Meteorology, and its effects upon the animal and vegetable world, together with such collateral subjects for observation as might present themselves at the time to the notice of the observer.
Since the eclipse returns had been received from thirty or forty stations situate between Braemar and Guernsey, which had been divided into three groups: the first consisting of those stations north, the second of those on and near, and the third of those south of the line of annularity. By tables and diagrams, it is shown that the depression of temperature during the eclipse was about 2 1/2° at stations north of the line, and nearly 6° at stations on and south of the line of central eclipse. That at places where the usual diurnal increase had taken place in the morning, the depression of temperature during the eclipse was greater, and that at places where such increase had not taken place it was less than the above numbers. Also at places where the sky was uniformly overcast, the decrease in the readings of a blackened bulb thermometer was less than 12°, whilst at places where the sky was partially clear, the depression was from 17° to 19°, and that what temperature soever the blackened-bulb thermometer indicated in the morning, fell during the eclipse to that of the air at all places. The humidity of the air was such that at places north of the line the wet-bulb thermometer read 2.6° less; on and near the line the depression was 3.2°, and south of it was 3.7° below the adjacent dry-bulb thermometer. At some places the humidity of the air increased at the time of the greatest eclipse, but this was far from being universal.
The sky was partially clear at some places on the east and south coasts, in the Channel islands, and north of Scotland, and it was for the most part overcast elsewhere. Near the southern extremity of the central line, the sky was partially clear; and at its northern limit, near Peterborough, the clouds were broken; at most intermediate places the sky was wholly overcast. The complete ring was seen at Charmouth and neighbourhood, near Lyme Regis, and at Peterborough, but, so far as I can learn, at no other places. My own station was on the calculated line of central eclipse, near Oundle, in Northamptonshire; and here I saw the moon's and sun's apparent upper limbs coincident, or very nearly so, and therefore concluded that I was situated on or very near the northern limit of annularity, and distant from the central line by three miles.
Of the numerous and remarkable phenomena attendant upon large solar eclipses, only a few were witnessed at any part of the country, or, we might say with greater justice, were indicated. Independently of the particular character of the day, the time of year was against the favourable witnessing of all that class of effects which relate to the fluctuations of light, the form of shadows, the changing colour of the increasing gloom, and other kindred observations. For the same reason the intensity of the gloom was difficult to estimate, and when at its maximum was much less appreciable than it would have been had the sky been clear and the decrease of light been effected by the diminishing of the direct beams from the sun as the obscuration proceeded, in which case it is likely that the gloom would have been at once estimated, arising from the mere force of contrast; thus we all know that in summer time the approach of a storm or dark cloud, sufficient to create a gloom, which should render it difficult to pursue any occupation, however trifling, would be a matter of note, and would command our immediate attention, whilst in the ordinary dull days of winter we suspend, when obliged to do so, various occupations by reason of want of light, with scarcely a passing consideration of the cause.
The general feeling of disappointment as to the depth of gloom at the time of the greatest obscuration is in part attributable to this fact and to the clouds which interposed like a screen to diffuse by reflection some portion of the direct rays absorbed by them over the surface of the earth. Of the information forwarded to me by members of the Society regarding general appearances at the time of the eclipse, there is a striking degree of accordance, which is very confirmatory of the truthfulness of the impressions recorded, and we all know how difficult it is to obtain simple and accurate testimony relating to facts which are chiefly dependent upon personal experiences and sensations.
My own impressions recorded at the time I have since found to be accordant with those of other observers, making allowance for the more or less departure from the central line, and the different states of the weather at the several places. For the sake of estimating general effects as accurately as possible and to the widest extent, I took up my own position to command an uninterrupted view of the adjacent country, and an extensive horizon. In the position chosen, which combined the required advantages, I may add there was nothing to stimulate the imagination or aid the effects likely to be resultant from the approaching phenomena. At the distance of about a mile before me due north was the spire of Oundle Church, and beyond it and on either side were fields and pasture lands, bounded with hedge-rows and tall elms, with little undulation, as far as the eye could reach. The weather for some time previous to the commencement was raw and very ungenial, even for the time of year, and communicated a dreary aspect to the country; in some places beside the hedge-rows on one side lay a thin line of snow, the remains of a recent fall, and near the place where we were stationed a drift still lingered to the depth of a foot. The wind was gusty, and the sky overcast chiefly with cirro-stratus, and dark scud hurrying past before the sun's place from the northwest, the clouds occasionally giving way and allowing the sun to be dimly seen through momentary breaks. At intervals they became less dense, and for a time gave promise of permitting a clear and uninterrupted set of observations. The sun, however, continued visible only by snatches, until after the greatest phase of the eclipse, when the sky became uniformly overcast, and a small steady rain set in for a considerable time. It was long before I could perceive any departure from the usual amount of light.
At 12h. 32m. it might have been an ordinary dull day, birds were then no way affected and were singing cheerily. I estimated about two-thirds of the sun to be obscured. At 12h. 39m. there was a very perceptible gloom to the north, and the sun's crescent shone out with a bright silvery light between breaks clearer than it had been before. At 12h. 43m. the gloom was general, excepting around the sun, which appeared the centre of a circle of light, and illuminated with fine effect some bold irregular masses of cumulus in its vicinity. At 12h. 45m. the gloom increased, slight rain fell, and the wind rose higher. Birds were now heard chirping and calling. At 12h. 53m. a severe storm might have been supposed impending, and numerous birds were flying homewards; the deepening of the gloom was gradual but very slow, and between 1h. 0m. and 1h. 1m. was at its greatest intensity; but even at this time the obscurity was not sufficient to require that any employment should be suspended. I myself, situated in an open garden at the time, found no difficulty in reading ordinary type at any ordinary distance. Messrs. Adams and Symons, situated 5 feet from a shed in a brickfield adjoining, spoke of the gloom as very intense for a period of 10 seconds, and sufficient to render it difficult to take the readings of the thermometer.
At the time of greatest obscurity a body of rooks rose from the ground and flew homewards, also a flock of starlings rose together, and various small birds flew wildly about. A hare was seen to run swiftly across a neighbouring field, as though it were daybreak. Straw rustled, and the silence was peculiar and intense, broken only by the hollow sound of the wind as it whirled in gusts between the trees; the darkness and intervening lull was that of an approaching thunder-storm. The sky was overcast in the neighbourhood of the sun principally with cirro-stratus. Directly after the greatest intensity, the gloom was sensibly and instantaneously diminished, and in a very short time the day was restored to its ordinary appearance.
Probably in consequence of the sky being overcast, I could perceive no flickering or unsteadiness of light, and none of the effects of colour described as attendant upon previous eclipses. The clouded sky, excepting that part near the sun, was one leaden grey or slate colour, and quite in accordance with the raw ungenial character of the day, nor could I perceive that the clouds appeared lower, or in fact that there was any very noticeable departure from the gloom we usually experience during dull winter weather. After 12h. 50m. the lark ceased to rise and did not sing; at 1h. 10m. it rose again and was heard. The information I have been able to collect indicates that birds and animals, but particularly the former, were affected in some degree at most places, and that it is probable to suppose the gloom was referred by them to the approach of evening, and this not so much from the simple fact of the gloom as from the manner of its approach, without the accompanying signs of atmospheric disturbance which usher in a storm, and to which all birds and many animals are keenly sensitive.
Throughout the time of the eclipse it occurred to me, apart from the causes before mentioned, that the degree of direct illumination was far more than commensurate with the amount of sun's disc illuminated. Even up to the time of the greatest phase, when clear, the unobscured part of the disc emitted direct and divergent beams of considerable brilliancy, which marked out a luminous tract in the surrounding gloom, and were clearly and well defined in extent and figure. As the eclipse proceeded the illuminated crescent of the sun assumed a pure silvery brightness like that of Venus after inferior conjunction with the sun. The absence of all yellow in its brightness was remarkable, and at the time when the annulus was nearly formed it appeared like a line of silver wire.
The clearing up of the gloom directly after its greatest intensity, and almost immediate return to the general effect of an ordinary dull day, was very marked, and could not fail to be observed by every one. After the reversal of the cusps, it became impossible to determine whether the light received was communicated by a fraction of the sun's disc behind a thin layer of cloud or by the sun's perfect disc behind a heavy amount of cloud. From this time all special effects were lost in the neighbourhood of our place of observation, alike to the astronomer and the general observer. When, only a small fraction of the disc was visible, the departure from the amount of light we are accustomed to receive on an average dull day in winter was so inconsiderable that we might infer very approximately the real amount of sunlight our average daylight under a cloudy sky is equivalent to. From all that I have been able to collect, I think it reasonable to infer that the great paucity of effects and special phenomena in relation to the occurrence, not excepting places where the sun was visible, is due to atmospheric conditions, alike attributable to climate, time of year, and unfavourable state of the weather, and does not tend to lessen our confidence in previous descriptions of the grandeur and beauty of the attendant phenomena upon large eclipses. Optical phenomena, we all know, must be dependent upon the medium through which we view them for the nature and power of the effects produced.
As a test of the gradual decrease in intensity of the sunlight, strips of photographic paper were exposed for equal intervals of time every five minutes. The result was a scale of tints which exhibited very clearly the progress towards the time of greatest obscuration and the period of its actual occurrence, the paper a little time previous to the greatest intensity ceasing to colour in any appreciable degree. The range of tints is low, owing to the cloudy state of the sky, but this disadvantage does not interfere with the proportionate depths of tint. Early in the morning powder was fired by a 3-inch lens in four seconds, and once subsequently in seven minutes. As the weather became more unfavourable and the eclipse proceeded, it ceased to fire at any time within a minute.
The effect of the unwonted darkness upon birds would seem to have been general: mention is made all over the country of the return of rooks to their rookery; starlings were seen in many places taking flight, whole flocks of them together; near Oxford, Dr. Collingwood remarked that a thrush commenced its evening song. At Grantham, pigeons returned to their cote during the time of the greatest obscuration. At Ventnor, Dr. Martin notes the fact that a fish confined in an aquarium, and ordinarily visible at evening only, was in full activity about the time of the greatest gloom. In Greenwich Park the birds were hushed, and flew low from bush to bush, and at most places during the darkness the song of many birds was stilled. At Campden Hill it was observed that the crocus closed about the same time, and at Teignmouth that its colour changed to that of the pink hepatica, a fact, writes Charles Lake, Esq., "which I did not notice myself, probably because I did not look for it, but which I have since heard confirmed by others."
The darkness was not sufficient at any place to prevent moderate-sized print being read at any convenient distance from the eye out of doors, but difficulty was sometimes experienced in reading the instruments. At Grantham the darkness is described to have been about equal to the usual amount of light an hour before sunrise; near Oxford, as about equal to that just after sunset on a cloudy day. The general impression communicated was that of an approaching thunder-storm. The sudden clearing up of the gloom directly after the greatest phase, was likened by more than one observer to the gradual but somewhat rapid withdrawal of a curtain from the window of a darkened room. The darkness is described to have been generally attended by a sensation of chilliness and moisture in the air.
Published on 3 April 1858 in The Athenaeum Journal of Literature, Science and the Fine Arts.
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