On the Grossglockner also, by observing the zenith and the lowest visible point of the horizon, we found a striking coincidence with the law above mentioned; we there found— But between these limits the increase was not quite so regular. Thus we found at a zenith distance of 50°, 70 per cent. of cobalt, whereas the calculation gives only 59; an error; however, which at this altitude scarcely exceeds that due to a single degree of Saussure's cyanometer. When the firmament is observed from deep valleys, the lateral intensity of the blue colour is very irregular. In this case local vapours amass themselves, and cause the observed depth of hue to be much less than that obtained from calculation. Pursuing the method already described, we have also attempted to ascertain the quantity of yellow and reddish colour (ochre) which enters into the composition of the sky. The quantity of the latter naturally depends on the colour of the cobalt and white, as these themselves are not absolutely pure colours. This combination of more than simple blue and white has also the advantage, that by it we are enabled to determine the brightness itself with greater certainty. The coincidence of shade between the actual and the artificial colours greatly facilitates the comparison of both. No. Table of Observations with the Tricoloured Cyanometer (No. II.). The foregoing table shows, that for different points of the same vertical circle the decrease of the blue is accompanied by a decided increase of the ochre. The ochre appeared strongest at the horizon of the Grossglockner. The atmosphere in this portion had therefore a slight tint of green*, which resulted from the blending of the three colours, blue, yellow and white. White objects seen from a distance have always a yellowish or reddish tint imparted to them by the atmosphere. This is plainly observable on clouds, houses, snow-covered slopes, &c. It is a general rule, that, in the painting of such objects, a little ochre must be added to the white. The colour of the brightest cloudmasses, even when the sun is in a high position, contains generally from 1 to 2 per cent. of ochret. Distant mountains often appear blue when the sun is opposite; their own colour seems to have some influence in this case, as the same mountains in winter when covered with snow show a reddish-white colour‡. Those summits of the Alps which are covered with perpetual snow, when seen from a great distance in direct sunlight, exhibit this reddish tinge blended with the whiteness. The colouring of the light by its transmission through the atmosphere is peculiarly remarkable in the hues exhibited by the sky at daydawn and at sunset-the morning and the evening red. Forbes was the first to connect these beautiful colours with the existence of watery vapour in a certain state of condensation. The phænomenon of the morning and evening red is of too intense and changeable a nature to be investigated by means of our cyanometer. The evening glow of the Alps is peculiarly well known as a splendid exhibition of the evening red. It begins soon after sunset; the precipices and snow-crowned summits assume a dazzling glow, which disappears almost instantly when the shadow of the earth has attained the heights. A second glow is often observed, particularly in the more southern alpine groups; on Mont Blanc, Monte Rosa, &c. it is exhibited * A strong green colour (grass- or bottle-green) may sometimes be observed, at considerable elevations above the horizon, on clouds and nountain peaks when glowing with intense red. This has been often observed by Brandes and others. The phænomenon is merely a subjective colouring, occasioned by the wearying of the eye in gazing on the shining red. The complementary green is observed more frequently and plainly when the eye is directed, not on the firmament, but upon white objects. The clouds sometimes exhibit a very dark hue-thunder-clouds, for example. Sometimes the very finest of them cause important alterations in the colour of the heavens, without being recognizable as distinct groups. Humboldt has also observed such masses.- Voyages, vol. iii. p. 318. 4to. Compare also Saussure's Voyages, vol. iv. § 2088, note 1. "On the colour of Steam under certain circumstances."-Philosophical Magazine, vol. xiv. pp. 121, 419; and Pogg. Ann., vol. xlvii. p. 593; and supplementary volume, vol. i. 1842, p. 49. The last memoir contains an extensive collection of the earlier notions entertained upon this subject. in great splendour. On the masses of dolomite in the Fassathal we observed the same twice. A related phænomenon, which we had the opportunity of observing, deserves to be mentioned here. From the crest of the Wildspitze, on the 18th of September 1847, we had a fine prospect towards the north. The entire series of the northern limestone alps, from Salzburg to the Bodensee, was unfolded before us with extraordinary clearness. In the mean time a storm blowing towards the north increased in violence, and before we attained the summit (11,489 P. F.) the northern mountains exhibited an extraordinary colour. They had obtained a decidedly red tone, although the sun stood high, it being but 3 o'clock in the afternoon. We had left the summit scarcely half an hour, when immense cloud-masses were driven upon us from the side at which the red colouring had been observed. During this time the barometer fell considerably. It seemed as if the watery vapour of the atmosphere, during its gradual condensation to mist, had occasioned the redness in the same manner as the morning and the evening red is produced. For the observation of this phanomenon, it is first of all necessary that large masses of air should lie between the observer and the object; in the present case the distance amounted to eleven or twelve miles (German). It is only from a high position that objects distant enough, and with surfaces large enough to exhibit the modification of colour, can be observed. Opportunities to see the phænomenon occur but rarely, as it is but seldom that the observer finds himself at such elevations during similar states of the weather. The colour was not the shining red of the evening, but more of a purplishblue tinge, undimmed by fog of any kind, and in the production of which the gray colouring of the limestone masses had a share. Direct sunlight, when it passes through mist, has also a red tone imparted to it; but the colours of rocks, &c. being the products of reflected light, disappear long before the red tone can be assumed. We have in some cases endeavoured to determine the intensity of the red which occurs on the passage of the light through fog. Colour of Fogs with transmitted Light. The red colour in the present instance appeared sometimes more intense than is generally observed on plains. When the light falls upon the fog, it exhibits the usual uniform gray, similar to a mixture of 91 white with 9 per cent. black. A few words now remain to be said upon the duration of the twilight. It is everywhere known in the Alps, that on high mountains the duration is longer, although this is sometimes over-estimated. It may be almost regarded as a tradition repeated for every mountain, even when it is but a few thousand feet high, that the evening and the morning twilight touch each other at midnight. Though this is an exaggeration, a difference in the duration of the twilight is very appreciable in the higher regions. As the horizon expands from an Alpine summit, it is evident that the higher we ascend the greater will be the arch which separates sunrise from sunset, and hence the longer the day. In valleys, on the contrary, it often occurs that the direct sunlight is held back by interposed mountains, and hence is present only a few hours of the day. The feeble twilight is also considerably diminished by the same cause; thus the position of valleys with regard to the horizon may be such, that night sets in very soon after the setting of the sun. The twilight, in our latitude, continues on an average upon the plains until the sun has descended 18° under the horizon. Upon mountains the sun attains a much greater depth before the twilight departs*. It is difficult to express this with exactness, as the alterations in the transparency of the atmosphere on different days exercise so considerable an influence. XV. On a Method of obtaining a perfect Vacuum in the Receiver of an Air-pump. By THOMAS ANDREWS, M.D., F.R.S., M.R.I.A.† THE space left vacant in the upper part of a long glass tube, which after being filled with mercury is inverted in a basin of the same metal, affords the nearest approach to a perfect vacuum which has hitherto been obtained. It is true that it contains a little mercurial vapour at the ordinary temperature of our summers, and probably also at lower temperatures; but the quantity is exceedingly small, and its influence in depressing the barometric column must be altogether inappreciable. Besides the mercurial vapour, a trace of air may generally be detected even in tubes which have been carefully filled, and in which the air interposed between the glass and mercury has been expelled * Compare also Martin's Monit. Univers 1844, p. 2796; and Kæmtz, Lehrbuch de Meteorol., vol. iii. p. 50 and following. + Communicated by the Author. by ebullition. This is best observed by inclining the tube till the mercury comes into contact with the upper end, when any air that may have been diffused through the vacuum will be seen collected in a small bubble, but greatly rarefied. It is easy to calculate approximately the depression of the mercurial column produced by this residual air. For this purpose the tube must be inclined till the bubble is exposed to a pressure of a few inches of mercury, measured in a vertical direction. In this position its apparent diameter is measured, as also the pressure to which it is exposed. For the object in view, the volume of the bubble may be calculated on the assumption that it is a sphere. The space occupied by the vacuum must also be estimated; and with these data, the depression of the mercurial column may easily be calculated. Let V be the volume of the space above the mercury when the tube is vertical; Then p, the pressure under which the diameter of the bubble of air has been measured; r, the semidiameter of the bubble; x, the depression of the mercurial column. If the diameter of the bubble 2r be 0.02 inch, the pressure p 2 inches, and the space V 1.2 cubic inch, the value of x is nearly 0.00001 inch; or the depression of the mercury, in consequence of the vacuum not being absolutely perfect, amounts only to dth of an inch. It is easy in actual practice to realize this close approximation to a perfect vacuum. The quantities now stated apply, in fact, to a barometric tube employed in an experiment which will be subsequently described. 1 100,000 The Torricellian vacuum leaves therefore scarcely anything to be desired in point of completeness; but it is unfortunately applicable to very few physical investigations. No instrument of any kind can be introduced into it, nor even any substance which is acted on by mercury. The vacuum obtained by the exhausting pump is not liable to these objections; but even with machines of the most perfect construction, and in the best order, a very imperfect approach can be attained to a complete exhaustion. A good ordinary pump with silk valves seldom produces an exhaustion of 0.2 inch.; and it is very rare indeed, if the manometer is properly constructed, to have it carried to 0.1 inch. In his "Etudes Hygrométriques" (Ann. de Chim. 3rd Series, vol. xv. p. 190), M. Regnault has given the following method for pushing the exhaustion further after the valves have ceased to act. In a large glass globe of from 20 to 25 litres capacity |