can accompany or follow them in this work; but its result, the distance of the earth from the sun, is intelligible to any schoolboy when fairly established and plainly stated.

The making of a railway is a very tedious and costly task; but the travelling over it a swift and cheap one. It is the same with the truths of science. The exclusive pedant would drag you through his details of discovery and demonstration, pretending that you cannot be a passenger in the triumphal car of science without being also an engineer.

These notes are intended to carry ordinary passengers along the path of scientific discovery without requiring them to excavate their own tunnels or drive the engine.

A selection of subjects will be carefully made month by month, and only those of general interest will be treated: others that are specially technical, or interesting only to a small section of experts, will not be touched at all.

Where preliminary explanation is necessary, it will be given in as few words as may be consistent with clear and readable exposition.

M.

A NEW DEVELOPMENT OF PHOTOGRAPHY.

JANSSEN has announced a very curious, and at present a mysterious, discovery in photography. He has lately produced some magnificent photographs of the face of the sun, displaying the spots, the faculæ, the "mottling" or "rice grains," or "willow leaves," as they have been fancifully called, and the other details of solar physiognomy, in a manner that affords to all and sundry fair opportunity of studying these stupendous irregularities. In the further prosecution of this admirable work, M. Janssen found that prolonged exposure destroys the picture, nothing appearing on applying the developer. Careful observation showed that this disappearance was gradual, as might be expected. So far there is no particular novelty in the observations, but by continuing the exposure beyond the period of disappearance an unexpected transformation is displayed. Instead of an ordinary negative picture with lights represented by shades, and the shades by light, a positive picture is now displayed on development; the bright body of the sun shown white and the spots black, as to ordinary vision. With careful manipulation this direct positive has all the distinctness of a fine photographic picture. About 1000 of a second was the time allowed for an ordinary picture of the sun, and with the gelatinobromide process of this time is sufficient. The direct positives

were obtained by continuing the exposure from half a second to a second.

What is the chemistry of this second action? To answer this question satisfactorily, further, and probably rather extensive, investigation is demanded; a research that must include the whole philosophy of the wondrous phenomenon of photographic development.

If among my readers there are any who have not witnessed this magical process, he or she should visit a communicative photographer, and ask for an introduction to his darkened chamber, where a glass plate, presenting only a surface of dirty-looking collodion film, is subjected to an incantation by enchanted waters, and forthwith appears a spectral image of the observer or any other person or thing the operating wizard has chosen to call forth. Nothing narrated in the chronicles of witchcraft is more weird and wonderful than this.

THE

ARCTIC BALLOONING.

'HE Central Arctic Committee, after careful consideration and some discussion, passed unanimously the following resolution: "That, in the opinion of this committee, the plan of using three connected balloons, as tested at the Alexandra Palace, does not warrant the committee in following out further that suggestion, but leads them to revert to the original idea of using single balloons as auxiliary to the work of the new expedition."

It is quite evident, from the valuable practical instruction derived from the rude experiment to which allusion is made in the above, that more experiments are required. The Government had done something in the study of military ballooning, but not nearly enough. Though somewhat blasé in reference to the putting forth of original projects, I am sorely tempted to revive one of the devices of my youth, suggested in the course of some struggles over Alpine glaciers, especially that of the Bossons. It was to attach to the upper part of the back, by a system of shoulder straps, a small balloon capable of lifting one's knapsack and from a half to three-fourths of the weight of the body; and thus, relieved of so much encumbrance, to skip merrily over the Alps, especially up the snow slopes and glaciers, tripping lightly from ridge to ridge of the craggy glacier ice, and crossing its blue crevasses by easy flying leaps.

Such an arrangement, carrying a fortnight's supply of food in addition, might enable an exploring party to approach the Pole in spite of the so-called "paleocrystic ice," provided the gas would not ooze through the balloon faster than it became relieved of ballast by VOL CCXLVII. NO. 1797.

BB

the consumption of the provisions. A gale of wind might possibly be inconvenient; while, on the other hand, a favourable breeze, rather stiff, would be equivalent to the "seven-league boots" of the nursery hero. The rate of progress, in any case, should be very different from the one mile per day of Markham's sledge parties.

M.

A STEAM-ENGINE WORKED BY THE SUN.

MOUCHOT, of Algiers, has fully carried into practical

I say

effect an oft-repeated philosophical dream, viz. that of using the sun's rays directly as a source of mechanical power. "directly," because, as is now pretty generally understood, the combustion of coal, wood, &c. is but an indirect application of ancient bottled sunbeams to modern use.

M. Mouchot's engine has been long at work. In November last his solar furnace raised above 7 gallons of cold water to the boiling point in 80 minutes, and at the end of another hour and a half raised its steam to a pressure of eight atmospheres.

On De

cember 24 he distilled 5 gallons of wine in 85 minutes. Since the spring of last year he has been working a horizontal engine at the rate of 120 revolutions per minute, with a pressure of 3 atmospheres, pumping 250 gallons of water per hour, at 4-feet pressure. This was done from 8 A.M. to 4 P.M. without sensible hindrance by passing clouds. He has also sublimed sulphur, distilled sulphuric acid, purified linseed oil, concentrated syrups, carbonised wood in closed vessels, fused and calcined alum, &c.

The solar heat is concentrated by means of mirrors, and the boiler is enclosed in glass through which the solar rays pass readily for heating the water, while the radiations of the obscure heat from the boiler itself are obstructed by the glass. This difference of the penetrability of glass by rays of differing intensity may be easily proved by holding a piece of glass between the sun and one's face, and then repeating the experiment before a domestic fire. The glass is no screen to the solar rays, but an effective one to those from the fire. Greenhouses and cucumber-frames are heated thus. The solar heat freely enters these glass traps, but cannot readily get out again. It should be noted that M. Mouchot's success in Algiers by no means proves that his engine would work in England. We are still likely to remain dependent on our underground fossil sunbeams, but for the inhabitants of tropical countries M. Mouchot's invention opens out quite a new era of physical civilisation.

A

BALLOON PHOTOGRAPHS.

NOTHER application of photography and a new use for balloons has been opened by M. Paul Desmarets. He makes a hole in the bottom of the car and there places a camera; and thus, by instantaneous process, obtains a map-like portrait of the country below, which may be printed by the autotype or some other of the many processes now in operation. These photographs admit of considerable magnifying of details, which greatly adds to their interest and value. M. Desmarets's pictures were taken over Rouen, and arrangements are now in progress for the systematic photography of Paris from above. It remains to be seen whether practically useful maps may be thus produced, and to what extent existing maps may be corrected by these interesting sun-pictures. The most curious element of the invention is that it should be a novelty, that so obvious and simple an idea should not have been carried out long ago.

Commander Cheyne must not omit the hole in the bottom of his balloon car, and must carry suitable photographic apparatus. He should take lessons in photography forthwith. The copyright of midnight sun-pictures of the Pole, if well worked, might pay the expenses of his expedition.

W

GEOLOGICAL CONVULSIONS.

E all know that land and sea have changed places, and that even the tops of high mountains were once under the sea. I have myself found fossil remains of marine shells on the summit of Mount Pilatus, which is more than 7,000 feet above the present sea level; but these belong to a geological era long passed away, and their inhabitants were probably animals that dwelt in shallow waters near the shore.

Mr. Gwyn Jefferys has examined some fossil shells found in Calabria and Sicily at heights of more than 2,000 feet above the sea level, and finds them of the same species as others that are now living at depths of not less than between 9,000 and 10,000 feet below its surface, and dredged up during the expeditions of the Lightning and the Porcupine. If the inference that they cannot, as well as do not, live at less depths is correct, their existence in this position indicates an upheaval of eleven to twelve thousand feet within a period which, geologically speaking, is but recent. The probability of this great change is increased by the fact that the whole region between Vesuvius and Etna is still a literal hot-bed of volcanic activity, and

that even monuments of human handiwork thereabouts have been sunk below the sea and afterwards raised above it, as shown in the celebrated instance of the temple of Jupiter Serapis at Puzzuoli, the remaining columns of which bear the perforations of marine stoneborers up to a height of 23 feet above the present sea level, marking this as the depth of their submergence and re-elevation, not merely since the temple was built, but since it became a ruin. This is shown by the fallen columns, which are not perforated in the same manner as those which stand upright.

EYE-MEMORY.

L

OOK steadily at a bright object, keep the eyes immovably on it for a short time, and then close them. An image of the object remains; it becomes, in fact, visible to the closed eyes. The vividness and duration of such impressions vary considerably with different individuals, and the power of retaining them may be cultivated. Besides this sort of retinal image thus impressed, there is another kind of visual image that may be obtained by an effort of memory. Certain adepts at mental arithmetic use the "mind's eye" as a substitute for slate and pencil by holding in visual memory pictures of the figures upon which they are operating, and those of their results.

In my youthful days I was acquainted with an eccentric old man who then lived at Kilburn Priory, where he surrounded himself with curious old furniture reputed to have originally belonged to Cardinal Wolsey, and which, as I was told, he bequeathed to the Queen at his death. He was the then celebrated, but now forgotten, "Memory Thompson," who in his early days was a town traveller (for a brewery, if I remember rightly), and who trained himself to the performance of wonderful feats of eye-memory. He could close his eyes and picture within himself a panorama of Oxford Street and other parts of London, in which picture every inscription over every shop was so perfect and reliable that he could describe and certify to the names and occupations of the shopkeeping inhabitants of all the houses of these streets at certain dates, when Post-Office Directories were not as they now are.

Although Memory Thompson is forgotten, his special faculty is just now receiving some attention, and it is proposed to specially cultivate it in elementary schools by placing objects before the pupils for a given time, then taking them away and requiring the pupil to draw them. That such a faculty exists and may be of great service is