upon it, but owing to the transparency of the iodide to radiant heat, it requires some exposure to produce the thermograph. This red substance is far less absorbent of radiant heat than white paper, and hence it is sometimes easier to obtain a thermograph of the carbon points by exposing to the radiation from the lamp the back of the paper on which the iodide is strewn, than by exposing the face covered with the iodide. It is often, indeed, more easy to burn a thermograph through the paper than to discharge the colour of the iodide. Hence, white paper may be protected from radiant heat by being covered with a substance like the iodide of mercury.

We are here naturally reminded of the experiments of Franklin, which consisted in placing cloths of various colours upon snow, and observing the depth to which they sank in the snow when exposed to direct sunshine. Franklin concluded that the lighter the colour of the body the less is its power of absorption. The generalizations founded on this experiment are for the most part fallacious. Results long ago obtained, establishing the vast influence of chemical constitution on radiant heat, led the speaker to contrast iodine, an element, with alum, a body of highly complex character. Both substances were in powder, the one being dark, the other white. Exposed to the radiation from various sources, the white powder proved itself in all cases the most powerful absorber. The dark powder of amorphous phosphorus was also compared with the hydrated oxide of zinc, but the white powder was the best absorber. Bodies of the same colour compared together showed similar differences. The red oxide of lead for example was contrasted with the red iodide of mercury, and the oxide proved the most powerful absorber. So also the white chloride of silver was compared with the white carbonate of lead, the lead salt proved by far the most powerful absorber. In this way it was proved that as regards the absorption of radiant heat, white in some cases exceeds black, black in some cases exceeds white, and the other colours are equally capricious; all evidently depending on the chemical constitution of the substances. Here, as in other cases moreover, radiation and absorption go hand in hand, the substance which absorbs heat most powerfully radiating the same heat most copiously.

In the case of Franklin's white cloth exposed on snow to sunshine, there is no reason why it should sink at all; there is, on the contrary, reason to conclude that it must rise relatively to the snow surrounding it. For, as regards the luminous rays of the sun, they are alike powerless to warm the cloth or to melt the snow. Whatever effect is produced is therefore due to the dark solar rays. Now, snow absorbs these rays with greater greediness than any other substance; hence the white cloth, which absorbs less than the snow, really defends the snow underneath it from the action of the sun, and owing to this protection the cloth, if exposed for a sufficient time, will rise in relation to the surface round, just like a glacier table.

But though the cloth is not so good an absorber as the snow, it is nevertheless a very powerful absorber; it comes near the snow in this

respect. And when, as in the case of the black cloth, we have added to the absorption of a large portion of the dark rays by the cloth, the absorption of the whole of the luminous rays by the dye, the sum of the absorption of both classes of rays exceeds the absorption by the snow of the dark rays alone. The black cloth will therefore sink in the snow. This is the explanation of Franklin's experiment.

The speaker concluded by referring to various experiments on the transmission of radiant heat through rock salt; to the influence of science as a means of intellectual culture; and to the necessary defects of any system of education in which the study of nature is neglected or ignored.

[J. T.]

WEEKLY EVENING MEETING,

Friday, January 26, 1866.

SIR HENRY HOLLAND, Bart. M.D. D.C.L. F.R.S. President, in the Chair.

SAMUEL W. BAKER, Esq.

On the Sources of the Nile.

THE primary object of geographical exploration is the opening to general intercourse such portions of the earth as may become useful to the human race. The explorer is the precursor of the colonist, and the colonist is the instrument by which the great work must be constructed, that greatest and most difficult of all undertakings, the civilization of the world.

The progress of civilization is dependent upon geographical position. The surface of the earth presents certain facilities, and obstacles to general access; those points which are easily attainable must always enjoy a superior civilization to those that are cut off from association with the world.

We may thus assume that the advance of civilization depends upon means of transport; countries remote from the sea may through the ingenuity of man be rendered accessible; the natural productions of those lands may be transported to the sea coast in exchange for foreign commodities, and commerce thus instituted becomes the pioneer of civilization.

Rivers are the natural arteries of the world; providing channels of communication, they have been from the earliest ages the most important geographical phenomena.

Countries that are blessed with navigable rivers invariably prosper;

the ancients were so cognizant of this fact that all first settlements were made upon the banks of rivers, and the origin of the human race dates from an Eden between the Tigris and Euphrates.

London upon the Thames is a mighty fact significant of the importance of river communication. America, that giant offspring of Great Britain, owes her unexampled prosperity to her navigable rivers. Accepting the theory that rivers are the most important geographical points, we turn with increased interest to that great artery of Africa now before us; that mysterious stream which has ever absorbed the attention of both ancient and modern geographers-the Nile.

Egypt has been created by the Nile. The great Sahara, that frightful desert of interminable scorching sand which stretches from the Red Sea to the Atlantic, is cleft by one solitary thread of water. Ages and ages before man could have existed in that inhospitable land, this thread of water was at its silent work; year after year it flooded and fell, leaving a rich legacy of soil upon the barren sand, until the delta was created; and man, at so remote a period that we have no clue even to an approximate date, occupied the fertile lands thus born of the river Nile, and that corner of savage Africa rescued from its barrenness became Egypt.

The Egyptians claimed the most ancient origin; their geographical position gave them extraordinary advantages for commercial enterprise. Bounded on the east by the Red Sea, on the north by the Mediterranean, while the fertilizing Nile afforded inland communication, Egypt became in the earliest era the most civilized and prosperous country of the earth.

Egypt not only was created by the Nile, but the very existence of the Egyptians depended upon the annual inundation by that river; thus, all that pertained to the Nile was of vital importance to the people; it was the hand that fed them.

Two thousand five hundred and eighty years ago a graduated pillar was erected as a nilometer in a well on the island of Rhoda, opposite old Cairo, and the rise and fall of the river was measured and watched with equal anxiety to the present day.

A century later, a canal was commenced by Pharaoh Necho to connect the Nile with the Red Sea; the undertaking failed, but was subsequently completed after the Persian conquest, and water communication was thus established between the Red Sea and the Mediterranean. Egypt was the Nile and the Nile was Egypt.

Depending so entirely upon the river as Egypt did, it was natural that the mysterious origin of those sacred waters should absorb the speculations of all thinking men. It was unlike all other rivers. In July and August, when European streams were at their lowest in the summer heat, the Nile was at the flood!

In Egypt there was no rainfall. Not even a drop of dew in those parched deserts through which the glorious river flowed for 860 thirsty miles of latitude without one single tributary. Licked up by the burning air, and gulped down by the exhausting sand of Arabian

deserts, nevertheless it flowed; supporting all losses by evaporation and absorption, the noble flood shed its annual blessings upon Egypt. An anomaly among rivers; flooding in the dry season; everlasting even in sandy deserts; where was its hidden origin? where were the "Sources of the Nile?"

The "Source of the Nile" was, from the earliest period, the great geographical question to be solved. Thus, in the ancient days of Egypt, her kings and priests endeavoured to unravel this great mystery, and to explain the cause of the annual flood, upon which the existence of the Egyptians depended.

The most important event of Jewish history commenced about 1706 B. C., when the family of Joseph came from Canaan into Egypt to purchase corn during the seven years of famine; and thus commenced the settlement of the Israelites among the Egyptians. The colonization was due to the river Nile. Seven years of unprecedented fertility must have been due to seven seasons of extreme inundation, which so filled the granaries of Pharaoh, that he supported the whole population of Egypt during seven succeeding years of famine. That famine must have been caused by the low level of the river Nile. There was famine, i. e. want of rain in Canaan, and this drought must have extended to the mountain range in Abyssinia, and have exhausted the great tributaries to the Nile from that country. These tributaries are the Atbara, Settite or Taccazy, Salaam, Angrab, Rahad, Dinder, and the great Blue Nile.

We thus find, as far back as 3570 years, the proof that the population of Egypt had originally been induced by the Nile, as the fertility produced by that extraordinary river not only supported the people of Egypt, but attracted in times of scarcity emigrants from far countries, as exemplified by the colony of Israelites. Everything connected with Egypt was so dependent upon the Nile, that even one of the seven plagues was a disturbance of its waters.

"And the fish that was in the river died; and the river stank, and the Egyptians could not drink of the water of the river; and there was blood throughout all the land of Egypt."

"And all the Egyptians digged round about the river for water to drink; for they could not drink of the water of the river."

Doubtless this plague was caused by so low a level of the Nile that the current ceased, and the water stagnating gave birth to a mass of animalculæ of a crimson colour, resembling blood. This is proved by the words "they digged round about the river for water to drink."

This custom prevails at the present day among the Arabs, who refuse to drink the water of the White Nile from the river when low, on account of minute worms that affect the stomach.

Following this curse upon the river in the reign of Pharaoh, is the plague of frogs: "And the river shall bring forth frogs abundantly,"†

&c.

* Exodus, vii. 21-24.

Exodus, viii. 3.

The appearance of frogs in such myriads is a further proof of the general stagnation of the river, caused by an extraordinary dry

season.

These striking facts, handed down to us by Sacred History, give a peculiar interest to the Nile. Civilization and learning had sprung from the banks of the Egyptian river-we hear of Moses, that he was "learned in all the wisdom of the Egyptians.' But although Egypt was at that period the centre of the world in all that related to science and civilization, and dependent upon the river for its actual existence, not all the learning and wisdom of the Egyptians could determine the "Sources of the Nile."

Time will not permit me to allude to the expeditions that, during some thousand years, have been framed by various nations for this discovery. All failed until our countryman, Bruce, discovered the source of the Blue Nile about ninety years ago. He was the first Englishman who had started upon this mysterious mission. Bruce confessed that the Blue Nile was not the great river, but merely a tributary to the main stream known as the White Nile. Numerous expeditions have started since that time to explore the acknowledged Nile, to seek for the sources of the White river. All failed; a fatality hung over those marshy regions of inner Africa, and the Nile sources seemed doomed to remain for ever sealed. There appeared to be insurmountable difficulties from the north, and it was reserved for the honour of England to send an expedition to the south via Zanzibar. In command of this expedition, struggling forward with British tenacity of purpose, our gallant explorers, Speke and Grant, reached the first Nile source in the Victoria Lake.

In the very heart of Africa, in N. lat. 4° 55', I met those great men with open arms. Gaunt and lean with toil and bad climate, but elate with having accomplished that which had baffled the whole world, on the 15th February, 1863, they marched into Gondokoro at the head of the faithful eighteen followers who had started with them from Zanzibar.

It would be vain to attempt the description of this meeting. For nearly two years I had been exploring the tributaries of the Nile, absorbed with the one idea of discovering its true source, and of meeting my old friend Speke in some dilemma from which I had a Quixotic hope of extricating him.

Having completed the exploration of all the great rivers from Abyssinia, I had descended the Blue Nile to Khartoum, at which place I had organized a powerful expedition for the White Nile. To overcome the great difficulty of Central Africa, "want of transport," I had purchased a number of beasts of burden, including horses, camels, and asses, all of which, to the number of twenty-nine, I conveyed up the White Nile in three vessels, and I landed them safe in Gondokoro in lat. 4° 55' on 3rd Feb., 1863.

The voyage from Khartoum to Gondokoro occupied forty-five days. Nothing can be more tedious than this journey through inter