group. This is very strikingly illustrated by the observations of Dr. Lloyd on the rain-fall of Ireland. Stations situated on the southwest side of a mountain range showed a quantity of rain far in excess of that observed upon the northeast side. The winds in passing over the mountains were drained of their moisture, and were afterward comparatively dry.

7. Two or three years ago I had an opportunity of witnessing a singular case of condensation at Mortain in Normandy. The tourist will perhaps remember a little chapel perched upon the highest summit in the neighborhood. A friend and I chanced to be at this point near the hour of sunset. The air was cloudless, and the sun flooded the hillsides and valleys with golden light. We watched him as he gradually approached the crest of a hill, behind which he finally disappeared. Up to this point a sunny landscape of exquisite beauty was spread before us, the atmosphere being very transparent; but now the air seemed suddenly to curdle into mist. Five minutes after the sun had departed, a dense fog filled the valleys and drifted in fleecy masses up the sides of the hills. In an incredibly short time we found ourselves enveloped in local clouds so dense as to render our retreat a matter of some difficulty.

8. In this case, before the sun had disappeared, the air was evidently nearly saturated with transparent vapor. But why did the vapor curdle up so suddenly when the sun departed? Was it because the withdrawal of his beams rendered the air of the valleys colder, and thus caused the precipitation of the moisture diffused through the air? No. We must look for an explanation to a more direct action of the sun upon the atmospheric moisture. Let me explain. The beams which reach us from the sun are of a very composite character. A sheaf of white sunbeams is composed of an infinitude of colored rays, the resultant effect of all upon the eye being the impression of whiteness.

But, though the colors and shades of color which enter into the composition of a sunbeam are infinite, for the sake of convenience we divide them into seven, which are known as the prismatic colors.

9. The beams of the sun, however, produce heat as well as light, and there are different qualities of heat in the sunbeam as well as different qualities of light-nay, there are copious rays of heat in a sunbeam which give no light at all, some of which never even reach the retina at all, but are totally absorbed by the humors of the eye. Now, the same substance may permit rays of heat of a certain quality to pass freely through it, while it may effectually stop rays of heat of another quality. But in all cases the heat stopped is expended in heating the body which stops it. Now, water possesses this selecting power in an eminent degree. It allows the blue rays of the solar beam to pass through it with facility, but it slightly intercepts the red rays, and absorbs with exceeding energy the obscure rays; and those are the precise rays which possess the most intense heating power.

10. We see here at once the powerful antagonism of the sun to the formation of visible fog, and we see also how the withdrawal of his beams may be followed by sudden condensation, even before the air has had any time to cool. As long as the solar beams swept through the valleys of Mortain, every particle of water that came in their way was reduced to transparent vapor by the heat which the particle itself absorbed; or, to speak more strictly, in the presence of this antagonism precipitation could not at all occur, and the atmosphere remained consequently clear.* But the moment the sun withdrew, the vapor followed, without opposition, its own tendency to condense, and its sudden curdling up was the consequence.

* At this time I was brooding over experiments on the absorption of radiant heat by aqueous vapor.

11. With regard to the air, its temperature may not only have remained sensibly unchanged for some time after the setting of the sun, but it may have actually become warmer through the heat set free by the act of condensation. It was not, therefore, the action of cold air upon the vapor which produced the effect, but it was the withdrawal of that solar energy which water has the power to absorb, and by absorbing to become dissipated in true vapor.

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12. I once stood with a friend upon a mountain which commands a view of the glacier of the Rhone from its origin to its end. The day had been one of cloudless splendor, and there was something awful in the darkness of the firmament. This deepening of the blue is believed by those who know the mountains to be an indication of a humid atmosphere. The transparency, however, was wonderful. The summits of Mont Cervin and the Weisshorn stood out in clear definition, while the mighty mass of the Finsteraarhorn rose with perfect sharpness of outline close at hand. As long as the sun was high, there was no trace of fog in the valleys, but as he sloped to the west the shadow of the Finsteraarhorn crept over the snow-fields at its base. A dim sea of fog began to form, which after a time rose to a considerable height, and then rolled down like a river along the flanks of the mountain.

13. On entering the valley of the Rhone, it crossed a precipitous barrier, down which it poured like a cataract; but long before it reached the bottom it escaped from the shadow in which it had been engendered, and was hit once more by the direct beams of the sun. Its utter dissipation was the consequence, and, though the billows of fog rolled on incessantly from behind, the cloud-river made no progress, but disappeared, as if by magic, where the sunbeams played upon it. The conditions were analogous to those which hold in the case of a glacier. Here the ice-river is incessantly nourished by the mountain snow it moves

down its valley, but does not advance in front. At a certain point the consumption by melting is equal to the supply, and here the glacier ceases. In the case before us the cloud-river, nourished by the incessant condensation of the atmospheric vapor, moved down its valley, but ceased at the point where the dissipating action of the sunbeams equaled the supply from the cloud-generator behind.

Professor John Tyndall.

LAKE TAHOE.

1. A LAKE, six thousand feet above the sea, thirty miles long, sixteen miles wide, surrounded by mountains from which no summer melts all the snow, walled round the edges by firs and pines, set at the rim in a Mosaic of polished pebbles and brilliant flowers-is not that a lake to be loved? And I have not yet said a word of its water, which is so blue that it seems impossible it should not stain, and so clear that one can see fishes swimming more than a hundred feet below his boat, and so cold that ice would not cool it. For its water alone it could be well loved, if it lay in a desert. It has had some hard fortune in way of names. A German once named it Lake Bompland, and a militia general named it, after a governor, Lake Bigler. But ten years ago, by some marvelous good luck, it was rechristened by the old Indian name, Tahoe, pronounced by the Indians Tah-oo, and meaning "Big Water."

2. To find Lake Tahoe, one must journey on the Overland Railroad six days west from New York, or one day east from San Francisco, and leave the cars at Truckee. Truckee is as odd as its name. It looks so much as it sounds that one wonders if it could have been named beforehand. Truckee has one street. It is a broad, rocky,