THE POPULAR SCIENCE MONTHLY.
next reconnoitered the upper course of the Marañon River on the eastern slope of the Cordillera in the Peruvian north, whence the reports about the ruins of Kue-lap had created great interest. He passed the historically celebrated town of Cajamarca; traversed in the department of Amazonas an exceedingly broken and uneven country; and secured a complete plan of Kue-lap, with a number of details, furnishing data to correct previous accounts and surveys. He also gathered a number of traditions relative to occurrences anterior even to the time when the Incas began to make raids across the Marañon. After exploring many other ruins, the political conditions in Peru becoming unpleasant, Mr. Bandelier went into Bolivia, where he spent some time on the island of Titicaca, established the height of about 14,500 feet as the uppermost limit of sedentary occupancy in ancient times on the southern side of Illimani, and examined the slopes, up to 15,400 feet, of the great peak of Kaka-a-ka, or Huayna Potosi. At last accounts he was preparing for a journey to Pelechuco, in the northwestern corner of Bolivia.
Temperature Levels in Lake Mendota.—Prof. E, A. Birge, of the University of Wisconsin, has been pursuing studies of the "Plankton" of Lake Mendota in that State, with a prime view to making a contribution to the natural history of an inland lake as "a unit of environment." He finds that during the summer the difference in temperature between the surface and the bottom may amount to 10°, 12°, or even 15° C, but the decline in temperature from surface to bottom is not uniform as the depth decreases. If a series of temperatures is taken about the 1st of August it will be found that there is a layer of surface water from about twenty five to forty feet in thickness, the temperature of which is nearly uniform. Immediately below this mass of warm water lies a stratum in which the decline of temperature is extremely rapid. This stratum may be from about six to ten feet thick, with a decline of nearly as many degrees centigrade per yard; or it may be only about a yard thick. This layer in which the temperature decreases rapidly may be known as the thermocline—the Sprunsgchiet of German authors. Below the thermocline the temperature decreases toward the bottom at first more rapidly and then more slowly as the depth of water increases, but never showing the sudden transitions which are characteristic for the thermocline. The thermocline was first noticed by Richter in 1891 in a study of the Alpine lakes. Its origin was attributed by him to the alternate action of the sun warming the surface in the day, followed by a cooling at night. The alternation of the conditions resulted in the formation of a layer of water of nearly uniform temperature above the colder bottom water. In Lake Mendota the concurrence of gentle winds and hot weather is essential to the formation of the thermocline. The warmth of the surface water, received from the sun, is distributed through a certain depth of the lake, a depth which is proportional to the violence of the wind and the area of the lake. In a lake of the size of Mendota the water would be of uniform temperature from top to bottom if it were always agitated by violent winds. On the other hand, if the weather were perfectly calm, the lake would be warmed only to the depth to which the rays of the sun could directly penetrate.
Curious Photographic Effects.—Since the rise into prominence in 1895 of the X-ray phenomena, there has been a greatly increased interest among physicists in the even more curious but apparently closely allied phenomena of normal physical emanations from certain surfaces which have the property of influencing the sensitive plate, and in some cases even impressing an image on such insensitive substances as glass, copper, etc. These phenomena have been variously labeled scotography, vapography, etc, but there has not as yet been sufficient insight gained into their causes to allow of a truly descriptive title. Dr. W. J. Russell, who has made this phenomenon the subject of his last two Bakerian lectures before the Royal Society, is authority for the following statements. He had previously found that zinc, other metals, wood, straw-board, and printed papers, when placed in contact with a dry plate, had a certain action on it, which enabled it to be developed as if it had been exposed to light. In his later lecture he recounts a number of additional experiments. Zinc and other materials,