Page:Popular Science Monthly Volume 45.djvu/338

 turn would exhibit peculiarities, but the purpose of the paper it is hoped has been attained—namely, to show engravings made from sun prints of thin sections of wood with the various elements of structure in the proper position and of natural size.

A single enlarged view of a section of the ash is herewith given, and both indicate the structure seen in Fig. 8 on a larger scale, and show that pictures of such objects may well be taken



with the light passing through the object falling upon the sensitized plate in the dark chamber of the camera. By a comparison of Figs. 8 and 3 it will be seen that the two show the same ash wood in transverse section. In fact, a small portion of Fig. 3 near its center was selected for the picture from which engraving 8 was made, and this last is therefore no exception, for it was also a catching of a picture by Sunshine through the Wood.

his subterranean explorations from 1888 to 1893, M. Martel has found that the temperature of natural caves is not equivalent to the mean annual temperature of the place, hut is inconstant; is not uniform in different parts of the same cave; and that the temperature of water in caverns is subject to the same variations as the temperature of the air, and is sometimes very different from the temperature of the air. The causes of these variations are not well understood, but as among them M. Martel mentions fissures admitting air from without; cavities in which cold air settles; and the influence of water, which cools the air through the evaporation of its oozings, or, when streams flow through the cave, brings in all the variations of the external air.