322 THE POPULAR SCIENCE MONTHLY. 



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 ele- 

 ments 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 



FIG. 8. CROSS SECTION OF ASH. Magnified. 



with the light passing through the object falling upon the sen- 

 sitized plate in the dark chamber of the camera. By a com- 

 parison 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. 



IN his subterranean explorations from 1888 to 1893, M. Martel has found that 

 the temperature of natural caves is not equivalent to the mean annual tempera- 

 ture 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 varia- 

 tions as the temperature of the air, and is sometimes very different from the tem- 

 perature of tlie air. The causes of the?e variations are cot 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. 



