-1859] WRITINGS OF JOSEPH HENRY. 19] 



principle for carrying back the series of records by means of 

 trees which are found in the deep bogs of different parts of 

 Great Britain. By searching for corresponding thick or 

 thin rings in the outer circumference of one tree and in the 

 inner of another, a number of trees may be arranged in a 

 series, and thus the record extended back into the geological 

 periods. Whatever may be the practical value of this plan, 

 it is certainly ingenious and worthy of attention. Since the 

 trees found in bogs are, we may suppose, the regular and 

 consecutive productions of the primitive forests, they would 

 probably represent the successive vegetation of a series of 

 centuries. 



The remains of plants found in the rocky strata indicate 

 that the same diversity of weather and the same changes of 

 seasons existed in the past geological ages as at the present 

 time. By carefully studying the rain marks on sandstone, 

 the direction of the wind during storms in the ancient periods 

 may be determined; and this will probably be found the 

 same as in thunder showers of the present day. The remains 

 of plants and animals of a tropical character found abun- 

 dantly in the northern regions assure us that the tempera- 

 ture of the surface of the whole globe has undergone remark- 

 able changes. 



Effect of different surfaces. — The rays of heat from the sun 

 which strike the earth are partly reflected into space and 

 partly absorbed b}'' the surface in producing an elevation of 

 temperature. The absorbent and reflective powers are com- 

 plementary to each other, and vary greatly in different sub- 

 stances, and as we have seen according to their color and 

 texture. Lampblack possesses this power of absorption in 

 the greatest degree; and if we represent this by 100, that of 

 common glass will be 90, and that of polished metallic sur- 

 faces about 6. Consequently, the latter have a high reflec- 

 tive power, while that of lampblack and other dark sub- 

 stances is very small. This is a matter of interest to the 

 agriculturist, since the amount of heat which may be re- 

 ceived by a given surface will depend very much upon its 

 color; and indeed in some cases, charcoal or other dark sub- 



