190 WRITINGS OF JOSEPH HENRY. [1855- 



to the horizon, and the latter, as in the other case, much less. 

 A field also which slopes in an easterly direction receives less 

 heat than another inclined towards the west, inasmuch as 

 more reaches the latter, since the maximum heat of the day- 

 takes place after the sun has passed the meridian; as it is, 

 each of these enclosures gets a less amount than one of equal 

 extent parallel to the horizon. 



Estimate of temperature by rings in trees. — It frequently hap- 

 pens that permanent records are found of the past condition 

 of our globe in the impressions retained in the rocky strata, 

 and that the yearly occurrences of certain phenomena such 

 as the annual deposit from the overflowing of rivers. Such 

 records may be rendered available in determining the time 

 of actions which may have long since ceased, or which con- 

 tinue to the present day. It is well known that the trees 

 of our latitude increase in size by the deposition of an addi- 

 tional layer annually between the wood and the bark, and that 

 a transverse section of such a tree presents a series of concen- 

 tric though irregular rings, the number of which indicates 

 the age of the tree. The relative thickness of these rings de- 

 pends on the more or less flourishing state of the plant in the 

 year in which they were formed, and therefore indicates the 

 relative state of heat and moisture during the same period. 

 Furthermore each ring in some trees may be observed to be 

 subdivided into others during the same year, indicating that 

 the vegetation was advanced or checked at intervals during 

 the season. Furthermore it has been found by observation 

 that even the motion imparted to a tree by the wind has an 

 influence on its growth, giving to its trunk an oval form, the 

 longer direction of which will be that of the prevailing wind. 

 A thin slice therefore cut from a large tree at right angles to 

 its axis, carefully polished and varnished, forms a natural 

 record of the weather well calculated to call forth admiration 

 and to impart instruction. It is scarcely necessary to remark 

 that the year should be carefully identified, corresponding 

 to a given circle, in order that the whole might be properly 

 numbered. 



Mr. Babbage has proposed an ingenious application of this 



