OF LAND AND WATER ON THE SURFACE OF THE GLOBE. 291 



however, no later accounts of such a determination of these quantities. 

 Several ijersons are said to have repeated the experiment, but as they 

 have not published its results, it seemed desirable to try it again with 

 that care, which might at least ascertain the reliance which can be 

 placed upon the method. The advanced state of modern geography 

 affords a more reasonable expectation of accuracy than could, under the 

 most favourable circumstances, have been attained in the time of Dr 

 Long; and the beautifully distinct manner, in which globe-plates of the 

 largest size have now been executed, gives great advantage to the trial. 

 In 1823 Mr Carey allowed me, for this purpose, to make use of the 

 plates of his 21-inch globe; and when I recently wished to check the 

 results, at which I had then arrived, Mr Addison obliged me with 

 those which he has had engraved for a globe three feet in diameter- 

 he took the trouble, likewise, not only of inserting all the latest dis- 

 coveries, but of having the impressions expressly worked off for me 

 with every precaution and attention. 



There are some difficulties in the pursuit of this inquiry, which 

 make it necessary to proceed with great care. Dr Halley observes, 

 "that the moisture of the air imbibed by the paper, did very notably 

 increase its weight, which made me very well dry the pieces before 

 I weighed them, that so I might be assured there was no error upon 

 this account; and in so doing, I found that in a very few minutes 

 of time, their Aveight would sensibly increase by their reimbibing the 

 humidity of the air." This effect is indeed so rapid that arttficial 

 drying is possibly the worst thing that can be done; it will occasion 

 the weights to vary while the paper is in the scale, and will thus 

 destroy the precision of the ratio, which may be derived from the 

 examination of parts of the same sheet. A much better method is 

 to lay the paper out for some time in a large room, where there is 

 no danger of much fluctuation in the state of the air, the materials 

 will then reach a nearly saturated state, in which they will generally 

 continue stationary during the time which they are in hand. Such 

 an exposure will of course on different days produce different degrees 

 of dampness; but uniformity in this respect, for any great length of 

 time, is unattainable, and if it can be secured for the interval, which 

 Vol.. VI. Part II. Pp 



