=r. [Book V. 



th.ic of the late ingenious Mr. Smeaton. He em- 

 ployed 5n its conftrucYion a flaxen firing of three 

 omrr.or.ly ufed in making nets, which, in or- 

 der so make it attract the 'moifture readily, he fteepecl 

 in- fait and water. This he extended along a board 

 properly </ and to one part contrived to at- 



tach an index, which ferved to fhew its variations. 

 M. Sauffure employed a hair for the fame purpofe, 

 which he fufpexided by a weight of three grains, and 

 contrived it to a^l upon an index pointing to a gra- 

 duated fcale ; and this was found to be a very delicate 

 and accurate inftrumenr. 



M. de Luc, however, conceived that a folid body 

 would afford the rooff: accurate and fteady meafure of 

 damp or drynefs, and was'lefs likely to be out of or- 

 der than fibrous and twifted fubftances. He fuccef- 

 fively employed ivory, box- wood, and whalebone; but 

 after feveral trials preferred the latter, becanfe of its 

 great power of expanfion, which fometimes exceeded 1 

 one-eighth of its length, and becanfe of its ' fteadincfs, 

 in always coming to the fame point in extreme moif- 

 ture. His hygrometer therefore eonfifts of a very thin 

 flip of whalebone, about 11 inches in length, and a line 

 in breadth, cut tranfverfely to the direction of the fibres. 

 This he extends by a fmall fpring, and the -variations 

 may be either meafured by a mark on the whalebone 

 qr by an index. This is at prefent the mod perfect 

 hygrometer, and that which is in mo-ft general ufe. 



In the higher regions of the atmofphere the cold is 

 found to be intenfr, and yet the moitture is generally 

 faid to be lefs abundant, than in thole nearer the fur- 

 face of the earth. This was experienced by all the 

 adventurers in air balloons, as well as by thofe tra- 

 vellers who have afcended to the tops of high moun- 

 tains. It is well known that the fummits of the alps 



and 



