VOL. LXXXI.] PHILOSOPHICAL TRANSACTIONS. 5 



duced is more or less compensated, is that operated on the width of the meshes; 

 and it is because the last of those effects is the only one that can affect the 

 length of the slips, that, in every change of moisture, they move evenly, with- 

 out any recoil. 



Mr. D. then finishes this paper by what he calls the conclusion ; where he 

 says that, having concentrated in these pages an account of 20 years assiduous 

 labour in hygrometry, mostly occasioned by the anomalies of the hygroscopic 

 threads ; the principal results have been, some determinations of the 4 principles 

 that directed him from the beginning, which are as follow: — 1st. Fire, as the 

 cause of heat, is a sure, and the only sure, means of obtaining extreme dryness : 

 this is produced by white heat in every hygroscopic substance that can bear it ; 

 and it may be thus transmitted to the hygrometer. 2d. Water, in its liquid 

 state, is a sure, and the only sure, nieans of determining the point of extreme 

 moisture on that instrument. 3d. It is not to be expected, a priori, of any hy- 

 groscopic substance, that its changes be proportional to those of moisture; but 

 it may be affirmed, that no fibrous or vascular substance, taken lengthwise, is 

 proper for the hygrometer. 4th. A means of throwing light on the march of a 

 chosen hygrometer, may be, to compare it with the correspondent changes in 

 weight of many hygroscopic substances. 



From those determinations in hygrometry some great points, he says, are al- 

 ready attained in hygrology, meteorology, and chemistry, of which he only in- 

 dicates the most important. 1st, In the phenomenon of dew, the grass often 

 begins to be wet when the air, a little above it, is still in a middle state of 

 moisture; and extreme moisture is only certain in that air, when every solid ex- 

 posed to it is wet. 2d, The maximum of evaporation in a close space, is far 

 from identical with the maximum of moisture; this depending considerably, 

 though with the constant existence of the other, on the temperature common to 

 the space and to the water that evaporates. 3d, The case of extreme moisture 

 existing in the open transparent air, in the day, even in time of rain, is ex- 

 tremely rare: he observed it only once, the temperature being 39°. 4th, The air 

 is dryer and dryer as we ascend in the atmosphere; so that in the upper attainable 

 regions, it is constantly very dry, except in the clouds. This is a fact certified 

 by M. de Saussure's observations as well as his own. 5th, If the whole atmos- 

 phere passed from extreme dryness to extreme moisture, the quantity of water 

 thus evaporated would not raise the barometer so much as half an inch. 6th, 

 Lastly, in chemical operations on airs, the greatest quantity of evaporated water 

 that may be supposed in them at the common temperature of the atmosphere, 

 even if they were at extreme moisture, is not so much as T-fo- part of their 

 mass. These last 2 very important propositions have been demonstrated by 

 M. de Saussure. 



