Il6 PHILOSOPHICAL TRANSACTIONS. [aNNOI/QI. 



all the hygroscopes of both classes : for, in respect of the threads, instead of 

 lengthening suddenly in that period of moisture, they have then a retrograde mo- 

 tion, either continuing or only beginning; and as for the slips, they, by lengthen- 

 ing in the same period, only follow their former laws : the slips which, com- 

 paratively to that of whalebone, have at first small steps, and which consequently 

 move in an increasing progression, continue only to follow that progression ; and 

 those which at first have greater steps, and consequently a decreasing march, have 

 then small steps conformable to their individual law ; therefore, none of those 

 hygroscopes of both classes have any sudden start, produced by any degree of 

 moisture in the medium, or by the application of concrete water ; each of them 

 follows, from one end to the other of its scale, its own progression ; and in re- 

 spect of slips, moisture is never extreme in the ambient medium, as long as, in 

 their respective progressions, they have not attained their greatest length. Our 

 common hygrometer must then be made of one of the slips ; but with that great 

 dissimilarity observed in their marches, which of them shall we choose as indi- 

 cating the real march of moisture ? None as yet from that consideration, which 

 he does not even think a primary one. Let us then examine which of the slips 

 possesses the most essential properties of an hygrometer, such as should be in 

 common use for comparative observations, and to which consequently future dis- 

 coveries in respect of the real proportions between the quantities of moisture it- 

 self would be applied. Steadiness is surely a first requisite for such an instru- 

 ment ; and in that respect no slip comes in competition with that of whalebone. 

 That property was the first motive of Mr. D.'s choice. Some other slips may be 

 brought to a certain degree of steadiness by studying what is the degree of stretch 

 which they may bear ; but that attention is not necessary for the slip of whale- 

 bone: if, for instance, when its point of extreme moisture has been fixed while it 

 was stretched to a certain degree, that stretch is much increased, it will acquire 

 some absolute length ; but it will be steady again for a new point taken then in 

 water. 



Another property of the slip of whalebone, which at first should seem con- 

 tradictory to the former, is its great expansibility, in which also it surpasses all the 

 substances that have been tried. Such a slip lengthens above -i- of itself from ex- 

 treme dryness to extreme moisture, which produces many advantages in the con- 

 struction and observation of that instrument. In respect to observation, when it 

 is exposed to the wind, the dift'erence between the chords of the arches of its 

 bends and its real length is so small, comparatively with its hygroscopic variations, 

 that the indetermination of its index will remain confined in a space of 1 or 2 de- 

 grees, when it becomes impossible to observe hygrometers whose substance has 

 but little expansion. Lastly, of all the substances which have been reduced to slips, 

 none is so easily made thin and narrow as whalebone. Mr. D. has found means 



