4 l>HILOSOPHICAL TRANSACTIONS. [aNNO 17gi. 



its increase in the air; and this we see to be the case, by comparing the marches 

 of the 3 kinds of slips with the correspondent increases in weight of the quill 

 and deal shavings, during the whole progress of the experiment. There are 

 differences in those marches as expected ; but they are not such as to give the 

 smallest reason to suspect that afterwards, during the period of the last 3 terms 

 of the table, in which we have no correspondent observations of increases of 

 weight in the shavings of deal and quill, the same law does not take place as in 

 the antecedent 17 terms. If the experiment was only made with one kind of 

 slip, it might be objected, that though that slip lengthens regularly during the 

 whole increase of moisture from its minimum to its supposed maximum, it is not 

 impossible but that immediately after, by some peculiarity of its nature, it will 

 lengthen, without any further increase of moisture in the medium. But that 

 surmise cannot be admitted when the slips of such dissimilar substances as whale- 

 bone, quill, and deal, sensibly agree in their motions at that period, and when a 

 number of other slips of the vegetable and animal kinds follow also the same 

 general march. 



The experiments here analyzed are only one set among others which, though 

 made with less accuracy, have given the same general results. These, relating 

 to various kinds of substances, Mr. D. intends to repeat, and to communicate 

 their results to the r. s. He then concludes this paper, with an immediate de- 

 monstration, that the hygroscopic motions of the slips are simple, while those of 

 the threads are the combined effects of 2 opposite causes. 



Mr. D. proceeds then to the recoil of hygroscopic threads. When formerly 

 he concluded, from the phenomena of the water thermoscope, that its conden- 

 sations were the combined effects of 2 opposite causes, which followed different 

 laws, it was not for having distinguished those 2 effects ; but only because of a 

 small retrogradation near the freezing point, preceded by a stationary state, 

 comparatively with the march of quicksilver ; but in the case of hygroscopic 

 threads and slips, in which we have the same phenomenon, the 2 opposite effects 

 are distinguishable in the threads, by one being operated more rapidly than the 

 other. If, for instance, we transport from a drier to a damper place, or in- 

 versely, the 2 kinds of quill hygroscopes, the slip proceeds in an even course to 

 a certain point, where it remains fixed ; but the thread moves in an interrupted 

 manner also to a certain point, whence it recoils. If that experiment is made 

 within the limits of the stationary state of the thread, it may recoil as much as 

 it has gone the other way, and be fixed at the same point in both places. The 

 case of the slip of quill is common to every slip, and that of its thread to all 

 others which have a quick motion. Here then we have separately the 2 effects 

 of moisture on the threads ; that on the fibres themselves is the soonest pro- 

 duced, and at first predominates : the slowest, by which afterwards the first pro- 



