172 PHILOSOPHICAL TRANSACTIONS. [aNNO 1685. 



a hook A. At the end of this whipcord hangs the weight C, about a pound or 

 something more; this weight is so fitted at the end as to receive and carry the 

 index D; under these is placed a graduated circle on the board EF, fixed by a 

 bracket against the wall. All things being thus adapted, the moisture of the 

 air twists the cord, and gives a motion to the index over the divisions in the 

 graduated circle ; and again, as the air grows more dry, the cord untwists, and 

 brings back the index by a contrary motion. 



What first gave me the idea of this, was the observing all cords tied at both 

 ends to be much more tight and stretched harder after rain has fallen on them, 

 than before; I concluded that if I could, as it were, tie a cord at both ends, 

 and yet give one end a liberty of circumvolution, it would perform the desired 

 effect. Now the weight C hung at the cord does this; for it fixes, as it were, 

 the end of the cord B, and yet it permits it to twist and untwist. And the 

 reason of this is plain; for the little particles of moisture, insinuating and soak- 

 ing into the cord, are like so many wedges, which must needs shorten it, as a 

 bladder is shortened by being blown up, and will lift a great weight. But the 

 easiest way for the cord AB to shorten, and raise up the weight C, is to do it 

 as a screw; for itself is a screw, its strands being twisted, and each particular 

 thread in it screw-wise, and consequently must give a circular motion to the index. 



To make an experiment of this, I wetted a cord and hung it up with the 

 weight at the end of it, and I perceived as it dried it untwisted, and that too 

 very quick, so as to be perceived by the eye; after the cord had so far untwisted 

 as I thought it had come to that degree of dryness that the present constitution 

 of the air would permit, I took a basin of warm water, which emitted a steam 

 and fume, and placed it under the cord ; immediately the cord began to twist 

 again very quick, and so continued till the water ceased fuming or was removed, 

 and then immediately it began to untwist again. I then breathed on it gently 

 with my breath, and found that 8 or 10 breathings would twist it 5 degrees of 

 a circle. I then left it to the air only, and for these last three weeks have ob- 

 served its motion as affected by its moisture and drought, and obeying its alte- 

 rations very nicely, not the least shower falling, at which it does not presently 

 twist; and when by rising clouds a fair day becomes overshadowed, the cord is 

 immediately sensible thereof, and again as sensible of their vanishing and change 

 to fair sunshine. So that I have seen it show alteration, when not the least 

 could be collecting from the sweating of stones, cracking of wainscot, &c. 

 And hence it seems to be the nicest hygrometer, that has ever yet been used. 



Even by applying a candle or heated iron near the cord, it makes it twist very 

 quick, contrary to Mr. Hook's oat-beard. 



One of the grand defects of most former hygrometers is, that they grow 



