Vol. lxi.] philosophical transactions. 127 



1770, of the highest and lowest states of the barometer and thermometer, with 

 the monthly quantity of rain, the sum of which for the whole year amounts to 

 44.64 inches. 



XX ir. Description of a New Hygrometer. By Mr. J. Smeaton, F.R.S. p. I98. 



Mr. S. having some years before attempted to make an accurate and sensible 

 hygrometer, by means of a hempen cord, of a very considerable length ; he 

 quickly found, that though it was more than sufficiently susceptible of every 

 change in the humidity of the atmosphere, yet the cord was, on the whole, in 

 a continual state of lengthening. Though this change was the greatest at first, 

 yet it did not appear probable that any given time would bring it to a certainty; 

 and it also seemed, that as the cord grew more determinate in mean length, the 

 alteration by certain differences of moisture became less. Now as, on consi- 

 dering wood, paper, catgut, &c. there did not appear to be a likelihood of find- 

 ing any substance sufficiently sensible of differences of moisture, that would be 

 unalterable under the same degrees of it; this led him to consider of a construc- 

 tion which would readily admit of an adjustment; so that, though the cord by 

 which the instrument is actuated may be variable in itself, both as to absolute 

 length, and difference of length under given degrees of moisture, yet that, on 

 supposition of a material departure from its original scale, it might be readily 

 restored to it, and in consequence that any numbers of hygrometers, similarly 

 constructed, might, like thermometers, be capable of speaking the same language. 



The 1 points of heat the more readily determinable in a thermometer, are the 

 points of freezing and boiling water. In like manner, to construct hygrometers 

 which shall be capable of agreement, it is necessary to establish 2 different de- 

 grees of a moisture which shall be as fixed in themselves, and to which we can 

 as readily and as often have recourse as poshible. One point is given by making 

 the substance perfectly wet, which seems sufficiently determinable; the other is 

 that of perfect dry; but which* he does not apprehend to be attainable with the 

 same precision. A readiness to imbibe wet, so that the substance may be soon 

 and fully saturated, and also a facility of parting with its moisture, on being 

 exposed to the fire to dry; at the same time that neither immersion in water, 

 nor a moderate exposition to the warmth of the fire, shall injure its texture; are 

 properties requisite to the first mover of such an hygrometer, that in a manner 

 exclude all substances that he was acquainted with, besides hempen and flaxen 

 threads or cords, and what are conr<pounded of them. 



Oh these ideas, in the year 1758, Mr. S. constructed 2 hygrometers, as nearly 

 alike as possible, that he might have the means of examining their agreement or 

 disagreement on similar or dissimilar treatment. The interval or scale between 

 dry and wet, he divided into 100 equal parts, which he calls the degrees of this 



