128 PHILOSOPHICAL TRANSACTIONS. [aNNO 1771. 



hygrometer. The point of O denotes perfect dry; and the numbers increase 

 with the degrees of moisture to 100, which denotes perfect wet. 



On comparing them for some time, when hung up near together in a passage 

 or stair-case, where ihey would be very little affected by fire, and where they 

 would be exposed to as free an air as possible in the inside of the house, he found 

 that they generally were within one degree, and very rarely differed 2 degrees; 

 but, as these comparisons necessarily took up some time, and were frequently 

 interrupted by long avocations from home, it was some years before he could 

 form a tolerable judgment on them. One thing he soon observed, not altoge- 

 ther to his liking; which was, that the flaxen cords which he made use of, 

 seemed to make so much resistance to the entry of small degrees of moisture, 

 such as is commonly experienced within doors in the situation above-mentioned, 

 that all the changes were comprised within the first 30 degrees of the scale; but 

 yet, on exposing them to the warm steam of a wash-house, the index quickly 

 mounted to 100. He was therefore desirous of impregnating the cords with 

 something of a saline nature, which should dispose them more forcibly to attract 

 moisture; in order, that the index might, with the ordinary changes of moisture 

 in the atmosphere, travel over a greater part of the scale of 100: how to do 

 this in a regular and fixed quantity, was the subject of many experiments, and 

 several years interrupted inquiry. At last, he tried the one hereafter described, 

 which seemed to answer his intentions in a great measure; and though, on the 

 whole, it does not appear likely, that this instrument will ever be made capable 

 of so accurate an agreement, as mercurial thermometers are made to be; yet if 

 we can reduce all the disagreements of an hygrometer within ^^ part of the 

 whole scale, it will probably be of use in some philosophical inquiries, instead of 

 instruments which have not as yet been reduced to any common scale at all. 



The description of the hygrometer, as to its mechanical structure, is then 

 given, but is not necessary to be here retained. 



The cord here made use of is of flax, and between -^l. and ^ of an inch in 

 diameter; which can readily be ascertained by measuring a number of turns 

 made round a pencil or small stick. It is a sort of cord used in London for 

 making nets, and is of that particular kind called by net makers, " flaxen three 

 threads laid." Mr. S. does not imagine that the fabric of the cord is of the 

 most njaterial consequence; but yet he supposes, when cords can be had of si- 

 milar fabric, and nearly of the same size, that some small sources of variations 

 will be avoided. In general, he thinks, that the more cords are twisted, the 

 more they vary by different degrees of moisture, and the less we are certain of 

 their absolute length; therefore those moderately twisted, he supposes are likely 

 to answer best. 

 I A competent quantity of this cord was boiled in one pound avoirdupois of 



