VOL. I.XI.3, i PHILOSOPHICAL TRANSACTIONS. 12^ 



water, in which was put 2 dwts. troy of common salt ; the whole was reduced by 

 boiling to 6§ avoirdupois, which was done in about half an hour. As this ascer- 

 tains a given strength of brine on taking out the cord, it may be supposed that 

 every fibre of the cord is equally impregnated with salt. The cord being dried, 

 it will be proper to stretch it; which may be done so as to prevent it from un- 

 twisting, by tying 3 or 4 yards to 2 nails, against a wall, in a horizontal posi- 

 tion, and hanging a weight of a pound or 2 to the middle, so as to make it form 

 an obtuse angle. This done for a week or more in a room, will lay the fibres of 

 the cord close together, and prevent its stretching so fast after being applied to 

 the instrument, as it otherwise would be apt to do. 



Adjustment of the hygrometer. — The box cover being taken off, to prevent 

 its being spoiled by fire, and chusing a day naturally dry, set the instrument 

 nearly upright, about a yard from a moderate fire; so that the cord may become 

 dry, and the instrument warm, but not so near as would spoil the finest linen 

 by too much heat, and yet fully evaporate the moisture; there let the instru- 

 ment stay, till the index is got as low as it will go, now and then stroking the 

 cord between the thumb and finger downwards, in order to lay the fibres close 

 together, and thereby causing it to lengthen as much as possible; when the index 

 is thus become stationary, which will generally happen in about an hour (more 

 or less as the air is naturally more or less dry) by means of the peg at top raise 

 or depress the index, till it lays over the point O; this done, remove the instru- 

 ment from the fire, and having ready some warm water in a teacup, take a 

 middling camel's hair pencil, and dipping it in the water, gently anoint the cord, 

 till it will drink up no more, and till the index becomes stationary, and water 

 will no more have effect on it, which will also generally happen in about an hour. 

 If in this state the index lays over the degree marked 100, all is right; if not, 

 slack the screw, and slide the scale nearer to or farther from the centre, till the 

 point 100 comes under the index, and then the instrument is adjusted for use; 

 but, if the compass of the slide is not sufficient to effect this, as may probably 

 happen on the first adjustment, slack the proper screws, and move the sliding 

 stud nearer to or farther from the centre of the index, according as the angle 

 formed by the index, between the points of dry and wet, happens to be too small 

 or too large for the scale; the quantity can easily be judged of, so as the next 

 time to come within the compass of the slide of the scale; the quantity of slide 

 being 4- of the length of the index, and consequently its compass of adjustment 

 4- of the whole variable quantity. Now as sliding the stud will vary the position 

 of the index respecting the point of O, this movement is only to be considered 

 as a rough or preparatory adjustment, to bring it within the compass of the slide 

 of the scale; which will not often happen to be necessary after the first time; 



VOL. XIII. S 



