173 Remarks on Hygrcme try. 



In Plate VI. Fig. 5, AD represents a scale made of brass, 

 writing slate, or marble ; in which the right line EF, nine 

 or ten inches long, is divided into one hundred equal parts* 

 This part of the instrument being prepared, take a square 

 rod of wood, cut across the grain ; it must be four or five 

 inches in length, and not more than a quarter of an inch in 

 diameter : this is represented in the plate by the rectangles 

 PL, OK and MN. One surface of this rod must be 

 marked longitudinally by a coloured line, which is de- 

 noted in the figure by EG and FH. After the rod has 

 been thus marked, it must be suspended for several days 

 in the air of a close vessel containing a quantity of quick- 

 lime or dry potash, either of which will extract the mois- 

 ture from the wood and bring it to a state of great dryness. 

 When the piece is removed from the vessel, apply the 

 end PE to the line PO, making E coincide with the top 

 of the divided line EF, and mark the place of G, the op- 

 posite extremity of the line EG. In the next place, sus- 

 pend the rod for several days in the air of a close vessel 

 containing a quantity of water, by which precaution the 

 wood will become saturated with moisture. This being done, 

 apply the end QF to the right line PO, making F coin- 

 cide with the bottom of the divided line EF, and mark the 

 place of H the other extremity of FH. Lastly, draw the 

 right line GH, which completes the hygrometer. As oft 

 as you wish to make an experiment with this instrument, 

 make the end PE of the rod PL coincide with the divided 

 line EF, taking care that the point G falls in the line 

 GH, and observe the degree marked by E, which denotes 

 the state of the hygrometer. The figure MN shows how 

 this is to be done ; and it is placed in a situation which 

 makes the point Ecoincide with 50°. 



I have not the presumption to compare this apparently 

 insignificant contrivance with the elegant hygrometer of 

 M. De Luc. It has, however, the recommendation of sim- 

 plicity ; and a common artist might construct a number of 

 thein with little trouble and at little expense. They should 

 all be cut from the same board, and made to the same scale; 

 after which, those rods should be furnished with plates re- 

 2 semolina 



