\813.] Scientific Intelligence. 4(19 



ivory, about an inch and quarter in length, is turned into an 

 elongated spheroid, as thin as possible, weigliing only eight or 

 ten grains, but capable of containing, at its greatest expansion, 

 about 300 grains of mercury and the upper end;, which is 

 adapted to the body by means of a delicate screw, has a slender 

 glass tube inserted six or eight inches long, with a bore of nearly 

 the 15th part of an inch in diameter. The instrument being 

 fitted together, its eliptical shell is dipped into distilled water, or 

 lapped round with a wet bit of cambric, and after a considerable 

 interval of time filled with mercury to some convenient point 

 near the bottom of the tube, where is fixed the beginning of the 

 scale. The divisions themselves are ascertained by distinguishing 

 the tube into spaces which correspond each to the thousandth 

 part of the entire cavity, and therefore equal to tbe measure of 

 about three-tenths of a grain of mercury. The ordinary range 

 of the scale will include 70 of these divisions. To tlie upper 

 end of the tube is adapted a small ivory cap, which allows the 

 penetration of air, but prevents the escape of mercury, and 

 thereby renders the instrument quite portable. 



This hygroscope is largely, though rather slowly, affected by 

 any change in the humidity of the ambient medium. As air 

 becomes drier it attracts a portion of moisture from the shell or 

 bulb of ivory, which, suffering in consequence a contraction, 

 squeezes the mercury so much higher in the tobe. But when 

 the air inclines more to dampness, the "thin bulb imbibes 

 moisture, and swells proportionally, allowing the quicksilver to 

 subside towards its enlarged cavity ; but these variations are very 

 far from corresponding with tlie real measures of atmospheric 

 dryness and humidity. Near the point of extreme dampness the 

 alterations of the hygroscope are much augmented, but they 

 diminish rapidly as the mercury approaches the upper part of the 

 scale. The contraction of the ivory answeripg to an equal rise 

 in the dryness of the air is six times greater at the beginning of 

 the scale than at the 70th hygroscopic division, and seems io 

 general to be inversely as the number of hygrometic degrees, 

 reckoning from 20 below. Mr. Leslie, therefore, places another 

 scale along the opposite side of the tube, the space between 0 

 and 70° of the hygroscope being divided into 100°, and cor- 

 responding to the unequal portions from the number 20 to 120 

 ©n the logarithmic line. 



VI, Moisture aLsorled ly various Bodies from the Ah\ 



Mr. Leslie has ascertained by experiment that 100 grains of 

 the following bodies in the like circumstances absorb the follow- 

 ing quantity of moisture from the atmosphere;-— 



