METEOROLOGICAL INSTRUMENTS. 137 



currents of air. Should the wind be strong upon the instru- 

 ment, the " degrees of dryness observed," multiplied by two, 

 gives the " absolute dryness" (the " excess of dryness" being 

 omitted in the calculation), because a strong current of air 

 makes the instrument indicate the excess of dryness, which 

 is necessary to be added in a calm atmosphere. 



If the absolute dryness of an apartment be required, the 

 instrument must be placed in the shade and the dew-point 

 found, which, subtracted from the temperature of the apart- 

 ment, will give its absolute dryness. The reason is obvious, 

 and arises from this law, namely, that air has its dryness 

 doubled for every increase of temperature corresponding to 

 21 of Fahrenheit's thermometer, and in proportion for all 

 intermediate temperatures. 



It will detect the dampness of an apartment or bed. 



The facility of registering meteorological observations by 

 this instrument, will probably induce many to avail them- 

 selves of its use, and tend to enlarge that branch of science. 



In regulating the hygrometrical state of the air in hot- 

 houses, green-houses, &c., as well as in manufactories, ware- 

 houses, malting-houses, and in the laboratory of the chemist, 

 its use will be duly appreciated. 



The advantages it offers at sea cannot be too forcibly 

 pointed out. The objections made by naval men to the 

 barometer, leaving them in doubt whether to expect wind 

 or rain by the fall of the mercury, would in a great measure 

 be obviated by the joint observation of the hygrometer, 

 because, if it indicate a relative degree of dryness, wind alone 

 may be expected ; if the contrary, then rain, or rain with 

 wind, follows. 



N. B. The silk which covers the wet bulb, and thread 

 which conveys the water to it, requires renewal about every 

 month, and the fountain to be filled, when requisite, with 

 distilled water, or water that has been boiled and allowed 

 to cool, by immersing it in a basin of water till the aperture 

 only is just upon the surface, and the water will flow in. 

 If the hygrometer is placed out of doors in frosty weather, 

 the fountain had better be removed, or the freezing of water 

 within it may break it ; in this case a thin coating of ice 

 may soon be formed on the wet bulb, which will last a con- 

 siderable time, and may be renewed when requisite. 



Price, - - $3.50 to $5.00. 



" best article in rosewood case, lined with velvet, 17.00. 

 12* 



