62 



DESCRIPTION OF A HYGROMETER. 



is nearly uniform, will, if the air becomes 

 too dry, restore it to that degree of moisture 

 that has been previously determined upon as 

 the most desirable. 



In the annexed figure. A, B, and C, repre- 

 sent standards of wood which support the ap- 

 paratus ; E is a lever having its fulcrum at 

 F. Upon the top of the perpendicular rod 

 H, which rests upon the horizontal lever, a 

 light open-mouthed vial or cup, I, is placed, 

 having an orifice and lip at its side near the 

 top ; this contains about a gill of w'ater, its 

 weight and the weight of the long arm of the 

 lever being balanced by the counterpoise Gr, 

 J is a slip of bass wood, (Tilia Americana,) 

 four feet in length, two inches in breadth, and 

 one-eighth of an inch thick, the grain running 

 in the direction of its breadth, or horizontally. 

 This wood should be perfectly sound, of 

 straight grain, but not of very close texture, 

 its exterior being made rough by the use of 

 coarse glass paper, in order to expose a larger 

 surface to the air. At the l>ottom of the 

 standard B, is a nut and adjusting screw, W, 

 to which the slip of bass wood is attached ; a 

 plate of brass is screwed to the top of the 

 wood, and is fastened by a pin to the lever 

 E ; from this lever a silk thread is carried 

 around the pulley L, in a spiral groove ; the 

 axis of this pulley passes a dial plate and car- 

 ries an index. At N is a vessel of water, 

 having a proper supply pipe leading to it, the 

 water being retained at a uniform level by the 

 waste pipe P, and connected with the water 

 in the small vessel I, by a syj'hon. The re- 

 spective length of the legs of the syphon is 

 immaterial, as the flow of the water depends 

 upon the relative level of the surface of the 

 water in the resei'voir, and that in the cup ; 

 both legs being immersed, the syphon re- 

 mains constantly filled with water. 



To put the instrument in operation, let the 

 apartment be kept at its mean temperature, 

 and at the degree of moisture rec{uired, both 

 of the vessels and the syphon being filled 

 with water. After the instrument has been 

 exposed to this atmosphere a short time, turn 

 the screw at W until the orifice in the vessel 

 I is level with the top of the waste pipe P ; 

 then turn the pully L, and set the index at 



zero. The instrument being thus adjusted, 

 if the air becomes more dry the slip of wood 

 contracts, the small cup descends, and water 

 flows into it from the stationary reservoir ; 

 the water then escapes from the orifice in the 

 side of the cup, falls into a funnel, and thence 

 into a pipe, which, when the insti'ument is 

 used in green-houses, conveys the water to 

 evaporating pans placed upon the flues, or to 

 a horizontal pipe, having openings at its up- 

 per surface at suitable intervals, from which 

 the water falls at any point desired. As the 

 water thus discharged evaporates, the sur- 

 rounding air is moistened, the wood is ex- 

 panded, and when it attains its original 

 length, the orifice in the cup is again raised 

 to the level of the surface of the water in the 

 reservoir, and the flow of the water ceases. 

 If, from any cause, the air should become too 

 moist, the only efiect produced is, that the 

 cup continues to rise, and a small C|uantily o^ 

 water flows back to the large reservoir. 



If, by accident, the air should become very 

 dry, the further contraction of the wood 

 causes a larger quantity of water to issue 

 from the cuj) than is due to this change in the 

 hygrometric state of the air, the discharge 

 being accele7-ated as the difference of level 

 between the two surfaces increases ; thus the 

 return of the air towards its normal condition 

 is the most rapid at the time when it is of the 

 most importance that the moisture should be 

 restored. The same degree of moisture may 

 be made to discharge a greater or less quan- 

 tity of water, by moving the fulcrum of 

 the horizontal \e\ev to the right or to the 

 left. 



The opening and closing of valves and stop- 

 cocks is attended with nuich friction, but in 

 this instrument, it will be observed that the 

 water flows through the syphon with a very 

 small amount of friction, and that little power 

 is required to depress the cup. This plan 

 may, therefore, be used to advantage in many 

 instruments in which the motive power is 

 small, the weight of the water discharged 

 being applied to produce more extended or 

 more fitrcible movements. 



f John M. Baciielder. 



Boston, ilfajy 22, 1S50. 



