H YD 



319 



H YG 



column, forcing up the pulse-valve, 

 shuts it. Practice shows that a ten-feet 

 fall vi'ill raise a column of water one 

 hundred and fitly feet high, at the rate 

 of five quarts per minute, or one part 

 raised to eleven wasted, where the ram 

 is only supplied by a two-inch pipe. I 

 may further add, that theory teaches 

 that a ten-feet fall will raise water three 

 hundred feet high — of course, in a very 

 small quantity. Mr. II. P. M'Birkin- 

 brine, of Philadelphia, has been very 

 successful in the construction of this 

 valuable power. 



IIYUROCHAPJS morsmran(r. Hardy 

 aquatic. Seed and runners. Stillwater. 

 HYDROLEA. Two species. One 

 stove evergreen shrub, and the other 

 stove herbaceous. Cuttings. Loam 

 and peat. 



HYDROPELTIS purpurea. Half- 

 hardy aquatic. Offsets. Still water. 



HYGROMETER is an instrument 

 deserving of employment in the stove, 

 green house, and conservatory nearly 

 as much as the thermometer ; for the 

 correct degree of dampness of the air is 

 of very great importance in the cultiva- 

 tion of plants, and scarcely less than 

 that of the temperature in which they 

 vegetate. 



The perspiration from the leaves of 

 plants increases with the air's dryness, 

 and decreases w-ith its moistness. If it 

 be excessive, not only are their juices 

 too much reduced, but the very texture 

 of the leaves is destroyed. If, on the 

 other hand, tlie perspiration is prevent- 

 ed, the juices are too watery, and the 

 secretions and assimilations arc devoid 

 of consistency, rendering the plants too 

 succulent and weak. 



" It is impossible for any one to know 

 what degree of moisture he really main- 

 tains in a forcing-house without an in- 

 strument by which to measure it : that 

 instrument is the hygrometer, which 

 might as well be called the 'water- 

 gauge,' which is what the first word 

 really means. Of the many contriv- 

 ances to effect this end, the best for all 

 practical purposes, is Daniell's Hygro- 

 meter, of which the annexed cut (Fig. 

 93) exhibits the general appearance. 

 It measures the moisture in the air 

 quickly and precisely, and is not sub- 

 ject to get out of order. 



" If moisture is brought into contact 

 with a substance sufficiently cold, a part 

 of the moisture is condensed, and is so 



converted from a state of invisible va- 

 pour into water. 



" Thus, in a cold day, the glass roof 

 of a green-house may be seen streaming 

 with water, which runs down and forma 

 'drip;' and in this often unsuspected 

 manner air is rendered dry, notwith- 

 standing the operations of syringing, 

 steaming, &c. Daniell's Hygrometer is 

 constructed with reference to this cir- 

 cumstance. The figure represents two 

 hollow glass balls containing ether, and 

 communicating by the glass tube which 

 rests on the support. The ball which 

 forms the termination of the longer 

 leg is of black glass, in order that the 

 formation of dew on its surface may be 

 the more perceptible. It includes the 

 bulb of a delicate thermometer dipping 

 in the ether, its scale being inclosed in 

 the tube above the ball ; and whatever 

 change takes place in the temperature 

 of the ether is indicated by this thermo- 

 meter. The other ball is covered with 

 muslin. In making an observation it is 

 first necessary to note down the temper- 

 ature of the air ; next turn the instru- 

 ment, so that when the muslin-covered 

 ball is held in the hand, the ether may 

 escape into the blackened ball ; and it 

 should also be held till the included 

 thermometer rises a few degrees above 

 the temperature of the air, when it 

 should be replaced on the support. 

 Then drop, or gently pour, a little ether 

 on the muslin. The evaporation of this 

 extremely volatile substance produces 

 cold ; and attention must be instantly 

 directed to the black glass ball and in- 

 cluded thermometer. The latter will 

 be seen falling rapidly ; and at length 

 a ring of dew will appear at the line 

 which runs across the black ball — 

 quickly, if the air is very moist, slowly, 

 if the air is dry. If the air is very dry, 

 no moisture will be thus deposited till 

 the thermometer falls to, perhaps, 10", 

 20^, or 303 below the temperature of 

 the air. But at vv'hatever temperature 

 the dew forms, that temperature should 

 be noted as the dew-point; and the dif- 

 ference between it and the temperature 

 of the air, at the time, is the degree of 

 dryness according to the indications of 

 this hygrometer. Thus, in a moderately 

 dry day, let it be supposed that the 

 temperature of the air is 6')^ in the 

 shade, and that the muslin requires to 

 be kept moist, before dew is formed, 

 till the blackened ball containing the 



