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The Sling Psychrometer.
and the temperature of complete saturation or dewpoint, by measuring the reduction of temperature by evaporation.
"A sling psychrometer can easily be made, as follows For the frame, take a board eighteen inches long, two inches wide, and one-half inch thick, with a hole bored in one end to hang the apparatus on a nail when not in use. Get two all-glass thermometers with cylindrical bulbs, and the degrees Fahrenheit engraved on the stem. Cover the bulb of one thermometer with a thin piece of cotton cloth, fastening it securely by a thread. When this cloth covering is wet with water and exposed to evaporation in the air, it constitutes the 'wet-bulb thermometer'; the other thermometer has no covering on its bulb, is not wet at any time, and constitutes the dry-bulb thermometer'.
"Securely lash the thermometers on opposite edges of the narrow board, leaving the graduations on them plainly in sight, and the bulbs extending a short distance below the end of the board. To use the instrument, wet the cloth-covered bulb ith water, leaving the other bulb dry, and then swing the apparatus freely through the air for three to five minutes, or until the wet-bulb thermometer ceases to fall fall in temperature, and then read the temperature of each thermometer. Unless the air is saturated with moisture the wet-bulb will always show a lower temperature than the dry-bulb. Subtract the degrees of wet-bulb from those of the dry-bulb, and the remainder will show the degrees
Suppose the dry and the wet bulb 56°, then 56° = 9°, or the cold produced by evaporation. This swinging and reading of the pyschrometer are done in the shade in the open air when the temperature of dew-point is sought; it should be done rapidly and the thermometers read promptly The dry-bulb gives the temperature of the open air, and dew-point is determined by reference to tabulated figures." A common form of psychrometer is shown in Fig. 11, but inasmuch as this has a cup of water connected with the wet-bulb, it is not so handy for whirling. Such an instrument may be fanned instead of whirled.
One form of wet- and drybulb thermometer.
Hammon gives the following directions and figures for determining the dew-point : "To obtain
the dew-point from the wet-and dry-bulb hygrometer or psychrometer, moisten the muslin on the
Finding the Dew-point.
wet-bulb and then whirl or fan the instrument, when the temperature will fall. Continue the ventilation until the wet-bulb thermometer ceases to fall, when the two thermometers should be read. Subtract the reading of the wet-bulb thermometer from that of the dry. Find this difference in the column at the left of the table. The dew-point will then then be found at the intersection of the line opposite this difference and the column which is headed by the number nearest the air temperature (dry-bulb reading). Examples are given below:
Difference of reading of dry and wet bulbs.
Dew-point from table, between 27° and 33°, about..
38 43 48 53 58 63 69
35 41 46
10 17 24 30 36 42 48 53
16 24 30 37 43 49 55
20 27 34
16 24 31
40 46 51
41 47 53
28 36 43 49
39 45 51 57
25 33 40 47 53
30 38 45 51
17 27 35 42 49
11 23 32 40 47
5 18 28 37 45
12 24 34 42"
THE TILLAGE OF FRUIT LANDS.
THE study of the evolution of the ideas respecting the tillage of the soil opens one of the most interesting chapters in history. The subject is all the more suggestive because tillage is such a commonplace and almost universal labor that no one thinks of it as having had a history. Yet the practice of the simple stirring of the soil has been slowly evolved, like all other methods and institutions, through a long period of time, and as the result of many forces which were unobserved or even unknown at the time. We think of tillage as a custom; and if one considers the condition of farming at the present moment, he would seem to be warranted in such an association, for a custom is a habit which is not suggested by reason and inquiry. Perhaps the only reason which most persons could give for the tillage of the land is that they are obliged to do it. It would seem to be the simplest and dullest thing to till the soil. It is simply the driving of the animal and the holding of the plow, or taking care that the harrow scarifies the entire surface; it may be only the subborn wielding of the hoe or rake. This view of the matter is wholly correct