334 



THE CANADIAN HORTICULTURIST. 



fall rapidly until dew begins to gather, at 

 which point a further drop in temperature 

 is prevented or retarded by the condensa- 

 tion of moisture. The dew point, therefore, 

 that is, the temperature at which dew be- 

 gins to gather, is a good indication of the 

 probability of frost. If the dew-point is 

 40 degrees F. or above at sunset, there is 

 not likely to be frost that night. If, how- 

 ever, the dew-point is below 40 degrees, 

 precautions should be taken to prevent the 

 injury of tender flowers, vegetables, or fruit 

 buds. 



FINDING THE DEW FOINT. 

 Following is a method for determining 

 the dew point by means of the wet and dry 

 bulb thermometers or sling psychrometer : 

 Two ordinary all-glass thermometers, with 

 cylindrical bulbs, are strapped securely to 

 the edges of a thin board about 12 inches 

 long and two and a half inches wide, so that 

 the bulbs project below the board. About 

 the bulb of one of the thermometers one fold 

 of a fine piece of cotton or muslin is wrap- 

 ped closely and tied tightly at the top. To 

 use the psychrometer the covered bulb 



should be moistened with water, and by- 

 means of a cord fastened to the top of the 

 board. The psychrometer is swung to and 

 fro for a minute or so in the shade in the 

 outside air, until the reading of the wet 

 bulb is stationary. The two thermometers 

 should be read quickly after the swinging 

 ceases and the readings recorded. The 

 depression of the wet bulb reading below 

 that of the dry is the amount of cooling pro- 

 duced by evaporation and indicates the 

 humidity of the air. From these readings 

 the dew point can be determined from the 

 table given below. 



To illustrate the use of the table : Sup- 

 pose that the air temperature, the dry bulb, 

 reads 55 degrees, while the wet bulb reads 

 47 degrees, a depression of 8 degrees, the 

 table gives a dew point of 39 degrees. If 

 this condition exists at sunset, with a clear 

 sky and little or no wind, there is likelv tO' 

 be frost. 



THE ELECTRIC ALARM. 



Fig. 2 illustrates a simple and inexpensive 

 arrangement for giving warning when a 

 certain temperature, say ^2 degrees, has"- 



Thermometer 

 Dry- Bulb 



65 deg 



64 deg- 



63 deg 



62 deg- 



61 deg 



60 deg 



59 deg 



58 deg 



57 deg 



56 deg 



55 deg 



54 deg 



53 deg 



52 deg 



51 deg 



50 deg 



49 deg 



48 deg 



47 deg 



46 deg 



45 deg 



44 deg 



43 deg 41 



42 deg 40 



41 deg 39 



40 deg 38 



DEPRESSION 



OF THE WET BULB THERMOMETER. 



4' -o- 6' 7= 8° 9" 10^ 



41 

 40 

 39 

 38 

 37 

 36 



41 



40 

 39 

 38 

 37 

 36 

 35 

 33 



41 

 40 

 39 

 38 

 37 

 36 

 35 

 34 

 33 

 29 



41 

 40 

 39 

 38 

 37 

 36 

 35 

 33 

 32 

 31 

 29 

 26 



41 

 40 

 39 

 39 

 37 

 36 

 32 

 33 

 32 

 30 

 29 

 28 

 26 

 23 



41 

 40 

 39 

 38 

 37 

 37 

 34 

 33 

 29 

 30 

 29 

 27 

 26 

 25 

 23 

 19 



41 

 40 

 39 

 38 

 37 

 36 

 34 

 33 

 31 

 30 

 25 

 27 

 26 

 24 

 23 

 21 

 19 

 15 



42 

 41 

 40 

 39 

 37 

 36 

 34 

 33 

 31 

 30 

 28 

 27 

 22 

 24 

 22 

 20 

 19 

 17 

 15 

 9 



42 

 41 

 40 

 39 

 37 

 36 

 34 

 33 

 31 

 30 

 28 

 27 

 25 

 23 

 19 

 20 

 IS 

 16 

 14 

 12 

 9 



ir 



42 

 41 

 40 

 39 

 38 

 36 

 35 

 33 

 31 

 30 

 28 

 27 

 25 

 23 

 21 

 19 

 17 

 15 

 13 

 11 

 8 

 6 

 3 

 -6 



12' 



41 

 40 

 39 

 38 

 36 

 35 

 33 

 31 

 30 

 28 

 27 

 25 

 23 

 21 

 19 

 17 

 14 

 12 

 10 

 7 

 4 

 1 

 _o 



-6 



-22 



40 



39 



38- 



37 



35 



33 



32 



30 



28 



26 



25 



2? 



2a 



18- 



16 



\i 



11 



9- 



6 



3 



-1 



-5 



-9 



-15 



-22: 



