MISCELLANEOUS ARTICLES USED IN HORTICULTURE. 127 



most satisfactory to learn that Mr. Glaisher's mode of calculating the 

 dew point from their two readings of temperature, has been found 

 substantially to agree with the best hygrometers, up to balloon eleva- 

 tions of 20,000 feet above the level of the sea. 



The bulb of one thermometer is kept wet by being covered with 

 thin muslin, round the neck of which is twisted a conducting thread 

 of common darning-cotton, floss-silk, or lamp-wick ; this passes into a 

 vessel of water, with a conducting thread of about three inches long. 

 The cup or glass should be placed on one side or a little beneath, so 

 that the water may not be placed so close to the dry bulb as to affect 

 its reading. The following minute instructions, and the position and 

 precautions for using the wet and dry bulb thermometer, are taken 

 from Mr. Glaisher's Tables. 



" The instrument should be mounted in an open space with the bulb 

 raised about four feet above the soil, in the shade, at some little distance 

 from walls, trees, &c. The water-vessel or reservoir should always be 

 supplied with rain or distilled water. If the temperature of the air be 

 below 32, it will frequently happen that the wet bulb thermometer will 

 for a time read higher than the dry bulb ; such observations must not 

 be recorded ; when the water surrounding the wet bulb has begun to 

 freeze, the proper readings will take place. In frosty weather the 

 water in the reservoir will be frozen, but this is no reason for sus- 

 pending observations ; if the water upon the muslin be frozen at the 

 same time, the readings are perfectly available. If the muslin be dry 

 it is necessary that it be wetted by means of a sponge or brush 

 by the observer, who should leave it a sufficient time to allow the 

 water to become frozen, and who (having satisfied himself of the 

 fact) will proceed to take the reading in the usual way ; unless this 

 caution be attended to, the wet bulb will read higher than the dry. 

 When the weather is frosty, the muslin should be wetted a sufficient 

 time before the appointed hour of observation, and as a rule in frosty 

 weather it is desirable to immerse the bulb and conducting thread in 

 water after every observation. If the temperature of the air should 

 have ascended above 32, immerse the wet bulb thermometer for a 

 short time in warm water, so as to melt any ice which may remain ; 

 unless this be attended to, the wet bulb will read 32 so long as any 

 ice is in contact with it. Before use the cotton lamp-wick should be 

 washed in a solution of carbonate of soda, and pressed while under water 

 throughout its length. In use it should be of such extent that the 

 water conveyed be sufficient in quantity to keep the muslin on the 

 bulb as moist as when the air is saturated with vapour. The amount 

 of water supplied can be increased or diminished by increasing or 

 decreasing the extent of the conducting thread. 



" In observing, the eye should be placed on a level with the top of 

 the mercury in the tube ; and the observer should be careful to 

 abstain from breathing while taking the observation." 



Temperature of the air and of evaporation are given by the readings 

 of the two thermometers. 



