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the amount of humidity in the atmosphere. Considerable care 

 should be bestowed upon the wet bulb in order to obtain accurate 

 indications. A strip of fine mull muslin about four inches long and 

 just wide enough to go once round the bulb, with a quarter-inch 

 overlap, should be provided. Obtain the services of an assistant to 

 hold the thermometer, and after wetting the muslin wrap one end 

 round the bulb, leaving the other end to hang down. Now tie a 

 piece of cotton round the stem (and muslin) above the bulb, and 

 moderately tightly round the muslin underneath the bulb. The 

 muslin should fit tightly and not be " baggy." The loose end which 

 hangs vertically should be placed in a cup of water. The surface 

 of the water should be i^ inches below the bulb, and the cup should 

 be frequently refilled as the liquid evaporates. The muslin must be 

 kept clean and renewed frequently (at least once a week in summer). 

 This hygrometer will be found a valuable auxiliary to the barometer, 

 but notwithstanding this the u farmer's barometer," where the 

 three instruments are combined, cannot be recommended, as the 

 dry and wet bulb require to be exposed outside, whereas a 

 barometer is usually mounted inside the house. It will be noticed 

 that on a dry summer day when the dry bulb reads 100 deg. the 

 difference between the dry and wet amounts to as much as 30 cleg. 

 This, of course, indicates that the atmosphere is very dry, and as a 

 matter of fact, under these conditions it is capable of holding nearly 

 five times as much aqueous vapour before condensation occurs. As 

 a general rule the less the difference between the dry and wet bulbs 

 the more humid is the atmosphere, but the actual amount of humidity 

 can be obtained from Symon's hygrometrical tables. 



In addition to the hygrometer it is well to have a pair of self- 

 registering thermometers, maximum and minimum, and I should 

 recommend Negretti and Zambra's form of maximum (in fact I can 

 confidently recommend these makers for all thermometers), and 

 strongly advise that they be graduated on the stem, not on the 

 mount. In this instrument there is a slight contraction of the tube 

 just above the bulb. The thermometer is mounted almost 

 horizontally, but with the bulb end slightly lower (about { inch) than 

 the other, and acts as an ordinary dry bulb as long as the tempera- 

 ture is increasing, but the moment it starts to decrease a small break 

 is noticeable in the mercury column at the point of contraction. 

 The force of cohesion being insufficient to overcome this extra 

 friction, the mercury behind the break flows gradually down toward 

 the bulb, whilst that in front remains in position, thus indicating the 

 highest point reached. The minimum thermometer has a spirit 

 column in the place of a mercurial, and a black glass index moves 

 freely in the spirit, but cannot, without violence, enter the vacuum 

 at the top of the tube. As the temperature decreases, the index is 

 carried down by the spirit until the lowest point is readied, and as 

 the atmosphere then gets wanner the spirit Hows past, leaving the 

 index to record the lowest temperature attained. The question of 



