16 SCOTT. — METEOROLOGICAL INSTRUCTIONS. 



By this method, even in full sunshine, a very close approxima- 

 tion to the true temperature of the air may be made. 



Radiation. — The radiation thermometers should be placed in the 

 clips sent with them, and attached to an upright post at the 

 height of 4 feet above the ground. Alongside of the black bulb 

 thermometer in vacuo should be placed a bright bulb thermometer 

 also in vacuo, which should be read at the same time, so as to 

 obtain a measure of the radiation by the diiference between the 

 simultaneous readings of the two instruments. 



It will further be interesting to erect a black board, say 2 

 feet square, at a level of 4 feet from the ground, and to place 

 a black bulb in vacuo a few inches above this board, which will 

 thus intercept all heat radiated from the ground. 



For terrestrial radiation, a board about 2 feet square should 

 be placed upon the ground, its upper surface being fully exposed 

 to the sky. A slight groove in the board should mark the place 

 where the bulb of the thermometer, a minimum thermometer, 

 should be placed. 



Hygrometry. — Two hair hygrometers are supplied, which 

 should be erected in the thermometer screen. They should be 

 read at the same hour as the ordinary instruments. 



The principle of these instruments is as follows, the hygrome- 

 trical condition of the air is given by the elongation or contraction 

 of a hair, according as the air is moist or dry. 



They bear two scales on the same arc. 



The lower scale (Saussure's) divided into equal parts — 100 

 gives the so-called scale of absolute humidity ranging from per- 

 fect dryness to saturation. The upper scale, divided into unequal 

 parts, — 100 gives the relative humidity in per-centages. The 

 upper scale is that which is alone to be used. 



The following is the mode of setting the instrument; inasmuch 

 as it easily gets out of adjustment in carriage, the following rules 

 are to be observed : On a day of heavy rain or thick fog, when the 

 air is perfectly saturated with moisture, the screw at the top is to 

 be turned gently until the index comes to stand at 100 on the 

 scale. If perfect saturation does not occur, the index should be 

 brought by turning the screw to the point on the scale which gives 

 the per-centage of relative humidity shown by careful observation 

 of the wet and dry bulb hygrometer. 



The instrument is to be suspended at some little distance, say 

 4 inches, in front of the upright to which it is attached. When 

 the instrument has been originally set to be correct at 100 or at any 

 definite degree of relative humidity, its indications should be 

 carefully checked for a few days, and the position of the index 

 corrected by means of the screw, when it will soon attain the 

 requisite degree of consistency in its behaviour. 



The regulations for observing the instrument, as in force at the 

 Russian stations, where hair hygrometers are generally in use in 

 winter, are as follows : Whenever the temperature is above 32°, 

 the hygrometrical observations entered in the register, are to be 

 recorded by means of the wet and dry bulb hygrometer exclusively, 

 and the degree of humidity given in the Tables (Glaisher's 5th 



