THE THERMOMETER 13 



For meteorological purposes it is the temperature of the 

 air which is required. At night, or on a cloudy day, when 

 the air is moving, the temperature may be easily found ; but 

 when the day is calm, and the solar radiation intense, it is no 

 easy matter to obtain the temperature of the air. Uniformity 

 of exposure therefore becomes very important, so that we may 

 at least compare the temperature of different places. 



Stevenson's screen for thermometers is now very generally 

 adopted. It is a light wooden box, whose sides are made of 

 double jalousie- work, so that the rays of heat from the sun or 

 ground cannot reach the thermometers, and so that the air 

 can freely circulate about them. The screen is supported by a 

 firm wooden stand; it is freely exposed to the air on a grass 

 lawn ; and its height is such that the bulbs of the thermometers 

 are 4 feet G inches above the surface of the ground. 



There are two important modifications of the ordinary 

 thermometer which may be briefly alluded to ; the first is 

 adapted for the self-registration of the highest temperature 

 reached by the thermometer in any given interval of time. 

 In the ordinary thermometer a small piece of steel or enamel 

 is placed in the empt}- part of the tube ; this index is free to 

 move in the tube ; it is brought into contact with the mercury 

 during the cooler part of the day, and the tube is placed in a 

 horizontal position. As the mercury expands the index is 

 pushed forward to the highest point, and is left remaining 

 there after the mercury contracts. This modification is 

 called the maximum thermometer, and it is registered once 

 in every twenty-four hours, so as to obtain the highest 

 temperature of the day. There are, however, several forms 

 of this instrument. 



The other modification is called the minimum thermometer, 

 as it registers the lowest temperature of the day. In this 

 instrument alcohol is used instead of mercury, and the index 

 is placed in the alcohol in the tube ; as the alcohol contracts 

 it drags the index down to the lowest point, and as it expands 

 it passes by the index, leaving it stationary at the lowest 

 point. The difference between the highest and lowest 

 temperatures of any day is called the Eange. 



Now, the mean temperature of any day may be found with 

 considerable accuracy by merely subtracting V from half the 



