Devices and Helps in Nature Study. 199 



by its centre so that the sides and ends hang downward. 

 It may be crumpled into smaller space, but it must be loose, 

 so that the air may reach all parts of it. 



The paper should then be suspended by a thread to the 

 pointed end of the stick. Then the device should be care- 

 fully balanced. The heavy end of the stick will bring the 

 centre of gravity well toward that end. When the appara- 

 tus has been properly balanced, it should be suspended so 

 as to hang in a horizontal position. After hanging a short 

 time the balance will be destroyed. The moisture of the 

 air penetrates the paper, making it heavier. It therefore 

 sinks, and, since the stick is long, the depression is readily 

 seen. Every hydrographic change in the atmosphere may be 

 thus observed and recorded. A very sensitive piece of appa- 

 ratus can be used to measure the moisture in the air of out- 

 doors as compared with that inside of the house. When taken 

 into the cellar, the paper end will soon be depressed. Near 

 a stove it will again rise. This simple device will often 

 indicate the coming of rain also for the same reason. An 

 ingenious pupil arranged a sort of protractor at the point of 

 suspension so that the number of degrees of depression 

 could be determined. Such a device is a sort of hygrometer. 

 A better one consists of balancing a leaden weight at one 

 end and a large sponge at the other. The sponge should be 

 first wet in strong brine and then dried. The salt within 

 the sponge makes it more sensitive to the presence of 

 atmospheric moisture. 



The above device together with the thermgmeter, 

 weather-vane, and weather-pole are very helpful in making 

 observations on the weather. 



The following list of books will be helpful to teachers, 

 but let it not be forgotton that the greatest book on nature 

 study is Nature herself. 



