766 The Outline of Science 



when the meteorograph falls down to earth. The instrument is 

 protected from injury hy being enclosed in a light bamboo frame- 

 work, while the remnants of the balloon fabric act as a para- 

 chute, so that the records obtained are usually recovered in good 

 condition. A notice is attached to the instrument entitling the 

 finder to a reward on handing the instrument in at any Post 

 Office. The records made include pressure, temperature, and 

 humidity, and the whole is contained on a small rectangular plate 

 of silver-plated metal not much bigger than a postage stamp. 

 A microscope is required to enable the traces to be read with 

 accuracy, but the results are very dependable. 



The other type of balloon is much smaller, usually only 18 

 to 24 inches in diameter when inflated, and is made of thin 

 rubber, generally dyed some dark colour to render it easily 

 visible. The balloons are termed "pilot balloons," and are sent 

 up in considerable numbers every day at stations all over the 

 country. They are inflated with hydrogen, and after being re- 

 leased they are observed in their flight by theodolites at either 

 one or two stations. If two theodolites are used, the stations 

 chosen are separated by half-a-mile or more, and the length of 

 the base-line so formed affords a datum from which the balloon's 

 position can be accurately determined. Both observers at the 

 two stations take simultaneous readings every minute of the 

 angular altitude of the balloon and of its azimuth (i.e. its hori- 

 zontal angular position from the base-line), and from these read- 

 ings the position of the balloon at each minute is obtained. A 

 far more rapid method, however, is to use only one theodolite for 

 taking the necessary position readings, and to assume that the 

 balloon rises at a steady rate, which has been determined experi- 

 mentally and found to be between 450 and 500 feet per minute 

 for balloons of the size above mentioned. Knowing the height 

 and the azimuth and elevation angles, the balloon's position can 

 be rapidly determined by the use of a slide-rule. As a matter 

 of fact, this second method is the one commonly employed at 



