I9C9-] BY MEANS OF KITES AND BALLOONS. 13 



The self-recording instruments used in kite and sounding balloon 

 work are numerous in variety. Many observatories have instru- 

 ments made from special designs. All are built on essentially the 

 same plan. A clockwork rotates a cylinder which is covered with 

 either a sheet of paper ruled to scale or a sheet of smoked paper 

 or aluminium. Upon this sheet the pens or points, as the case may 

 be, connected with their respective elements, trace the conditions. 

 Paper scales are the more convenient and are used when the tem- 

 peratures to be recorded are not so low as to freeze the ink. The 

 instruments are made as light as possible, aluminium being the metal 

 used in the construction wherever it can be adapted. From 750 to 

 1,500 grams is the usual weight of an instrument, those for use in 

 kites being more substantially built than those for use in balloons. 

 The anemometer usually consists of a small aluminium pin wheel 

 mechanically geared to the pen — some are electrically connected. 

 The hair hygrometer is the only form yet available for self-recording 

 purposes that is light enough. The temperature is measured with 

 either a bimetallic element or a partially coiled tube containing 

 toluene. The barometer is of the aneroid type. The order of accu- 

 racy of these instruments is not high. Difficulty is experienced in 

 keeping the anemometer properly oriented while the kite is flying. 

 The hair hygrometer, if kept in good condition, probably records 

 within less than 5 per cent, of the correct value. Records of pres- 

 sure are, in nearly all cases, correct to within 2 mm., in many to 

 within I mm. The temperature may be relied upon to one degree 

 Centigrade in the records obtained from most kite flights, to less in 

 many. When used in sounding balloons at very great altitudes the 

 absolute error in any element is of course greater than those men- 

 tioned. In this case no anemometer is used, the wind velocity 

 being determined from observations on the drifting balloon with 

 one or more theodolites. 



The differences in the various instruments consist chiefly in the 

 way of exposing the elements so as to best obtain true records of 

 the conditions in the vicinity of the instrument. It is essential that 

 the temperature element especially be properly ventilated and insu- 

 lated. The method of ventilation is of course different in sounding 

 balloon and kite instruments. The former, being carried by the 



