248 PROCEEDINGS OF THE AMERICAN ACADEMY. 



in fair weather the days are damper than the nights. " Cold and warm 

 waves " commence in the upper air, as is proved by the temperature de- 

 creasing faster than normal, or even increasing abruptly, with altitude 

 before the fall or rise of temperature commences at the earth's surface. 

 Several ascents through clouds have shown the air above them to be 

 usually warmer and drier than the air below. Kites furnish a ready 

 and accurate method of measuring the heights of certain low and 

 uniform clouds, which could not easily be measured otherwise in the 

 daytime. It is interesting to note that this method was used by Espy, 

 about 1840, to verify his calculations of the height at which conden- 

 sation begins.* Changes of wind direction in the different air strata are 

 determined from the azimuths of the kites, and this change sometimes 

 amounts to 180°. The wind velocity usually increases with altitude, 

 and vertical currents commonly prevail near cumulus clouds. During 

 high flights the wire is s*^^rongly charged with electricity, but no measure- 

 ments of its kind or potential have lately been attempted. 



The writer is glad to acknowledge his indebtedness to his assistants, 

 Messrs. Clayton and Fergusson, who have devised and constructed im- 

 proved kites and apparatus, and during his absence have taken entire 

 charge of the work. To them and to another assistant, Mr. Sweetland, 

 is largely due the success which has been attained in this novel branch of 

 research. For still higher ascents there will be required a steam engine 

 to operate the windlass, and a meteorograph with a lower pressure scale. 

 With these appliances, for whose purchase a grant has been asked from 

 the Hodgkins Fund of the Smithsonian Institution, it is probable that 

 records can be obtained three miles above Blue Hill, and possibly 

 higher. 



To reach much higher altitudes, unmanned free balloons, or " ballons 

 sondes " as they are called, have been considerably used both in France 

 and Germany. These balloons, which carry self-recording apparatus, rise 

 until equilibrium is attained in the rarefied air. When they lose their 

 buoyancy and fall to the earth, most of them have been recovered, with 

 the instruments and records uninjured, by the senders, who have been 

 notified by the finder of the place of descent, which is often at a great 

 distance from the starting point. The altitudes are calculated from the 

 barometric pressure, according to Laplace's formula, but the impossi- 

 bility of knowing the mean temperature of the whole mass of air makes 

 the determination inexact. Theoretically, in order to ascend ten miles 



* Philosophy of Storms, 1841, p. 75. 



