214 Instructions for the Scientific Expedition 



Though out of the question in the circumstances immediately un- 

 der contemplation, it may not be amiss to remind aeronauts, that 

 observations of the actinometer may, no doubt, be made with con- 

 siderable ease and precision in the car of a balloon, and if accom- 

 panied with good barometric and hygrometric simultaneous obser- 

 vations aloft and below, w^ould in every point of view be most pre- 

 cious, thus adding one to the many useful subjects of inquiry in 

 those hitherto almost useless adventures. 



4. Radiating Thermometers. 



As, however, the actinometer can only be observed at intervals 

 in perfectly clear weather, additional information with regard to solar 

 radiation, of much interest, though not of so precise a nature, may 

 be obtained, by the daily register of the maximum temperature of a 

 register thermometer, with a blackened bulb exposed to the full ac- 

 tion of the sun's rays. It may be placed about an inch above the 

 bare soil, and screened from currents of air. The maximum tem- 

 perature indicated by such a thermometer, even in clo\idy weather, 

 will generally be considei-ably above that of the air, and the maxima 

 and mean daily maxima of its indications will, after a long series 

 of observations, afford data of the utmost value to the history of 

 climates. The bulb of the thermometer should be about half an 

 inch in diameter, and it may be uniformly blackened with lamp- 

 black and varnish. The graduation should be made upon the glass 

 stem, to prevent any inconvenience from the expansion and warping 

 of the scale. 



The measure of terrestrial radiation is of no less importance to the 

 science of meteorology than that of solar radiation, but no perfect 

 instrument has yet been contrived for its determination. Very va- 

 luable information, however, may be derived from the daily register 

 of the minimum temperature of a register spirit-thermometer, the 

 bulb of which is placed in the focus of a parabolic metallic mirror, 

 turned towards the clear aspect of the sky, and screened from cur- 

 rents. The mirrors furnished to the observatories are of silver- 

 plated copper, but planished tin-plate or zinc might be substituted 

 without detriment. They are 6 inches diameter and 2 inches deep, 

 and the thermometers which are graduated upon the stems pass 

 through sockets in their sides, in which they may be accurately ad- 

 justed by corks. Their bulbs do not exceed half an inch in diameter. 



Even in the daytime a thermometer so placed, and turned towards 

 the clear sky, but away from the rays of the sun, will fall several de- 

 grees below the temperature of the surrounding air. 



5. Hygrometers. 



Times of observation. — Observations of the dew-point hygrometer 

 are as desirable at the regular hours as those of the other meteoro- 

 Icgicnl instruments; but, as more difficulty attends the observation, it 

 is mov," Inble to omission, and it is of great importance that when 



