EXPERIMENTS WITH BALLOONS. 59 



the lower level is determined by contact of a point with the surface of mer- 

 cury, are good comparative or differential instruments, but nothing more. 



Of the Thermometer. — The best and most convenient thermometers appear 

 to be those made by Greiner, of Berlin, witli a paper scale enclosed in an 

 outer tube, or a scale of milk-white glass. The bulbs are 

 exposed, and the scales cannot be injured by immersing the 

 bulbs, or whole instrument in water, or any other liquid, 

 for purposes of comparison. The graduation should extend 

 from —85° Fahr. to about -|- 100° Fahr. In Gay-Lussac's 

 ascent, the thermometer descended 40° 25". It is not likely 

 that any observers would ascend much higher than he did, 

 or that they would undertake an ascent Avhen the tempera- 

 ture at the earth's surface was less than 10° C. The ther- 

 mometers, during the ascent, should be enclosed in bright 

 tin tubes (having an opening through which the scale can 

 be observed), open at both ends, with a round disc of tin at 

 a little distance from the ends, to prevent the effect of radi- 

 ation. Thermometers thus protected were used at the Cambridge Observa- 

 tory, and found to answer well. The temperature of the air being already 

 known, one thermometer with a wet bulb will be sufficient to determine the 

 pressure of vapour at a given station. 



Directions for Observing. — When the motion of the balloon in a vertical 

 direction appears to be small. 1. Observe the thermometer attached to the 

 barometer. 2. Make the lower edge of the upper ring appear to touch the 

 upper end of the mercurial column. 3. Make the lower edge of the lower 

 ring appear to touch the lower end of column. 4. Observe the thermometer 

 in the tin case for temperature of air, and note the time. 5. Read off the 

 two verniers of the barometer. 6. Observe the psychometer (wet-bulb ther- 

 mometer) and Daniell's hygrometer. The observations at the surface of the 

 earth should be made in the same order. The observers should avoid as 

 much as possible approaching the thermometer and barometer, in order that 

 they should not influence the temperature. The aeronaut must be instructed 

 in making the contact between the ring and the end of the mercurial column, 

 also in reading a vernier correctly. 



Cost of Instruments. 



Two of Bunten's barometers, each 4/. Ss £ 8 1 6s. 



Duty, 25 per cent 2 4 



Two thermometers, each 1/. lis. 6rf. 3 3 



Duty, 25 per cent 16 



Same, to be used with wet bulbs 3 3 



Duty, 25 per cent 16 



£18 18s. 

 Tin cases for thermometers, Daniell's hygrometer, &c. 

 It would be manifestly imprudent to commence operations with only just a 

 sufficient stock of such fragile instruments as barometers and thermometers. 

 Duplicates of every one should be provided. This would make the cost of 

 the instruments amount to about 50/. To the above might be added, — a 

 sympezometer, constructed for the purpose, without sliding scale, — a maxi- 

 mum and minimum thermometer, about as large as a watch, constructed by 

 Breguet. 



(Signed) David Brewster, Edward Sabine, 



J. F. W. Herschel, W. Whevvell, 



J. W. Lubbock, W. H. Miller. 



T. R. Robinson, 



