D O C U M E N T 8 7 O C T O B E R 1 9 2 5 9 7 87. From Auguste Piccard Brussels, 14 Av. Ernestine, 14 October 1925 Dear Professor, Many thanks for your letter. I am very pleased that you find the balloon experi- ments interesting.[1] I share your opinion that this endeavor is daring, but I find that if there is a prospect of success, it must be dared. How large this prospect is can only be estimated once the apparatus has been assembled. After thorough consideration, I have resolved to build an initial instrument and postpone the final decision until afterward. Today I would like to take the liberty to inform you of my plans, from which you will gather that I have put some thought into some of the difficulties, without, how- ever, encountering any unsurmountable obstacle so far. I already told you that the balloon with its electromotor & propeller must be set in as rapid a rotation as possible I am hoping to obtain 1 rotation per minute. (The precise calculation of the forces has not yet been done it is not easy, but I am going to compile the necessary elements.) The motor is suspended in the net below the equator and hence cannot shake the basket. The temperature homogeneity must be extended quite widely. The effect estab- lished by Miller is of order of magnitude . The relative changes in length of the apparatus should hence not reach this level for brass it corresponds to a °C fluctuation in the temperature difference. Such homogeneity is ¢easily² attained without difficulty in an ice thermostat. As the air temperature at night in March at a few thousand meters altitude does not differ much from 0°, a relatively low volume of shielding suffices to maintain the ice throughout one night. 5–10 cm ought to suffice for the ice cladding. This thermostat weighs about 100 kg, though. But the ice & water will serve as a maneuvering ballast the next morning during landing, so they can be subtracted from the regular ballast. Alterations in apparent gravitation. 1.) Changes in the vertical speed of the balloon at night, in calm weather & at a stable height, produce vertic[al] accelerations of order of magnitude g. 2.) Swinging motion of the balloon: g vertically. 3.) Changes in altitude of the balloon: at most 30 m/min, corresponding to . 4.) Horizontal winds & the curvature of the Earth: but only over longer periods of time. Δl l ---- - 5.10–10 ≅ 0 8 , 10000 -------------- - 2.10 4– 0 5.10 5– , Δg g ------ - = 10 5– g min -------- - g 1 3 -- - 10 4– Δ