April 21, 1910] 



NA TURE 



221 



when in use. The instrument is tied in the middle of 

 the kite, partly for convenience, partly, presumably, to 

 shelter it from direct sunshine. 



The record is made on the cardboard disc shown in 



A Fir,. 2. 



Fig. I, A. The difHcuIty of procuring- cardboard which 

 is not warped by var}ing- humidity, and the occasional 

 thickening of the trace through the running of the 

 ink, suggest that it would be an improvement to use 

 a smoked metal sheet. 



The anemometer deserves special mention because 

 it is simple and unique. It consists of a light cellu- 

 loid ball of 3 inches diameter suspended by about 40 

 feet of fine cotton, and the velocity of the wind is 

 deduced from the tension in the cotton. The effect of 

 the wind on the cotton is neglected, but it appears 

 doubtful if this is justifiable. With cotton o'2 mm. 

 in diameter, the area exposed to the wind is nearly 

 one-third that of the ball used. 



Pilot balloons are usually small balloons 2-3 feet 

 in diameter, which are sent up to determine the wind 

 at different altitudes. Observations are made by 

 theodolites at the end of a base line, or at times by 

 one theodolite, the rate of ascent in that case being 

 calculated from the free lift and diameter of the 

 balloon. 



It is assumed that the rate of ascent is given by 

 L = kpv^r^ — fiv'-r^, 



where L is the free lift, r the radius of the balloon, 

 f the upward velocity, p the density of the air, and fe 

 a constant. The values of /*, calculated from ten sets 

 of observations given in the report, show great irregu- 

 larity, varying between 5'5xio-* and i5"6xio""^ in 

 C.G.S. units, or between 1/480 and 1/170 in the units 

 used in this part of the report (grains, feet, minutes). 

 Unfortunately, no information is given, and no reasons 

 are put forward, to account for the variations, beyond 

 a vague suggestion of convection currents. It would be 

 interesting to know how much of the variation could 

 be attributed to (i) the deposition of dew on the 

 balloons, (2) the effect of solar radiation on the 

 balloon's temperature, (3) differences in the wind, (4) 

 errors in the observations. As they stand, the results 

 indicate that observations made with one theodolite 

 may give very erroneous values for the wind. 



An interesting table gives the values of /i~' obtained 

 by six different methods. From his experiments with 



XO. 21 12, VOL. 83] 



a whirling machine, Dines found 240. Observation 

 of a small 3-inch ball, falling through 200 feet, gave 

 280. Dines 's theodolite observations give 290, those of 

 C. H. Ley 330, and twelve ascents of registering 

 balloons 322. Stanton found a still higher 

 value, 354, from laboratory experiments on 

 a i^-inch ball. It is of interest to note 

 the close approximation to the correspond- 

 ing value, 380, deduced from the value of 

 k given by Allen's experiments with steel 

 balls in water. 



Registering balloons of about i metre 

 diameter, having a free lift of 200 to 300 

 grammes, are used to carry a meteoro- 

 graph, which weighs, with its case, 60 

 grammes only (28 grammes without the 

 case). About 60 per cent, of the balloons 

 sent up are recovered, and it seems re- 

 markable that the proportion is higher in 

 winter, the season of high winds, than it 

 is in summer. The explanation given is 

 that when the meteorographs fall in stand- 

 ing crops they frequently come into con- 

 tact with mowing machines, and as the 

 instrument case resembles an old tin can it 

 is not surprising that Hodge neglects to 

 gather up and return the fragments. The 

 difficultv might be overcome by attaching 

 a partially filled small, cheap , balloon » 

 which would act as a signal for some time 

 after the instrument reached the earth. 

 An addition of 5 per cent, to the number recovered 

 would compensate for the extra cost. 



The meteorograph is shown in Fig. 2, and dia- 

 grammatically in Fig. 3. The aneroid box A expands 

 under decreasing pressure 

 and opens the frame in which 

 it is fixed, so that E, L move 

 across the plate beneath them. 

 If the temperature is constant 

 they make two parallel 

 traces; if the temperature 

 falls, the German silver strip 

 M contracts more than the 

 invar strip HC, and rotates 

 DE about C. Thus, the 

 abscissae of the trace give the 

 pressures, and the distance 

 between the traces the corre- 

 sponding temperatures. The 

 instrument furnishes no in- 

 formation as to the rate of 

 ascent. 



It is a great advantage that 

 the calibration of the instru- 

 ment is made on the actual 

 plate, which is fitted ready for 

 the ascent, and that the pres- 

 sure and temperature are 

 varied together. It ought, 

 however, to be explained why 

 the instrument is tested down 

 to —40° C. and to 100 mm. 

 only, when it is to be exposed 

 to temperatures of —60° C. 

 and pressures of 50 mm. or less. 

 The heights have been ob- 

 tained from the recorded 

 pressures by the use of dia- 

 grams, and more recently by 

 means of semi-logarithmic 



squared paper. The need for great care in dealing 

 with this problem is illustrated by the errors in the 

 table on p. 7. On July 29 the difference of pressure 



Fig. 



