Nov. 20, 1884] 



NA TURE 



67 



has occasionally been checked by observations taken at the ends 

 of a base of S91 feet, and with such satisfactory results, that in 

 tin present case, where the observations being relative, great 

 accuracy in measuring the absolute heights is not required, I 

 have decided to adopt it in preference to the latter infinitely more 

 cumbrous method. 



Numerous observations have been made without success, acci- 

 dents of all kinds happening both to kites and instruments, suffi- 

 cient to deter any one who was not imbued with a little faith 

 that all would eventually come right. I have therefore only 

 been able so far to collect the results of a select few, viz. 23 in 

 all, in which the conditions were favourable. 



Hitherto I have only used kites as a. point oVappuiiox obser- 

 vations on the differential velocity of the air at different heights, 

 EL purpose for which they are obviously exceptionally fitted. I 

 am hoping, however, during the coming year, with new and 

 improved apparatus and assistance, for which I have received a 

 Government grant, to employ the same means of elevation for 

 observations of temperature, pressure, height of cloud-strata, &C. 



Anemometer 1 n Kile-Wire 



The height of the place of observation is 500 feet above sea- 

 level. 



Of course it is not intended at this early stage to attempt to draw 

 any but the most temporary conclusions from such sparse data. 

 There is no doubt that if observations could be taken every hour 

 a distinct diurnal variation in the difference between the velocity at 

 two given heights would be obse ved, the velocity at the greater 

 altitude probably tending towards a minimum about the same time 

 that the velocity at the earth's surface reached its maximum. This 

 would, however, only be found to be the case when the heights 

 were about 1000 feet or more. Apart from actual determination 

 by help of the instruments, however, the existence of such a 

 diurnal variation has been several times forcibly brought to my 

 notice by the fact that while during the middle of the day the 

 kite frequently flies with great difficulty owing to the presence 

 of vertical ascending and descending currents ; towards evening, 

 when the wind at the surface has often died away altogether, the 

 kite flies at a higher altitude and pulls harder and steadier than 

 it did during the day. This has so often occurred that I have 

 ceased noting it as anything extraordinary. I may observe that 

 such a condition is precisely what one would expect if the theory 

 of the diurnal variation in the velocity of the surface wind 

 given by Dr. Koppen in the Zeitschrift der Oesterreichschen 

 Gesellschaft fur Meteorologie for 1879, be accepted. According 

 to this theory the expansion of the lower strata by solar action 

 during the day, causes an intermixture of the air ('11ft austauscA) 

 to take place between the upper and lower layers, by which the 

 velocity of the lower layers is increased by the greater velocity 

 which the descending air brings with it from above, while the 

 upper layers have their velocity decreased by the smaller velocity 

 with which the ascending lower air retarded by the asperities of 

 the earth's surface, is endowed. Thus while the mean velocity 

 of the atmosphere might remain about the same, the differences 

 between the velocities above and below should undergo a diurnal 

 period, the minimum difference occurring somewhat after mid- 

 day. I was glad to see the other day that some observations on 

 the velocity of the wind at some lofty observatory (I think Pike's 

 Peak) showed that the diurnal period in the wind velocity at 

 8000 or 9000 feet, in exact opposition to what occurs at the 

 earth's surface, exhibited a minimum about midday. 1 



Another feature that has been brought out by observing the 

 flight of my kites, which frequently fly at heights of from 1300 

 to 1500 feet above the sea and thus enter the clouds, is the 

 existence of a courant ascendant under cumulus and cumulo- 

 stratus clouds. When such a cloud comes over, the kite rises 

 up until the string is at an angle of 60° or more ; but in propor- 

 tion as it rises, so its pull becomes weak ; the ite in fact lies on 

 its face, and thus losing nearly all the horizontal component, the 

 curvature of the string increases very much, and if an instrument 

 is attached to it, it is sure to come down. After such a cloud 

 has passed I have frequently noticed the apparent existence of a 

 downward current which causes the kite to descend and at the 

 same time increase its pull by the pressure being exerted more 

 against a vertical surface. 



Regarding the observations themselves, I am not aware that 

 any similar ones have previously been made, except by Mr. 

 Stevenson of Scotland. His plan was to fix anemometers to a 

 pole 50 feet high or place them at different heights up a moun- 

 tain. In the latter case it is not certain that the velocities 

 represent what would occur in the free atmosphere at the same 

 level. In the former, one is limited to poles of moderate height, 

 and I do not at present see that anything else can compete with 

 a kite-wire for greater heights ; balloons, captive or otherwise, 

 being of course out of the question where wind is concerned. 

 Mr. Stevenson seems finally to have adopted a very simple 

 formula for the increase of the velocity with the height, viz. that 



it is exactly prop irtional, or 



H 



That though this might 



be true up to 50 feet it is certainly not true for greater heights 

 I showed pretty conclusively in NATURE for March 29, 1883 

 (p. 506), where a discus ion of Dr. Vettin's cloud observations 



favoured the formula — ( V through a range of mo e than 



20,000 feet. 



Though I do not wish to try and deter nine any formula at 

 this preliminary stage, it may be interesting to note the exponent 



1 This seems also to be the case on Ben Nevis, regarding which Mr. 

 Buchan says, " In each of the months the maximum velocity is during the 

 night, and the minimum during the day, being thus the reverse of what occurs 

 at low levels and on plains " (vMe Journal of the Scottish Meteorological 

 Society, 3rd series, N j. 1, p. 17). 



