24 MR JOHN AITKEN ON THE NUMBER OF DUST PARTICLES IN THE 



the hygroscope, which is generally hung alongside the thermometers. It was not a little 

 gratifying to find that these instruments also showed the air had entirely changed. The 

 temperature had risen 3 degrees since the first test was made, having risen from 71° to 74°. 

 But not only had the air risen in temperature, it had also become greatly reduced in 

 humidity. The wet bulb was now 3° lower than when the observations were begun, the 

 depression having increased from 11° to 17°, showing that the air was not only dryer owing 

 to rise of temperature, but that its absolute humidity was much less than at first. It was, 

 therefore, evident that some change had taken place by which air from a different source 

 was now coming to the place of observation, and that we were now testing quite a different 

 air from what was tested at first. On looking into the cause of this change it was found 

 that, when the tests were begun, the local wind was light and came from the westwards, 

 i.e., off the lake ; but afterwards it had changed first to S.W., then to S.E., and had, at 

 the place of observation, greatly increased in force. This rapid change in the direction 

 of the lower current seemed to be caused by the upper south-westerly wind striking 

 the face of the mountain, which is here nearly vertical in some places, and curving 

 ^downwards and outwards from the mountain to the lake, and in a direction nearly 

 opposite to the true wind. The trees on the face of the mountain were distinctly seen 

 bending in the strong wind ; their movements clearly indicated that the air coming to the 

 place of observation was upper air, forced down to the level of the lake by the upper 

 current meeting the face of the mountain. So long as this circulation was kept up, and 

 the air blew down from the mountain side and out to the lake, the number of particles 

 tended to get lower, the air also tended to get hotter and dryer, till at last the number 

 of particles was as low as 1700 per c.c, or one-sixth of what it was at first; the tem- 

 perature had risen about 4 degrees, and the wet bulb was fully 1 8 degrees below the dry. 



About an hour after the observations began it was noticed that the upper S.W. wind, 

 which was causing the lower counter-current, was gradually extending downwards. It not 

 only struck the mountain face high up, but began to affect the trees very little above the 

 level of the lake, and, at last, the counter-current ceased, and the S.W. current extended 

 quite down to the lake, and the air again came in off the water. When the wind returned 

 to its original direction, the quantity of dust rapidly increased, and became rather larger 

 than it was at first. The air rapidly fell in temperature at the same time to rather under 

 what it was at the beginning, while it also regained its original humidity. 



These Vitznau observations point an important lesson which one is apt to forget, 

 which is this, that in testing the air we are testing the condition of only a thin layer of 

 air resting on the ground. The very exceptional and favourable conditions which 

 existed for a short time at Vitznau, when the upper air was driven down to the place of 

 observation, show how much the upper and lower air may differ as to dust, temperature, 

 and vapour. If this be so, it may be objected that all tests of dust as ordinarily made 

 are valueless. It must, however, be remembered that almost all our meteorological 

 observations are made on the conditions of this same thin layer. If we are to abandon 

 all observations because we cannot get perfect conditions for our tests, there would be 



