METEOROLOGICAL OBSERVATORIES. 159 



at the observatory and the other one some distance away in the park. 

 A triangle could thus be constructed on a known base and with two 

 known angles, so that the cloud height could be calculated with suffi- 

 cient accuracy. Here is also found a glycerin barometer, in which 

 glycerin is used instead of mercury, and it has, in consequence, a tube 

 which is as many times the length of an ordinary barometer as the 

 number of times mercury is heavier than glycerin. This results in an 

 instrument over 30 feet in height, and with a much extended scale, 

 so that it is easy to study by its means all the smaller changes in the 

 density of the air, the surface of the colored liquid in the tube moving 

 under our eye with unusual disturbances. Magnetic observations are 

 also made here, much in the same way as those already described in 

 the Eoyal Observatory, Greenwich, so that the two sets of observations 

 usually check each other. Most of us remember the weekly weather 

 curves published in the Times, but now, unfortunately, discontinued. 

 These emanated from the Kew Observatory. 



There are now meteorological observatories in all civilized countries, 

 but the rough sketch I have given of these two of our own will enable 

 one to form some idea of the subjects investigated and the instrumental 

 means adopted. 



HIGH-LEVEL OBSERVATORIES. 



Let us now turn our attention to those observatories which are situ- 

 ated on the summits of mountains, and which by reason of their alti- 

 tude attack the problems of air study from a much higher point of 

 vantage. It must be clear to any person who has looked attentively at 

 the sky that the motions of the upper air as shown by its clouds are 

 very different to those of lower levels, and it is with a view of eliminat- 

 ing as far as possible the effects caused by inequalites of the ground, 

 by friction, and by local circumstances that mountain peaks have in 

 various countries been fixed on for the establishment of meteorological 

 observatories. 



MONT BLANC OBSERVATORY. 



To begin with the highest in Europe, I must take you in imagination 

 to Mont Blanc, which, as you know, is situated in France, and about 

 40 miles to the south of the Lake of Geneva. 



In 1887 M. Joseph Vallot ascended Mont Blanc and made some pre- 

 liminary studies on the summit, leaving some self-registering instru- 

 ments there during the summer; and it was then that he formed the 

 idea of erecting a permanent observatory on the mountain. 



There were many difficulties owing to the great number and variety 

 of the instruments which modern meteorological science demands, and 

 M. Vallot instances that Saussure, in his famous early ascent (in 1787), 

 contented himself with proving that carbonic acid existed in the air 

 at these heights. jSTow it would be necessary to measure the exact 

 quantity. M. Vallot pitched his tent at first on the summit, 15,781 

 feet above the sea, but afterwards, on a more careful survey in 1889, 



