458 ANNUAL REPORT OF THE Off. Doc. 



light is caused by the reflection of sunlight on the dust particles in 

 the higher atmospheie. The height at which meteors become visible 

 has also been used as a basis of calculation. This height can easily 

 be determined by two astronomers who decide to measure the angular 

 height of all the meteors which pass between them. If they each 

 see a meteor at exactly the same time moving in a certain direction, 

 it is likely to be the same meteor. Then knowing the distance be- 

 tween the stations, it is easy to calculate the height above the earth 

 when first seen. But through how many miles of air has the meteor 

 travelled before the friction made it hot enough to glow? That 

 no one can tell. From one to two hundred miles may be put down 

 as the depth of this ocean of air. Yet one-half of its bulk is within 

 three and one half miles of the sea level. That is why the air is so 

 rare on a mountain top that exertion brings on speedy exhaustion. 



The air is so mobile that only Avhen it or we are in motion do we 

 notice its presence. It is so evenly distributed that we do not realize 

 that it has any weight. When Torricelli first advanced the theory 

 that air had weight, the idea was laughed at as being the height of 

 absurdity, and his friends feared that he was losing his mind. They 

 thought it absurd to suppose that the weight of the air could force 

 the water up a pump stock, but that the water followed the valve 

 because ''Nature abhors a vacuum" was perfectly good logic. Even 

 the great Pascal seems to have had grave doubts that the air had 

 weight, but he realized that the fact could be proved or disproved by 

 taking a tube closed at the upper end, filling it with mercury and 

 placing the lower end in a cup of mercury. This instrument he had 

 carried to the top of a mountain near Paris. As the instrument was 

 carried up, the mercury descended in the top as it was brought down 

 the mercury went up again. This Pascal concluded could only be 

 caused by the weight of the atmosphere pressing on the mercury in 

 the cup. Since the date of this experiment it has been acknowl- 

 edged that the air has weight, hence must cause things which are 

 immersed in it to become lighter by the weight of the air displaced. 



The fact that the air has weight enables it to carry water in the 

 form of mist and cloud and invisible vapor. It makes rain and snow 

 possible and causes moisture to be carried to almost all parts of the 

 earth. The only places where there is no rain are such localities 

 as are deprived of it by local conditions. Deserts are generally due 

 to mountains which rob the winds of their moisture as they pass 

 over them. 



No one factor has more to do with the prosperity of the farmer than 

 the atmosphere unless it be sunshine. What causes the changes in 

 the atmosphere? Why do we have rain today and sunshine tomor- 

 row? Why is one summer wet, as the one which has just passed, and 

 another dry? What makes these changes? These questions have 

 come into the minds of men since the earliest times and yet they 

 are still waiting for an answer. 



In order the more thoroughly to study and understand the pheno- 

 mena connected with the atmosphere, weather records have been kept 

 for many years. At first by a few men now by many in all parts 

 of the world. There is no part of the earth today in which the wind 

 currents, storm and weather changes are not recorded and kept so 

 that they can be compared and if possible a science created by study- 

 ing these isolated facts. 



