18 Trans. Acad. Sci. of St. Louis. 



In the Pitot tube the pressure developed is that to which 

 the velocity of the stream may be considered due. The 

 air current may be assumed to be an efflux from a great 

 tank, filled with air of density 6' to a height h above the 

 orifice. The pressure due to this column would be gh8'. The 

 velocity of efliux would be v^\/2gh. By the elimination 

 of h these two equations give Newton's formula for the 

 relation between pressure and velocity. 



8' h 



2'^ ~ 2CT'' • 



In this equation the pressure is given in dynes per sq. cm, 

 V being in cm. per sec. O is the Boyle-Gay-Lussac constant 

 inC. G. S. units, which for air is 2.88 X 106. Tisthe absolute 

 temperature centigrade and 6 is the barometric pressure in 

 dynes per sq. cm. This barometric pressure is supposed to 

 be corrected for velocity effects due to the wind sweeping 

 against and through the shelter in which the barometer is 

 placed. These velocitj^ effects disturb all readings of pres- 

 sure indicators in a stream of air. The compression on the 

 windward side and the rarefaction on the leeward side of every 

 obstacle, whether that obstacle be a barometer case or the 

 building in which the barometer is shel'tered produce such 

 effects. Dr. Engelmann long ago (in 1861) called attention 

 to such fluctuations in the barometer reading due to rapidly 

 variable winds. (Trans. Acad, of Sc. of St. Louis II 153). 

 In addition the wind sweeping across openings leading into 

 the building or the barometer case, produces an atomizer 

 action causing a flow of air from the opening into the 

 moving stream. The air within a building is therefore both 

 compressed and rarefied by the wind blowing around it. 

 Different buildings behave very differently in this respect. 

 The result depends upon materials of construction and upon 

 the arrangement of windows and doors. 



In a former paper * the writer has shown how all such 

 velocity effects may be eliminated. The disk collector there 



* Trans. Acad, of Sc. of St. Louis VIII: 1. 



