Fluid Pressure 95 



From this similarity of behaviour of liquids and gases 

 it is to be expected that bodies weighed in a gas also 

 suffer an apparent loss of weight, this loss being caused 

 by the partial support of the body by the pressure of 

 the gas. This loss actually takes place, and its magni- 

 tude may be found by determining the weight of the 

 body in a vacuum and also its weight when weighed 

 in an atmosphere of the gas. This experiment is rather 

 beyond the scope of our work, but, as a result of 

 carefully carried out experiments, it has been shown 

 that the Principle of Archimedes holds good for gases 

 as well as liquids. 



Exercise for Student. 



Frame a statement of the Principle of Archimedes which shall 

 include the case of gases as well as that of liquids. 



All weighings are ordinarily carried out in air 

 a gas and, as a consequence, the weights obtained are 

 slightly smaller than the actual weights of the bodies 

 concerned. By " actual weight " is intended the total 

 pull of the Earth upon the body weighed. The weight 

 of a litre of air is about T293 grams. Since the loss 

 of weight is equal to the weight of displaced gas, it is 

 easy to calculate the error in the weight of a body 

 weighed in air, if the volume of the body be known. 



Exercise for Student. 



Why are the weights of the Standard Pound and Kilogram defined 

 as obtained in a vacuum ? 



The error involved in weighing the comparatively 

 small bodies dealt with in our work, due to the upthrust 

 of the air, is negligible when compared with other sources 

 of error in the weighings. It must be remembered, 

 however, that, in all scientific work of the highest 

 character, allowance is always made for the errors in 



