CONTRIBUTIONS FROM THE JEFFERSON PHYSICAL LABORATORY 



OF HARVARD COLLEGE. 



FRICTION AND FORCE DUE TO TRANSPIRATION AS 

 DEPENDENT ON PRESSURE IN GASES. 



By J. L. Hogg. 



Presented by J. Trowbridge, November 8, 1905. Received April 6, 1906. 



Problem in General and Results in General. 



This is a preliminary paper on an investigation whose object is 

 threefold. 



It is known that at pressures lower than about 1 cm. of mercury 

 the resistance which a solid body meets in passing through a gas is 

 noticeably smaller than it is when the pressure is that of an atmos- 

 phere. This is because the phenomenon of slip presents itself at the 

 low pressures. The first object of the investigation is to study fully 

 the relation between the friction of a gas on a solid body and the 

 pressure in the gas at those pressures where slip is appreciable. 



Again, it is known that, if a given volume of gas is divided into two 

 portions by means of a partition one side of which is kept hotter than 

 the other, and if a small opening of any form be made in the partition, 

 the gas will flow from the cold side to the hot side of the partition until 

 there is an excess of pressure in the chamber contiguous to the latter. 

 This excess of pressure depends on the difference of temperature of the 

 two sides of the partition and on the mean pressure. 



The second object is to examine in a particular simple case the 

 relation between the force on the partition (called the transpiration 

 force), resulting from this flow of gas towards the hot side of the parti- 

 tion, and the mean pressure in the gas. 



The third object of the investigation is to examine the feasibility of 

 measuring the pressure in a gas when either of these relations is known 

 without having recourse to the McLeod gauge. That this third object 

 is by no means the least significant will appear when the evidence as to 

 whether the McLeod gauge is reliable for the measurement of small 

 pressures is adduced. 



