NICHOLS AND HULL. — PRESSURE DUE TO RADIATION. 563 



Since we can neither do away entirely with the gas nor calculate its 

 effect under varying conditions, the only hopeful approach which remains 

 is to devise apparatus and methods of observation which will reduce the 

 errors due to gas action to a minimum. The following considerations 

 led to a method by which the elimination of the gas action was practi- 

 cally accomplished in the present experiments. 



1. The surfaces which receive the radiation, the pressure of which is 

 to be measured, should be as perfect reflectors as possible. This will 

 reduce the gas action by making the rise of temperature due to absorp- 

 tion small, while the radiation pressure will be increased ; the theory re- 

 quiring that a beam, totally reflected, exert twice the pressure of an 

 equal beam, completely absorbed. 



2. By studying the action of a beam of constant intensity upon the 

 same surface surrounded by air at different pressures, certain pressures 

 may be found where the gas action is less than at others. 



3. The apparatus — some sort of torsion balance — should carry two 

 surfaces symmetrically placed with reference to the rotation axis, and 

 the surfaces on the two arms should be as nearly equal as possible in 

 every respect. The surfaces or vanes should be so constructed that if 

 the forces due to gas action (whether suction or pressure on the warmer 

 surface) and radiation pressure have the same sign in one case, a reversal 

 of the suspension should reverse the gas action and bring the two forces 

 into opposition. In this way a mean of the forces on the two faces of 

 the suspension should be, in part at least, free from gas action. 



4. Radiation pressure, from its nature, must reach its maximum value 

 instantly, while observation has shown that gas action begins at zero and 

 increases with length of exposure, rising rapidly at first, then more 

 slowly to its maximum effect, which, in many of the cases observed, was 

 not reached until the exposure had lasted from two and a half to three 

 minutes. For large gas pressures, an even longer exposure was ne- 

 cessary to reach stationary conditions. The gas action may be thus 

 still further reduced by a ballistic or semi-ballistic method of measure- 

 ment. 



The results of ballistic observations of radiation pressure at different 

 gas pressures are given below in Table I, in which p indicates the pres- 

 sure of the surrounding gas in millimeters of mercury, and d the static 

 equivalent of the ballistic throws of the torsion balance. The results 

 were obtained with substantially the same apparatus and method de- 

 scribed on page 568 et seq. : — 



