302 Journal of the Asiatic Soc. of Bengal. [N.S., XV, 1919]. 
ment parallel to the surface at S, and no field in B. It was 
shown by Maxwell that such a field exerts a pressure on the 
surface at S, and is in this respect, similar to the compressed 
part of a medium. 
Consider as before the case of a compressed medium. 
When the surface has moved from § to §’ the medium between 
and S’ must now be unstrained, and must originally have 
occupied a space SC, where C is between S and 8’. In this 
case work has been done on the medium in A, and all strain 
energy has been taken from the medium in SC in its expan- 
sion to SS’. 
In the case of the electric field, if a part of the field occu- 
pying SC could give up all its energy in expanding to S89’, 
we should have conditions similar to those in the case of the 
compressed medium, and the work done by the surface S in 
moving to 8’ would be found in the same way. But the field 
in SC in expanding to SS’ will not lose the whole of its energy, 
unless SS’ is infinitely great, in comparison with SC, i.e. un- 
less C coincides with 8. Therefore if the surface at S in moving 
to S’, leaves no field behind it, the field in front must be 
moved as a whole through S89’. 
If then the method of treatment which has been used in 
which exists on one side only of the surface, exerting a 
pressure on it, and is moved forward as a whole by the surface. 
e work done on such a medium by a moving boundary 
would be measured by the pressure exerted on the boundary 
and the distance it has moved. If the field varies in intensity, 
the average rate at which work is done by a uniformly moving 
reflector, is a measure of the average pressure on it. 
It would appear therefore, that the assumption that the 
work done by the advancing reflector is done against a pres- 
sure exerted by the radiation reflected, is not valid in the case 
of strain waves in material media ; and that in the case of 
electrical disturbances, it can only be justified by further con- 
sideration of the properties of the electric field. 
SN Fee eR em mm mee i ee 
* 
