THERE are two points occurring in the review (p. 434) of “The Principles of Magnetism and Electricity” in which the author appears to me to be correct, though the examples are given as instancing errors into which he has fallen. The reviewer states, “The author measures magnetic force in dynes and difference of potential in ergs,” thereby apparently implying that this is incorrect. Surely these are usual units for expressing these quantities. Again, the author is taken to task for stating that in the case of a dynamo or motor armature, “owing to the self-induction of each section, a certain amount of energy is used twice in each revolution to establish the current in it. This energy is lost so far as the external circuit or the effective output of the machine is concerned.” Whilst with a dynamo running sparklessly with copper brushes this is only partially true, the difficulty of obtaining the return of the energy thus absorbed is in practice so great that we see on a very large proportion of machines that carbon brushes are used, the object of which is to enable this energy to be wasted without an actual spark, and it is well known that machines with carbon brushes thus working have a higher rise of temperature from the waste of power than when true electrodynamic commutation takes place. Actual measurements of power absorbed have also shown a waste of power from this cause sometimes exceeding 5 per cent, of the output of the machine.