OF THE expenses involved in the milling and mixing of rubber, the costs of materials and of labor are constantly subjected to the closest analysis. Economic drives are concentrated mainly upon them. Power, being a less tangible thing, often eludes the attention that is warranted by the profitable study of factors which may introduce savings. Frequently its cost equals or exceeds that for labor in a mill-room operation. When in a large factory there may be previously plasticized daily a half million pounds of crude rubber, when a half million pounds of master-batches are prepared, and when every day one and a half million pounds of compounded rubber stocks are mixed, power bulks enormously. Milling and mixing equipment to do that demands a power input of approximately 21,500 horsepower, or about 386,000 kilowatt hours. At one cent per K. W. H., as an example, this represents a daily cost of $3,860. With an average of 250 working days a year, the total reaches nearly a million dollars. The smaller manufacturer makes a proportionate outlay for power in these preparatory processes. So it would seem worth while to study power applications and those variables which may be turned to account in the search for greatest efficiency. Some points which may be mentioned are in part a review or confirmation of the findings of others which have appeared in published articles, other facts are from observations of very recent practice.