The moths couple as a rule about eight o'clock in the morning. About four in the afternoon they should be separated by taking them by the wings and drawing them gently apart. Each female should now be placed by herself in a cell, which is then closed by a pin as shown in the figure. Here she will lay her eggs and in due time die. The males may as a rule be thrown away, but it is wise to keep a few of the more active ones, in case there should be a superabundance of females the following day. When the females have finished laying their eggs, which operation occupies about thirty-six hours, they are ready to be shipped to this office. The cells, with their inclosed moths and eggs, should be placed in a strong box of wood or tin, being packed in such a manner that they will not be crushed, and mailed to the entomologist at the department. By using the inclosed return penalty slip, payment of postage may be avoided. The name of the sender should be placed in each box. The moths, as soon as received, will be examined microscopically, and the eggs of those which are found to be free from disease will be weighed and paid for at the rate of $2.50 per ounce of 25 grammes (about 6-7 of an ounce avoirdupois). Silk culturists are advised not to attempt the production of eggs unless they are adepts at the industry, and have had at least one season's experience. We would advise each person desiring to sell, to send a sample first, with a statement of the quantity offered. * * * * * Dr. Zintgraff of Bonn has taken a phonograph with him to Africa. He intends to bring home phonograms of the savage dialects which he will hire the natives to speak into the machine. * * * * * [NATURE.] DETERMINING THE MEAN DENSITY OF THE EARTH. In _Nature_ for March 5 (p. 408) Prof. Mayer suggests an improvement in our method of determining the mean density of the earth, from which it appears that our plan has not been properly understood. This misunderstanding, no doubt, has arisen from the incomplete description of our method given in the _Nature_ (Jan. 15. p. 260) report of the _Proceedings_ of the Berlin Physical Society, which report was probably the only source of information accessible to Prof. Mayer. We are led therefore to give a short description of our method. Let H I K L represent a section of a cubical block of lead, about two meters in the edge, and weighing 100,000 kilos. The balance, A B C, is placed in the middle of the upper horizontal surface. It bears the scale-pans, D and E. Under these scale-pans the block is bored vertically through, and two other scale-pans, F and G, are suspended below the block, attached to the balance by means of rods passing through these openings.