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HINTS TO CORRESPONDENTS. Names and Address must accompany all letters, or no aUenti will be paid thereto. This is for our information and not for publication. References to former articles or answers should give date of paper and page or numer of question. Inquiries not answered m reasonable time should be peated; crespondems will bear in mind that some answers require not a little research, and, thou deavor t o all e by leuer cr in thisdepent. each must talce histum. Special Written Information on otters of personal rather than neral interest cannot be expected without remuneration. Scientific Amerlcan Supttlement. referred to may be at the office. Price 10 cts each. Books referred to promptly supplied on receipt of price. IUlnerais sent for examination should be distinctly marked or labeled. (4537) W. B. R. asks : Why does the air end of an air pump such as is used on locomotive engines sometimes get hot when pumping air ? What makes the sound that we hear when a locomotive engine exhausts In the stack? A. The air pomp on a locomotive is an air compressor. The compression of air and gases develops heat. Any volume of air contains a certain amount of heat, the exponent of which is the temperature. When the volume Is suddenly compressed Into a part only of its original volume, the aame qoantity of heat remains, but in a smaller space. which causes the temperature to rise in proportion to amount of compression. The sound of the exhaust the moke stack is from the vibration of the air, due the soddennese of the operation, the exhaust actiug like an explosion. (4538) W. E. K. asks (1) how to obtain formic acid. A. In various ways; the dry distillation of oxalic acid is the simplest. It is made by the oxidation of gum and other organic substances by mixing 1 part of organic matter with 3 parts of manganese dioxide and 2 parts of water ; 3 parts of sulphuric acid 50 per cent are added and in a short time the mixture is . distilled. 2. How to take grease stains from the hande off the piano keys. A. Cover with glass and expose to I the sun. 3. A core for pimples. A. Consult a physician. Try washing the face with solution of Rochelle ! Balts. (4539) W. P. asks (1) for the best method to fasten rubber to steel without using rivets or similar devices. A. Use bicycle tire cement or marine glue. 2. Is there any difference between a series or shurt dynamo, provided everything is in the same proportion as near as possi ble to construct them, in regard to the E.F., that. is. let. bothmachines be of the same size and run at the same speed. A. The series . dynamo gives the highest E. M. F. on open circuit. the ' shunt on closed circuit. As regards maximum E.M.F.. they may be considered Identical.
