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.Tune 19, 1885. J THE PHOTOGRAPHIC NEWS. 387 £ I'ig. 1 frg. 2. Negretti and Zambia’s thermometer (fig. 1) is also in common use for registering the highest temperature in the proportion of 9-5. Hence we obtain the following rules for converting Fahrenheit into Centigrade degrees, and vice versa : — To convert Fahrenheit into Centigrade degrees, subtract 32, multiply the remainder by 5, and divide by 9. To convert Centigrade into Fahrenheit, multiply by 9, divide the product by 5, and add 32. It would be impossible to mention here all the different forms of thermometers in use at the present day, but one or two of the self-registering instruments, commonly used for meteorological purposes, must be briefly described. One of the oldest contrivances for marking the highest and lowest temperatures reached in any given interval of time is Six’s thermometer (fig. 1). This consists essen ¬ tially of a bent tube with a bulb at each end, one bulb being larger than the other. The two bulbs are filled with spirit, a bubble of air being left in the smaller one. The bend of the tube is occupied by mercury. At the top of each column of mercury is a steel pin. When the temperature rises, the spirit in the large bulb expands, and pushes the mercury and index before it. When the temperature falls, the spirit contracts, and the pressure of the air-bubble in the small bulb drives the mercury back again, leaving the index in each case at the highest point reached by the mercury, the maximum being on the side of the small bulb, the minimum on the side of the larger bulb. The instrument is set by bringing down the index to the top of each mercury column by means of a magnet. These thermometers have serious defects, and are not much used, except for green houses, and where great accuracy is not required. Phillip s maximum thermometer is an ordinary thermo- meter, in. which a small portion of the mercury is detached from the main column by an air bubble, the de tached portion remaining at the highest point reached by the column. This instrument is set by swinging it bulb downwards, until the detached mercury resume! its position as close as possible to the main column. Fiq. 3. reached. In this instrument there is a contraction near the bulb, so that while the mercury is allowed to pass in expanding, on contraction the column in the tube is broken off, thus registering the maximum temperature. Rutherford’s minimum thermometer is filled with alcohol, containing a small index, which is carried back by capillary attraction, and left at the lowest point reached by the alcohol, which on expanding flows past it without disturbing it. The great objection to this instru ment is the liability of some of the spirit to condense in the upper part of the tube, thus shortening the column and exaggerating the minimum readings. This probably ex plains many of the very low temperatures occasionally published in the newspapers. Negretti and Zambia’s minimum thermometer, which is similar in principle to the above, is represented in fig. 3. We now come to the consideration of the principal errors in thermometric readings, and the corrections to be applied. Any thermometer can be verified at the Kew Observatory for a small charge, and a certificate obtained of the necessary corrections. The Meteorological Office advises the rejection of all instruments having a greater error than 0°3 in the space of 10®. Even the best mer curial thermometers, however, are liable to a gradual con traction of the bulb, which causes the readings in the course of time to be too high. If a thermometer is heated even a little above the boiling-point, the zero point is some times considerably displaced. Crafts in one instance got a displacement of 2604 after a prolonged exposure to a high temperature. The amount of correction for this dis placement of zero may always be found by placing the instrument in melting ice or snow, and reading the height of the mercury. Any difference from the true reading (0°C or 32° Fahr.) should be noted down, and always subtracted from all future readings. But even with the most accurate observations, and with the most carefully-constructed instruments, it is a matter of great difficulty to obtain the true temperature of the air, owing to the influence of radiation from surrounding objects. To obviate this difficulty a thermometer screen (fig. 4) is employed. This is made of wood, painted white Fig. 4. both inside and outside, and louvred all round so as to secure a free circulation of air. The screen should be placed on posts upon an open grass plot, and should face the north in our hemisphere. If such a position cannot be obtained, the screen should be attached to a garden wall in such a way that the air can circulate freely behind it. For isolated observations of shade temperature, Arago’s method of swinging the thermometer, attached to a piece of string, is recommended, and gives correct results whether in shade or sunshine. It was found by Brewster that the mean temperature of the day can be very closely obtained by taking the average of two readings at nine o’clock morning and evening, and this method is recommended by the Meteorological Office wherever a continuous record cannot be kept. Of course,
