[Illustration: 18. Gridiron-bulb Thermometer. Scale about 1/5.] Circumstances sometimes arise in which it becomes necessary to convert readings from one scale into those of the others, according to the following rules:— 1. To convert Centigrade degrees into degrees of Fahrenheit, multiply by 9, divide the product by 5, and add 32. 2. To convert Fahrenheit degrees into degrees of Centigrade, subtract 32, multiply by 5, and divide by 9. 3. To convert Réaumur degrees into degrees of Fahrenheit, multiply by 9, divide by 4, and add 32.[6] 4. To convert Réaumur degrees into degrees of Centigrade, multiply by 5 and divide by 4.[7] [Footnote 6: 8 R = 50 F.] [Footnote 7: 8 R = 10 C.] For the production of _continuous_ records, the Meteorological Committee of the Royal Society have adopted an instrument called a Thermograph, or self-recording wet and dry bulb thermometer, which is largely aided by photography. The bulbs of the thermometers are necessarily placed in the open air, and at a suitable distance from any wall or other radiating surface; the tubes are of sufficient length to admit of their being brought inside the building, in due proximity to the recording apparatus placed in a chamber from which daylight is rigidly excluded. [Illustration: 19. Thermograph and Self-recording Hygrometer. Scale about 1/18.] The essential conditions in such an apparatus are:—1. A means of denoting the height of the mercurial column in the stem of a thermometer in relation to a fixed horizontal line. 2. A time scale denoting the exact moment at which the atmosphere reached the temperature indicated by the mark. 3. As the marks are produced chemically, and not mechanically (as in the Anemograph), a _dark_ room. A description of the drawing on page 23 will best show how very efficiently, through the ingenuity of Mr. BECKLEY, these conditions have been obtained:—S, wet bulb thermometer; T, atmospheric thermometer; B, screw for adjusting thermometers; C C, paraffin lamps or gaslights; D D, condensers, concentrating the light on the mirrors R R; R R, mirrors reflecting light through air-speck in thermometers V V; E E, slits through which light passes from mirrors R R; F F, photographic lenses, producing image of air-speck from both thermometers on cylinder G; G, revolving cylinder or drum carrying photographic paper; H, clock, turning cylinder G round once in 48 hours; I, shutter to intercept light four minutes every two hours; leaving white time-line on developing latent image. II.—EVAPORATION. Solar heat rarefies the air by driving its particles asunder; it also vaporises water from the surface of river, lake, and ocean, diffusing the vapour through the atmosphere.