_Differential Thermometer._--To graduate this instrument, you first maintain the two bulbs at an equal temperature, by which you determine the first fixed point, which is zero. Then, enveloping one of the two bulbs with melting snow, and elevating the other by means of a vessel with warm water, to a known temperature--to 20° Centigrade, for example--you fix a certain space, which you afterwards divide into 20 equal parts or degrees. The scale is continued by carrying successively to each side the known value of a degree. * * * * * GRADUATION OF RUMFORD’S THERMOSCOPE.--This instrument is graduated by dividing the tube which separates the two bulbs into equal parts, the number of which is arbitrary, though, in general, the thermoscope tube is divided into nine or eleven parts. There is always an odd number of degrees, and you manage so that the odd degree is found in the middle of the tube. It carries the mark of zero at each end, and the figures 1, 2, 3, &c. proceed from each end of this middle degree, and form two corresponding scales. * * * * * GRADUATION OF MARIOTTE’S TUBE.--You divide the little branch which is sealed at the end into a certain number of parts of equal capacity, and the large branch into inches and parts of inches. It is necessary to take care that the zero of the two ascending scales correspond, and are situated above the inferior bend formed by the two branches of the instrument. THE END. W. WILSON, PRINTER, 57, SKINNER-STREET, LONDON. End of Project Gutenberg's The Art of Glass-Blowing, by T. P. Danger