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508 THE PHOTOGRAPHIC NEWS. [August 7, 1885. of the camera and slides, by reason of the riders being placed well apart, and there being plenty of room, and full arrangements for carrying baggage between them, and immediately over the axle of the driving wheels where the weight is best disposed. (To be continued.) METEOROLOGY FOB PHOTOGRAPHERS. BY J. VINCENT ELSDEN, B.SC. (LOND.), F.C.S. Chapter IV.—Atmospheric Pressure—BAROSIETERS— Causes of Variation of Pressure—Sunshine and the Barometer. The fact that the atmosphere exerts pressure upon the earth’s surface scarcely needs proof. Owing to the equality of this pressure in all directions, we are not sensible of it under ordinary circumstances; but the moment this equality is disturbed, it becomes sufficiently evident. Thus, to borrow an illustration from a piece of apparatus familiar to photographers, the pneumatic plate holder consists of a flat rubber - disc, communicating with an air-ball. When the air is squeezed out of the ball and the disc applied to a glass plate, the atmospheric pressure upon that portion of the plate which lies opposite to the disc is no longer counterbalanced by the pressure of the air in the ball. The disc and plate are consequently pressed tightly together, with a force corresponding to the difference between the pressure of the residual air in the air-ball, and the prevailing pressure of the atmosphere. This pressure is generally about 14 7 lbs. upon every square inch of surface. Familiar as this property of the atmosphere appears to be at the present day, it was not until the 17th century that Torricelli, a pupil of Galileo, discovered it. Galileo himself had previously noticed that water would not rise in a pump beyond about thirty-four feet, and he ventured upon the somewhat strange explanation that a column of water, when lengthened beyond this point, at last breaks by its own weight. Torricelli, however, proved that liquids are held up in a vacuum by atmospheric pressure alone ; that the atmosphere presses downwards with a uniform force upon every part of the surface of liquids ; that this pressure is transmitted equally and in all direc tions through the body of the liquid ; and that if this pressure is relieved from any portion of such a surface, by placing an exhausted tube over it, the liquid will be forced into the tube, until the weight of the column, so forced up, is equal to the pressure of a column of the atmosphere of the same cross-section as the tube. This is the principle of the barometer. When we say, therefore, that the height of the mercurial column is thirty inches, we mean that thirty cubic inches of mercury (supposing the cross-section of the barometer tube to be one square inch) weigh as much as the pressure of the atmosphere upon every square inch of the earth’s surface at that locality. Pascal further demon strated the truth of this principle by showing that if a barometer be taken to the top of a mountain, the mercury sinks in proportion as the height of the atmosphere diminishes. The simplest form of barometer is a tube standing verti cally in a vessel of mercury, the height of the column being measured by a graduated scale beside it. The standard barometer used at Kew is of this kind ; but to avoid the necessity for what is called “capacity correc tion”—that is, the correction necessary to be made on taking an observation, on account of the alteration in the level of the mercury in the cistern—this form is usually somewhat modified. Thus, in Fortin’s barometer the bottom of the cistern can be screwed up, at each ob servation, so that the level of the mercury in the cistern just reaches an ivory pin, marking the starting point of the scale. An easier method is adopted in the Kew barometer, in which the inches on the scale are shortened from the upper part downwards, in proportion to the rela tive sizes of the tube and cistern, so that a single obser vation is enough for each reading. The same end is gained in the well-known Siphon barometer, in which the short leg is open, and the movement of the mercury in each leg is half as much as in Fortin’s instrument. As two readings must be taken each time with this instrument, the errors of observation tend to compensate each other. Another advantage of the Siphon barometer is, that they can be removeil, without risk, by sloping it until all the mercury passes beyond the bend into the long leg, and then carrying it with the bend upwards. The ordinary dial barometers are of this kind, the index being moved by means of a string and a float resting upon the mercury in the short leg. A pencil may be attached to this instrument so as to make a mark on ruled paper, moved by clockwork, thus giving an automatic record, and form ing what is termed a barograph; but automatic registra tion is better accomplished by photographic means, to be described hereafter. A convenient form of barometer as regards size and portability is the Aneroid (fig. 1), in which the pressure is measured by the elasticity of a thin metal case, partially exhausted of air. When the pressure in creases, the lid is forced inwards ; when it diminishes, a spring pushes the lid outwards. The movements are trans mitted by levers to a dial. The disadvantage of these instruments for very accurate work is the liability of the mechanism to get slightly out of order, rendering constant comparison necessary with standard mercurial barometers. (To be continued.) PHOTOGRAPHY AND THE SPECTROSCOPE.— Lecture I.* BY CAPTAIN W. DE W. ABNEY, R.E., F.R.S. Every lecturer, when he begins, must have a text of some de scription, and 1 propose to make my text for the lectures a plate exposed to the spectrum. You see before you a spectroscope comprising a collimator, two prisms, and a camera, with a lens of 13-inch focal length, and in this slide is a sensitive collodion plate. The spectrum of the hot carbons of the electric light is upon the focussing screen which you see before you, and I will simply expose this plate, and refer to it from time to time as my lecture continues. The plate is given six seconds’ exposure to the light of that spectrum, and now in the subdued light coming from this lantern, whose sides are covered with translucent orange paper, • Being the Cantor Lectures for 1885.
