184G.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



341 



BRITISH ASSOCIATION. 



Session- lOlh, held at Southampton, September, lSi6. 



{Continued from page 308.) 



Section A. — Matiiejutics. 



Self-Kegistering Instruments. 



Mr. Brooke stated that he had applied to the barometer, thermometer, 

 and psyclirometer, a new apparatus, by which also he had olitained a register 

 of variations of the declination magnet, a description of which he had trans- 

 mitted to the Roval Society. This may be hrietly explained to consist of 

 two concentric glass cylinders, which enclose between them a piece of pho- 

 tographic paper, and are carried round by clockwork once in twelve hours. 

 These are covered by a blackened case having a narrow slit parallel to the 

 axis of the cylinders, through which a small point of light, reflected from a 

 spherical concave mirror attached to the magnet, and then refracted through 

 a cylindrical lens, passes and impresses the paper. Some photographs were 

 exhibited, and were much approved of by the president, by Dr. Whewell, 

 and other leading members of the committee who were present: from these 

 the position of the magnet at any given time might generally be determined 

 within ten seconds, and frequently within five. Tlie self-registering baro- 

 meter was described as a syphon barometer, the extremities of which were 

 nearly an inch in diameter, and exactly the same size, being adjacent por- 

 tions of the same tube. A glass bulb having a tubular stem rests on the 

 surface of the mercury in the lower end of the tube, and is maintained in a 

 vertical position by small friction-rollers. On this stem rests tlie short arm 

 of a balanced lever, and the barometer is so placed that the long arm of the 

 lever carries a black paper screen between a lamp and the slit in the appa- 

 ratus above described ; the light of the lamp being condensed by a cylindri- 

 cal lens placed in front of and parallel to the slit in the apparatus. This 

 screen has a narrow slit in it, at right angles to the former; and the small 

 portion of light, transmitted through the point at which the two slits cross 

 each other, produces a trace upon the photographic paper. The ratio of the 

 arms of the lever may be determined at pleasure, so as to ma,^nify the varia- 

 tions from two to ten times; and as the line is very sharply defined, when 

 magnified five times the variations may be readily determined to the thou- 

 sandth part of an inch. The author stated that he had not had the oppor- 

 tunity of determining the errors of the instrument, arising from friction and 

 other causes, but he believed them to be very small. In the self-registering 

 thermometer and psycbrometer the mercury in a wide flat bore of the tube 

 intercepts a portion of the focal line of light formed by refraction throusb a 

 cylindrical lens placed as above. This does not require more particular des- 

 cription, as it differs only in details from similar apparatus, of which a de- 

 scription is in print. 



Mr. F. Ronalds, on presenting his third annual volume of observations 

 and experiments made at the Kew Observatory, described bis experiments 

 on the photographic self-registration of the electrometer, the barometer, the 

 thermometer, and the declination magnetometer, explained his existing appa- 

 ratus for these purposes, and exhibited the resulting photographs — but first 

 briefly adverted to bis previous proposals in 18-40 and 1841, and experiments 

 in 1341 relative to the subject. The principal characteristic of his improved 

 system is a peculiar adaptation of the lucernal microscope. An instrument 

 of this kind was employed in July 1843 to register the variations of Volta's 

 atmospheric electrometer. The pair of straws were properly insulated and 

 suspended within the body of tbe microscope, and towards its object end. 

 A condensing lens was placed at the end itself, and a good lamp stood be- 

 yond it ; a strong light was therefore projected upon those sides of the straws 

 which were turned towards the condensing lens, and the other sides were in 

 deep shade. Tbe lignt also impinged upon a little screen fifteil into the 

 back of a case about two feet long fixed to the eye end of the microscope, at 

 right angles with it, and vertically; through this screen was cut a very nar- 

 row curved slit, whose chord was horizontal, and radius equal to tbe length 

 of the straws. Between the electrometer and the screen an excellent com- 

 bination of schrometer lenses by Ross was accurately adjusted, to produce a 

 good chemical focus of the electrometer at a distance as much beyond the 

 external surface of the screen as the thickness of one of the plates of glass to 

 be presently mentioned. In the long vertical case was suspended a frame 

 about half the length of the case, provided with a rabbet, into which two 

 pieces of plate glass could be dropped, and these brought into close contact 

 by means of six little bolts and nuts. The frame could, be removed at plea- 

 sure from the line by which it was suspended, and the line, after passing 

 through a small hole stopped with grease at the top of the long case, was 

 attached to a pulley about four inches in diameter, on the hour arbor of a 

 clock. Lastly, counterpoises, rollers, and springs were used for insuring ac- 

 curate sliding of the frame, &c. A piece of photographic paper was now 

 placed between the two plates of glass in the movable frame, the long case 

 was closed so as to prevent the possibility of daylight entering it, the clock 

 was started, and the time of starting was noted. All that part of the paper 

 which was made to pass over the slit in the screen by the motion of the 

 clock, became now therefore successively exposed to a strong light, and was 

 consequently brought into a state which fitted it to receive a dark colour on 

 being again washed with the usual solution, excepting those small portions 

 upon which dark images of the lower parts of the straws were projected 



through the slit ; these parts of course retained the light colour, and formed 

 long curved lines or hands, whose distances from each other at any giveji 

 part of the photograph, i. e. at any given time, indicated the electric tension 

 of that time. Sometimes daylight was used instead of the light from alamp, 

 and in that ease, during the process some appearances of the sky were occa- 

 sionally noted, by which it was evident that in serene weather, when the sun's 

 light and heat varied, and the paper became consequently either more or 

 less darkened, the electric tension as shown in the photograph varied also, 

 increasing with tbe increase of light, &c. This fact has not perhaps before 

 been observed : but as the darkening effect on the paper could not always 

 be depended upon, separate notes were taken of the intensities of light and 

 the same results obtained. At the suggestion of the astronomer royal a dis- 

 tinguishing electrometer formed on the day pile system was afterwards era- 

 ployed, which exhibited in the photograph not only the tension but the kind 

 of electricity possessed by the electrometer at any given time. 



The dry thermometer was next tried ; it was of the horizontal kind, had 

 a flat bore, and its tube was introduced through the side of the microscope ; 

 the tube had a diaphram of very narrow aperture fixed upon it, and the slit 

 in the screen at the eye end of the microscope was now of course straight 

 and horizontal. The image was a little magnified, and the breadth of the 

 dark band or line in the photograph became tbe measure of temperature in- 

 versely at any time.* The barometer employed was of the syphon kind ; 

 the microscope was turned, in order to bring the long case and its sliding 

 frame into an horizontal position ; the clock was placed at one end, and a 

 little weight sufficient to keep the frame steady was suspended by a line 

 parsing over a pulley at the other end. The lower leg of the barometer was 

 introduced through the now bottom of the microscope, it was provided with 

 a similar kind of diaphram to that on the thermometer, and of course the 

 slit in the screen was now vertical. A very light blackened pith-ball rested 

 on the surface of the mercury, and its image was slightly magnified, but will 

 in future be much more so. The declination rnaf/net was one of two feet, 

 provided with a damper, and its mode of suspension was essentially similar 

 to that of the Greenwich declinometer. In order to adapt it for self-regis- 

 tration, a very light conical brass tube, projecting six inches beyond its north 

 end, was affixed to the lower side of the spur which carried it, aud to the 

 north end of that tube a small wire, called the index, was attached at right 

 angles ; this index descended through little slits in the bottoms of the two 

 cases, enclosed the magnet, &c., and took the place of the electrometer de- 

 scribed above in the lucernal miscroscope, which was placed below the cases, 

 and was now required to be much longer than before, in order that the 

 image and motion might be suliicieatly magnified, yet retaining a flat field. 

 Everything was very firmly fixed upon the two pillars which formerly carried 

 tbe transit instrument of George III. 



A great many photographs were obtained and sent for inspection to Green- 

 wich. Of some termday impressions, Mr. Glaisher, the magnetical and me- 

 teorological superintendent of the Greenwich Observatory, says, in an official 

 note, that " the beautiful agreement of those results with these at Green- 

 wich is highly satisfactory." This must be gr,itifying to Mr. Ronalds, who 

 has from the first so ably devised and conducted the experiments and obser- 

 vations at Kew. 



Mr. DoUond's atmospheric recorder registers simultaneously and continu- 

 ously on the same sheet of paper every variety of change in the barometer, 

 thermometer, hygrometer, electrometer, pluviometer, and evaporator: it also 

 records the force and direction of the wind. 



The barometer is upon the siphon principle, of a large bore. Upon the 

 surface of the mercury in tbe shortest leg is placed a float, very accurately 

 counterpoised, leaving only sufficient weight to compel it to follow the mer- 

 cury, and is correctly adjusted to that part of the apparatus which moves 

 the indicator, when the pressure of the atmosphere is at thirty inches. The 

 connexion of the float with tbe indicator is so arranged as to give a scale of 

 three to one. 



The if/iermome^r/cfl/ arrangement consists of lea mercurial thermometers 

 of a peculiar form. These are suspended upon an extremely delicate and 

 accurate balance. They are placed at the north end of the frame, and are 

 screened from the effects of the wind and rain by perforated plates of zinc. 



The hygrometer consists of a slip of mahogany cut across the grain. This 

 was placed in a cylinder filled with water, and suspended from the upper end 

 with a weight of two pounds at the other end, until it was found by repeated 

 examination to be completely saturated, and no longer to increase in length. 

 The length was then referred to an accurate scale, and the slip of mahogany 

 placed alongside the pipe of a stove, under the same suspension and weight, 

 until its shortest length was obtained. The difl^erence of the two results 

 being carefully taken, the scale was formed accordingly. It is placed in a 

 tuhe, open at both ends for a free passage of air, outside the observatory. 

 It is suspended and weighted as before, with full power to act upon the arm 

 of the indicator, quite free from the action of the sun or rain, and is found 

 to be extremely active and firm in its operation, showing upon an open scale 

 every hundredth of its extremes in dryness and moisture. 



The electroimter for thunder-storms and electric changes is constructed 

 by placing a well insulated conductor upon ihe higiiest convenient place, 

 from which a wire is brought down to an insulation on the top of the obser- 

 vatory, and from thence to a standard through another insulation to a metal 

 disc, between which and another fixed disc there is a movable disc attached 



* In order to cowvert this into tlie wet. bulb ' liygroraeter' nothing of course is necessary 

 but the application of tbe usual cup of water and the capillary threads. 



44 



