Sessional_Paper_1884-1885 — Page 130

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the airspecks is narrowed by slits, and the same arrangement is made to obtain the photographic record as in case of the barograph, but the photograph exhibits in this case two curves, which represent the heights of the dry and damp bulb thermometers interrupted by the two-hour lines. The record of the damp bulb is placed vertically under that of the dry, so that there is only one time-scale. One or two zero lines, from which to measure are obtained by allowing the light of either lamp to shine through', a small hole in either of the frames, in which the slits are cut.

27. The photographic sheets obtained from the thermograph are treated exactly as those obtained from the barograph. When they are dry the distances of the points on the curves from the zero line are read off by aid of glass scales graduated to degrees. The temperature in either case corresponding to the zero line is obtained daily by comparison with the simultaneous readings of the thermometers in the screen, which are corrected before being entered in the thermograph journal. The degrees on the reading scales should be larger than the degrees on the thermometers in the proportion, in which the images are magnified by the photographic lens. This has been attained in case of the damp bulb, but the degrees on the dry bulb scale must be multiplied by 0.980 in order to represent the readings of the thermometer. A correction is applied for this before the readings are entered on the tables printed in the monthly reports.

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28. In order to prevent by any possibility a mistake in the date of the photographic sheets, weekday, month and date are written on the back of every sheet, as it is removed from the cylinder.- Before my appointment the Crown Agents for the Colonies had ordered the barograph and the thermo- graph as well as the anemograph through the Meteorological Office. The tabulator, reading scales &c., were subsequently ordered at my suggestion as well as the pluviograph. Unfortunately a large stock of waxed paper had also been supplied by the Secretary to the Meteorological Office, who was not aware, that argento-bromide paper had for years been successfully adopted in India. Now the necessity for iodising and sensitising every photographic sheet has caused a deal of trouble during the damp and hot season. The sensitised sheets were found not to keep for two days on the barrel. The sheets had then to be changed every day. Even the iodised sheets did not keep for any length of time. Only freshly iodised sheets could be sensitised with any certainty of success, and this added enormously to the labour a great part of the time of the second assistant being taken up by this work. Even when every precaution was taken, the result was not nearly as good as during the winter. Tannin, as recommended by Chambers, was tried, but made no improvement here. A supply of MORGAN & KIDD'S argento-bromide paper has now been ordered, and thus the trouble of iodising and sensitising the sheets will be saved.-Another cause of occasional failure rests with the Kerosine lamps, but the new paper being so much more sensitive, the lamps are not likely to give any trouble, when the new process is introduced. It may also be found possible to secure Kerosine oil of superior quality. No great difficulty was encountered in keeping the damp bulbs constantly wetted, but occasionally the bulbs were found to be dry.

29. The clocks of the barograph and the thermograph were rated by shortening the pendulums, but it was found inconvenient to shorten them sufficiently. The outstanding error was corrected by laying suitable pieces of iron and a few small leaden weights on the flat upper surfaces of the bobs, the rates being subsequently kept constant by adding or removing one or more of the small weights. This arrangement proved so satisfactory, that the clocks when accurately started one morning were in by far the greatest number of cases found as accurate next morning, and the error seldom exceeded half a minute, and never 45 seconds.

30. The anemograph was erected in the course of January and worked without interruption since the 1st March. It is erected on a turret, built of strong teak-wood timber, fastened to the roof of the house by massive iron bolts. The turret rises 8 feet above the flat roof of the main building.

31. This instrument registers the number of miles traversed by the wind and also its direction. It consists of a ROBINSON'S anemometer of large size, the cups of which are 45 feet above the ground and 155 feet above mean sea level. The shaft carrying the cups is supported by friction balls running in a groove on top of the direction shaft and terminates in an endless screw, which working through toothed gearing drives a cylinder in the turret, round which a thin strip of brass forming a screw is wrapped. Round another larger cylinder, which is driven by a clockwork, is wrapped the metallic paper, on which the space traversed by the wind is recorded by the screw-shaped pencil, which rests on it with part of the weight of the cylinder round which it is wrapped. The pencil has only one turn on this cylinder and its pitch is 23 inches long, equal to a scale of 50 miles printed on the paper. ROBINSON'S Original factor-3 is adopted in our anemometric records. Whenever from further investiga- tion a new and reliable factor, dependent on the velocity of the wind shall have been determined for an instrument of exactly similar construction, it will be easy to alter the figures in our tables, but the action of the instrument is so perfect that no allowance need be made for friction.-In order to obtain a sufficiently distinct trace of the direction of the wind, the vane consists of two wind mill wheels, which keep their axis at right angles to the wind. With any change they move and carry with them a hollow brass tube, which contains, but is not connected with, the velocity shaft and acting through toothed gearing moves another thin screw-shaped pencil, which registers the direction on another part of the metallic paper. The pitch is equal to that of the velocity pencil and equal to a scale of the cardinal points of the compass printed on the paper. The clock moves the cylinder on which the paper is fastened 0.366 inch per hour.