Page:Madras Journal of Literature and Science, series 1, volume 6 (1837).djvu/81

No. 14, plate 15), Iron the pencil rod, because as the beam in rising above the horizontal or going below it, describes an arc, this rod with its weights, is moved more or less from the perpendicular, when both will have a tendency to bear on one of the guide rods YY. Supposing therefore the rod f s to terminate at the scale pan T, the register apparatus below may be moved further to the right. From this the pencil rod may rise to the beam, which of course must be prolonged, in order that the points on which the rod bears may be perpendicular above the register roller. The disadvantage of the pencil rod deviating from the perpendicular still remains in this arrangement:—but the end of a beam two feet in length, moving two inches upwards or downwards, deviates from the perpendicular only about quarter of an inch. Further, in the tropics the ordinary range of the barometer rarely exceeds a quarter of an inch (Daniell on Meteorological Essays), and during the storm that occurred at Madras 30th October 1836, which must be considered as an extreme case, the range of the barometer (as observed at the Madras Observatory, and recorded Madras Journal No. 14, p. 211) was only 1.135 inch; now, as the pencil rod will be very light, it is possible that the deviation under consideration, may occasion so very little difference between its absolute weight and its weight as bearing on the beam, that the objection thence arising may be more a theoretical than a practical one. It may, however, should the latter obtain, be completely avoided by placing an arc at the elongated end of the beam, the radius of which is the distance between it and the edge on which the balance turns. The pencil rod may be attached to this arc by means of a piece of platinum wire fixed to the top of the latter, which is of such fineness, that, while sufficiently strong to support the pencil rod, it is so flexible as accurately to adapt itself to the arc. It is evident that by this means the pencil rod will be moved up and down perpendicularly, as it will always be a tangent to the arc when this moves. If the arc be a perfect segment of a circle, the weight of the pencil rod will act with a uniform power on the balance, which of course it will not do if the arc be not such a perfect segment—and as there may be some difficulty in making this, it becomes a question, which of these two modes of attaching the pencil rod to the beam is the less objectionable. Should both be found practically defective, the pencil rod may be laid aside altogether, and the Mordan pencil furnished with its delicate spring, as mentioned in the original plan, be attached direct to the end of the beam at right angles to this. Thus situated, however, the pencil will require a plane surface on which to make the register, for in the case of the cylinder the pencil is a tangent to it only in a straight line, while the pencil, in the position stated, will describe an arc, as the beam moves up and down—consequently the hour lines instead of being straight as in the cylinder, must on this plane surface be seg-