Exp. 29. " If two plane polished plates of glass, three or four inches broad, and twenty or twenty-five long, be laid one of them parallel to the horizon, the other upon the first, so as at one of their ends to touch one another, and contain an angle of about 10 or 15 minutes, and the same he first moistened on their in ward sides with a clean cloth dipped into oil of oranges, or spirit of turpentine, and a drop or two of the oil or spirit be let fall upon the lower glass at the other end ; so soon as the upper glass is laid down upon the lower, so as to touch it at one end as above, and to touch the drop at the other end, making Is ith the lower glass an angle of about 10 or 15 minutes : the drop will begin to move towards the concourse of the glasses, and will continue to move with an accelerated motion till it ar rives at that concourse of the glasses. For the two glasses attract the drop, and make it run that way to wards which the attractions incline. And if, when the drop is in motion, you lift up that end of the glasses where they meet, and towards which the drop moves, the drop will ascend between the glasses, and therefore is attracted. And as you lift up the glasses more and more, the drop will ascend slower and slower, and at length rest, being then carried downwards by its weight as much as upwards by the attraction. And by this means you may know the force by which the drop is at tracted at all distances from the concourse of the glasses. Now, by some experiments of this kind, (made by Mr Ilauksbee.) it has been found that tl c attraction is almost reciprocally in a duplicate proportion of the distance of the middle of the drop from the concourse of the glasses. viz. reciprocally in a simple proportion, by reason of the spreading of the drop, and its touching each glass in a larger surface ; and again reciprocally in a simple proportion, by reason of the attraction growing stronger within the same quantity of attracting surface. The at traction therefore within the same quantity of attracting surface is reciprocally as the distance between the glass es. And therefore, where the distance is n small, the attraction must be exceeding great." New tons's Optics, p. Exp. 30. If the plates in Exp. 26. are inclined to each other at a small angle, and are immersed in water with the line of their intersection vertical, the water will ascend between them, and will form a beautiful curve. This experiment is represented in Plate CX. Fig. 6. By measuring the ordinates and abscissa of the curve a b, Mr Hauksbee found it to be the common Apollonian hyperbola. This indeed follows from Exp. 26, from which it appears that the altitudes are recipro cally as the distances of the plates. The heights of the fluid being the ordinates of the curve, and the distances of the plates being as the abscissa, the ordinates are in versely proportional to their respective abscissa, which is the property of the common hyperbola.
The preceding experiments, though numerous and varied, are obviously insufficient to form a proper basis for a philosophical theory. The enormous discrepancy among some of the results, as obtained by different phi losophers, is completely bewildering ; and, if we did not know that they were all made in the eighteenth cen tury, we should have imagined that they were obtained in the dullest age of experimental philosophy. The height to which water ascends in a capillary tube, the diameter of whose bore is of an inch, has been stated in the latest elementary works upon natural philosophy, at 5 inches and 3-tenths, according to the experimas of Mr Atwood; while, according to the more recent experiments of Messrs Hauy and Tremery, made at the desire and under the eye of La Place, the water rises only to the height of 2 inches and I-tenth. The difference between these quantities amounts nearly to two-thirds of the whole ; and what is still more perplex ing, we are unacquainted with the particular manner in which the experiments were made, • so that we are unable to discover any ground of preference, from the varlet's precautions which were probably taken by the different observers to ensure accuracy, and to avoid error.
Under these circumstances, the writer of this article resolved to repeat the experiments himself ; and he was particularly anxious to bring his apparatus into such a state, that lie could always obtain the same result by re peated trials. After much trouble, he succeeded in this attempt to such a degree, that the water uniformly stood at the same height in the tube ; and the same degree of accuracy was obtained for the other fluids, which were submitted to examination. To enable the reader to form a proper estimate of the degree of confidence which these results should inspire, we shall describe the precautions which mere found necessary in the course of the experi ments.
Haring obtained a glass tube 7.9 inches long, and of a iniform circular bore, we took a wire of a less dia meter than the bore of the tube, and formed a small hook :11 one of its ends. This hook was fastened to the middle of a worsted thread, of such a size, as when doubled, to fill the bore of the tube. The wire was then passed through the tube, and the worsted thread drawn after it ; and when the whole was plunge /I in an alkaline solution, tile worsted thread was fixed at one end, and the tube was drawn backwards and forwards, till it was complete!y deprived, by its friction on the thread, of anv gre-se or lercign matter which might have adhered to it. The tube and thread were then taken to clean water, and the same operation was repeated.
When the tube was thus perfectly cleaned, it was fixed vertically, by means of a level, Plate CX. Fig. 7. in the axis of a piece of wood D, supported by the arm A D, fixed upon a stand A B ; and it was also furnished with an index in n, which was moveable to and from the extremity b. On the arm C E, moveable in a vertical direction by the nut C, was placed a glass vessel F, containing the fluid, and nearly filled with it. The nut C, was then turned till the extremity b of the tube touched the surface of the fluid, which was indicated by the sudden rise of the liquor round its sides. The fluid then rose in the tube till it remained stationary, and the index in n was moved till its extremity n pointed out the exact position of the upper surface of the fluid. In this situation, the distance n b was a measure of the ascent of the liquid above its level in the vessel E. In order to ascertain, however, whether the fluid was sta tionary in consequence of any obstruction in the tube, or of an equilibrium of the attracting forces, the vessel with the fluid was raised a little higher than its former posi tion, by means of the nut C, and then depressed below it. If the fluid now rose to o, a little above n, and after wards sunk to p, a little below it, so as always to rise and fall with facility and uniformity along with the sur face of the fluid in the vessel, it was obvious that it suf fered no obstruction in the tube, and that n 6 was the accurate measure of its height. By separating the ex tremity b of the tube from the surface of the fluid, the fluid always rises above n; but upon again bringing them into contact, the fluid resumes its position at n. If there should be any portion of fluid at the end b of the tube, when it is again brought in contact with the fluid surface, the water would i•se around it before it had reached the general level, and therefore the height of the fluid, ob tained by measuring from the end of the tube, would be too small. In order to avoid this source of error, the index should have a projecting arm m r, Plate CX. Fig, 7. No. 2. carrying a screw st, whose sharp point can be easily brought on a level with the end b of the tube. ‘Vhen the extremity t, therefore, which can al ways be kept dry, comes in contact with the fluid sur face P Q, the extremity b must also be exactly in the same level, even though the fluid had already risen around it. The tube was then cleaned, as formerly, for a subsequent observation. The results which were thus obtained for a great variety of fluids, and with a tube 0.0561 of an inch in diameter, are given in the follow ing Table.