measured by the differences of the heights of the mercury at the two stations, and these differences being evidently always too small in this barometer, the correction is obviously always additive. As in constructing different barometers, the interior and exterior diameters of the tube will not always be exactly similar, though the cisterns may be turned always alike; this error, and of course the correction for it, should be in each instrument deduced from a comparison with a barometer of known accuracy at different heights. It will probably vary in different instruments from a ninetieth to a seventieth. Indeed, if it were always taken at an eightieth, in instruments constructed as above directed, the possible error could only amount to about one foot on a thousand; a quantity of very little importance. It now remains to say a few words on the necessity of two barometers for the mensuration of heights, and the probable error to be incurred by using a single one. There is 'no doubt, that when very great accuracy is required, two barometers ought to be used; but even with every precaution, altitudes cannot be taken by barometers sufficiently near for the purpose of carrying water, either by pipes or canals; and for the purpose of the geologist, military surveyor, or agriculturist, it is of very little importance whether a mountain is 1000 or 1010 feet high, though it is of the highest utility that he should know whether it is 800 or 1000. I have, during the course of many years, been in the habit of taking observations of altitudes by a single barometer, and have had many opportunities of repeating my observations on the same hills when the barometer has been at different heights, and either falling or rising during the time of observation; and more than once I have observed heights which had been trigonometrically taken by the best instruments; and I can safely say that the difference between these observations has seldom amounted to so much as two feet on a hundred. The mode I use is this:-At setting out, I take the height of the mercury, and note the time of observation; I likewise note the time of the second observation, and on returning to the first station, observe 3 again, again, and note the time. If the barometer has altered in the interval, a simple proportion corrects either of the three observations, and reduces the height to what would have been observed had the mercury been stationary. It is true that this method supposes the motion of the mercury to have been uniform during the interval of observation; but except in very variable weather, which does not often occur, particularly in summer, when the greater number of these observations will naturally be made, this supposition is liable to but small error. It is also true, that a traveller has often no opportunity of making a second observation at the spot he sets out from. Even in this case, a near approximation may often be made by observing, for example, at a streain on each side of the hill to be measured. If also he observes the barometer repeatedly in the morning before he sets out, and sees its tendency, and does the same at every halt, during the day, he will have data whercon to found a nearly accurate correction. But if all this should be out of his power, even under the most unfavourable circumstances, barometrical observations will give a much more accurate idea of the outline of a country than any we now possess; and it should be ever remembered, that observations though defective, if carefully made, and faithfully recorded, are valuable, and if repeated by different travellers the errors will, in most cases, compensate each other, and from the whole very accurate conclusions may be drawn. I have entered into a greater detail than would be necessary for a greater part of your readers, in the hope of being intelligible to those who are less acquainted with the subject, and who may wish to employ any instrument-maker for the construction of barometers similar to that which I have described. In justice to a very ingenious young artist, permit me to add, that I have employed in making those which I have, Mr. Thomas Jones, of No. 124, Mount-street, Berkeleysquare (pupil of the late Mr. Ramsden), and who makes them complete at the price of three guineas and a half, with a short scale reading from 25 to 31 inches; and four gui 4 neas neas for those with a long scale reading from 20 to 31 inches. I am, sir, your humble servant, H.C. ENGLEFIELD. P.S. On comparing several barometers made by Mr. Thomas Jones, since this description was first written, I find that in some of them the mercury does not take its true height on turning up the instrument, quite so quick as in the two which he first constructed for me. This difference is owing to the greater closeness of fibre in some picces of box wood than in others, but it does not affect the accuracy of the instrument. In order to give a quicker action to these barometers I advised Mr. Jones to bore a small hole or two in these cisterns, and insert a pin of open grained wood into them. This answered perfectly well; but a curious circumstance occurred when deal or willow wood pins were inserted, the mercury, when shaken for some time, passed through the pores of these woods in the form of a fine black powder, and it was necessary to substitute ashen pins to confine it in the cistern. It may not be superfluous to say, that the weight of this barometer is less than a pound and a half. The weight of Ramsden's last improved barometer is 4 pounds, and that of his earliest about 63 pounds. [ subjoin a few observations by which the accuracy of this barometer may be fairly estimated. On the Signal Hill at Brighthelmston. In 1796 with a barometer of Ramsden's. Signal House above High water mark In 1806, with my barometer Devil's Dyke, near Brighthelmston. In 1788, with a barometer of Ramsden's - 4 4 4 69 69 Lord Abercorn's Lodge, at Stanmore, above the S The observations from which this height was ded were made in the chaise, both on Jan. 3 and 7. XII. On E. V.'s Article "On the Means of gaining P in Mechanics." To Mr. Tilloch. A SIR, CORRESPONDENNT asserts in your Magazine for month*, that he has a machine with which, by the ap cation of 2lb. descending through three feet, he raises 20 through two feet. He wishes to have some person's o nion of it through the same medium. You possibly m *Phil. Mag. vol. xxix. p. 351. ha шапу, not forbear giving mine. As the gentleman has neither given a description nor rawing of his machine, it is impossible to give any other pinion of it than a general one, and that must be against : for it is not very likely that the gentleman should have iscovered any latent property in the mechanical powers; ince they have been tried in so many different ways and orms, and by so many ingenious persons, that, had there een such latent power, surely some of them would have iscovered it. But they have hitherto found, that these owers act according to certain immutable laws, beyond hich, not one jot can they be forced. The mechanical. owers, by a little consideration, may all be reduced to the fect of a lever, or a combination of levers; and I can, ith safety, affirm that the lever possesses no such power as at which he attributes to his machine. I should limit this last expression a little, for I do not tempt to deny what he so positively asserts, but this I can th equal confidence assert; that though his machine may rform as above related, it is impossible that it should conue to do so; that is, that it should continue to produce otary motion, with that power with which it set out. inust sooner or later come to a state of rest, and will then uire as much external power to restore the machine to former state, as it had apparently gained power beyond laws of mechanics, by its first effort. Then where is advantage of the machine? f the gentleman would be so obliging as to favour us na drawing and description of his machine, we may re of the fact much better; and if it convinces the world is great power, and overturns my argument, it will very h improve my knowledge in mechanics, and I shall gladly subscribe myself his much obliged humble servant, mmercial Academy, y, February 17, 1808. T. SWANWICK. XIII. Ex |