A continuation of new experiments physico-mechanical, touching the spring and weight of the air and their effects. The I. part whereto is annext a short discourse of the atmospheres of consistent bodies / written by way of letter to the right honourable the Lord Clifford and Dungarvan by the honourable Robert Boyle ...

EXPERIMENT XXII.

Wherein is propos'd a way of making Barometers, that may be transported even to distant Countries.

THinking it a desireable thing (as I have elsewhere intimated) to be able to compare together, by the help of Barometers, the weight of the Atmosphere at the same time, not onely in dif∣fering parts of the same Country, as of England, but in differing Regions of the World; I could not but foresee that 'twould be very difficult to accomplish my desire without altering the form of the Barometers I had hitherto made use of. For as these be

unfit to be transported far, because that stagnant Mercury would be so apt to spill. So the procuring them to be made in the pla∣ces where they are to be used, though it be no bad expedient, and such as I have divers times made use of, is liable to this inconve∣nience; that, besides that few will take the pains, and have the skill, requisite to make Baroscopes well, though they be suffici∣ently furnished with Glasses and Mercury for that purpose, be∣sides this, I say, except men be more than ordinarily diligent and skilful, (and perhaps though they be,) 'twill be very difficult to be sure that the Baroscope newly made in a remote Country, is as Good (and but as good) as that which a man makes use of in this; in regard that at the making of the former, they are supposed to have no other Baroscope to compare it with; and to be sure, they have not the same with which it is to be compared Here.

Being by these considerations invited to attempt the making of Portable or Travailing Baroscopes, (if I may so call them,) I thought it requisite to endeavour these three things: The first, to make the vessel that should contain both the sustained and the stagnant Mercury all of one piece of Glass, of a like bigness: The next, to place this vessel, when fill'd, in such a Frame, as may be easie to be transported, and yet in a reasonable measure defend the Glass from external violence, no part of it standing quite out of the Frame, as in all other Baroscopes: And the third, so to or∣der the vessel, that it may not be subject to be easily broken by the violent motion of the Mercury contain'd in it.

The first of these will not seem practicable to those that ima∣gine (without any warrant from the Hydrostaticks) that tis as well necessary as usual, that the stagnant Mercury should have a vessel much wider than the Tube, wherein the Mercurial Cylinder is sustain'd; but to us the difficulty seem'd much less to make the Glass part of our Tube of one piece, and of a convenient shape, than afterwards to fill it.

But to do both, we took a Glass Cylinder seal'd at one end, and of a convenient length, (as about 4 or 5 foot,) and caus'd it by

the flame of a Lamp to be so bent, that, to those that did not take notice 'twas sealed at one end, it seem'd to be a Syphon of very unequal Legs, the one being 3 or 4 times longer than the other; by virtue of which Figure the shorter Leg may serve in stead of the distinct vessel usually imployed to contain the stagnant Mer∣cury. To fill this, which is not easie, one may proceed after this manner. Take a small Funnel of Glass, with a long and slender Shank, so that it may reach 3 or 4 Inches, or further, into the shorter Leg of our Barometrical Syphon (if I may so call it;) and by this Funnel pour into this shorter Leg as much Mercury as may reach about 2 or 3 Inches in both Legs; then stopping the Orifice with your finger, and slowly inclining the Tube, the Mercury in the longer Leg will gently fall to the sealed end; and the Air that was there before, will pass by it, and so make it room. The Mercury in the shorter Leg (which Leg ought to be held uppermost) will by the same inclination of the Tube fall towards the Orifice, but, being by the finger that stops that, kept from falling out, if you do slowly reerect the Glass, and then make it stoop again as much as before, the Mercury will pass out of the shorter Leg into the longer, and joyn with that which was there before; and if all the Mercury do not so pass, the Orifice is to be stopt again with your Finger, and the Tube inclin'd as for∣merly. This done, the Tube is to be erected, and by the help of the Funnel more Mercury is to be poured in, and the foregoing pro∣cess of stopping the Orifice, inclining the Tube &c. is to be re∣peated, till all the Mercury pour'd into the shorter Leg, be brought to joyn with that in the longer; and then the open Leg is to be furnisht with fresh Mercury, observing this, that the nearer the longer Leg comes to the being fill'd, the less you must raise it from time to time, when you pour Mercury into the shor∣ter; as also, that when you see the longer Leg quite full of Mer∣cury, (though there be but litle in the shorter,) you need not pour in any more, if the longer do much exceed a Yard; because upon the restoring of the Tube to an erected posture there will

subside from the taller leg into the other a pretty quantity of Mercury, by reason of the space at the seal'd end, which will be deserted by the Mercury that was there. But because tis difficult by this way, as well as by that practised already, to fill a Tube with Mercury without leaving any visible bubbles; to free it from such (if any happen to be) you must once more stop the Orifice with your finger, and incline, and reerect the Tube divers times, till you have thereby brought most of the smaller bubbles into one greater; (which you may if you please increase, by letting in a little Air:) for by making this Great bubblle pass leisurely two or three times from one end of the Tube to the other, it will in its passage as it were lick up all the small Bubbles, and unite them to its self; which may afterwards by one inclination more of the Tube be made to pass into the shorter Leg, and thence into the free Air.

But there is another sort of Funnels, which if one have the skill and conveniency to make, (as I. M. easily doth,) one may very expeditiously fill the bended Tubes of our portable Baro∣meters. For if you make the slender part of the Funnel not streight but bended, in the form of an Obtuse Angle, and of such a length, that the part which is to go into the shorter Leg of our Siphon may reach to the Flexure (of the Siphon;) then you may, by so holding the Tube that the sealed end be somewhat lower than the other, and by pouring in Mercury at the Obtuse end of the Angular Funnel, easily make it run over the Flexure into the longer Leg of the Siphon; provided you do now and then, as occasion requires, erect a litle and shake the Tube, to help the Mercury to get by the Air, and expell it.

By such wayes as these we have found by Experience, that tis possible (though not easie) to do in such a bended Glass, as our purpose requires, what, besides a very late Learned Writer, the Diligent Mersennus himself, admonishes his Reader, that tis not a practicable thing to do in the Ordinary Glasses of the Torricel∣lian Experiment, viz. to free the Mercury of a straight Tube from

Air and Bubbles, (s as to be able by inclining the Glass to make the Liquor ascend to the very top.)

The First of our 3 above mentioned Scopes being thus attai∣ned, it was not difficult to compass the Second, by the help of a solid piece of Wood, which is to be somewhat longer than the Tube, and a good deal broader in the lower part than in the upper, that it may receive the shorter Leg of the Siphon. In such a piece of Wood, which was about an Inch thick, we caus'd to be made a Gutter or Channel, of such a depth and shape, that our Siphon might be placed in it so deep, that a flat piece of Wood (like a plain'd Lath) might be layd upon it, without at all pressing upon or so much as touching the Glass; so that this piece of Wood may serve for a Cover to defend the Glass, to be put on when the Instrument is to be transported, and taken off again when tis to be hung up to make Observations with; the Channel-piece of wood serving both for a part of a Case, and for an entire Frame; which may for some uses be a litle more commodious, if the Co∣ver be joyned (as it may easily be) to the rest of the Frame, by 2 or 3 litle Hinges and a Hasp, by whose help the Case may be readily opened and shut at pleasure.

The 3^d thing we proposed to our selves is nothing near so ea∣sie as the 2^d, nor have we yet had opportunity to try, whether the way we made use of will hold, if the Barometer be transported into very remote parts, though by smaller Removes we found cause to hope that 'twill succeed in Greater.

The Grand difficulty to be obviated was this; That though 'twere easie to hinder the spilling of the Mercury, by stopping the Orifice of the shorter Leg of our Siphon, yet that would not serve the turn; for the upper part of the Tube being destitute of Air, if the Mercury be by the motion of the Instrument put to vibrate, it will be apt (for want of meeting with any Air in the upper part of the Tube to check its motions) to hit so violently against the Top of the Glass as to beat it out, or to crack some of the neighbouring parts.

To obviate this great inconvenience our way is, to incline the Tube, till the Mercury be impell'd to the very top of it, and yet there will remain a competent quantity in the shorter leg of the Glass, if that be not at first made too short. This done, the re∣maining part of the shorter Leg is to be quite fill'd up either with Water or Mercury, and the Orifice of it is to be very carefully and firmly stopt, (for which purpose we use our strong black Ce∣ment:) for by this means the Mercury in the longer Leg, having no room to play, cannot strike with violence as before, against the top of the Glass. But though by many times successively sha∣king the Baroscope we did not perceive that 'twas very like to be prejudiced by the shakes it must necessarily indure in Transporta∣tion to remote places, if due care be had of it by the way, yet till further Tryal have been made I shall not pretend to be certain of the Event. But thus much of conveniency we have already found in this Contrivance, that we sent it some miles off to the top of a Hill, and had it brought home safe again, the phaenomena at the top and bottom of the Hill being answerable to what we might have expected if we had imployed another Baroscope.

When the Instrument is to be sent away, the height of the Mer∣curial Cylinder (to be measured from the surface of the stagnant Mercury in the shorter Leg) being taken for that place, day, and hour, and compar'd (if it may be) with that of another good Ba∣roscope, which is to continue in that place; as much of the Gutter as is unfill'd by the Glass may be well stuffed with Cotten, or some such thing, to keep the Glass the more firm in its posture; and that the Tube be not shaken or press'd against the Wood, some of the same matter may be put between the rest of the Frame and the Cover, which ought to be well bound together. And when the Instrument is arriv'd at the remote place where tis to be imployed, (for if it be to be sent but a litle way, it may be carried safely without using any adventitious Liquor,) the Water that is added, may be taken off again, by soaking it up with pieces of Sponge, Linnen, &c. but if in stead of Water you put in Mer∣cury,

as it ought to have been put in by Weight, so it is to be taken out, till you have just the Weight that was put in: and tis not difficult to take out the Mercury by degrees, by the help of a small Glass-pipe, since You may either suck up litle by little as much as remains of the additional Mercury, when by erecting the Barometer, and warily unstopping the Orifice of the lower Leg, as much Mercury as will of its self flow out is efflux'd; or else you may take out the superfluous Mercury, by thrusting the lower end of the litle Pipe into that Liquor, and when it has taken in enough, stopping the upper end close with your finger, to keep it from falling back again when you remove the Pipe.

NB. If it should happen in a long voyage, that by the nume∣rous Shakings of the Instrument there should from the additio∣nal Water or Mercury in the shorter Leg get up into the longer any litle Aerial Bubble, which seems the onely (but I hope not likely) danger in this Contrivance, he that is to use the Instru∣ment, at the end of the Voyage may, if he be skilful, free the Mercury from it by the same way, that we lately prescrib'd to free it from Air, when the Instrument was first fill'd.

I presume I need not tell Your Lordship, that the chief use of this Travailing Baroscope is, That he that uses it in a remote part, keeping a Diary of the heights of the Mercury, by compa∣ring these heights with those at which the Mercury stood at the same times in the Barometer that was not remov'd, the Agree∣ment or Difference of the weight of the Atmosphere in distant places may be observed. To which this may be added, the Con∣veniency, which the structure of these Instruments gives them to be securely let down into deep Wels or Mines, and to be drawn up to the top of Towers and Steeples, and other elevated places: not here to consider, whether by a convenient addition, these, as well as some other Barometers, may not be made to discover even very minute Alterations of the Atmospheres Pressure.

Whether this Travailing Baroscope, being furnish'd at its up∣per end with a very good Ball and Socket, and at the lower end

with a great weight, (which way of keeping things steady in a Ship has been happily used by the Royal Society on another occa∣sion,) whether, I say, our Instrument may by this Contrivance, or some other that might be suggested to the same purpose, be made any thing serviceable at Sea, notwithstanding the diffe∣ring motions of the Ship, I have had no opportunity to try: but whether it may or may not be useful in spite of the rolling of the Ship, it may at least be made use of in flat Calms, (which divers times happen in long Voyages, especially to the East-Indies, and to Africk,) and then the Instrument, which at other times may lie by without being at all cumbersom, may be made use of, as long as the Calm lasts, to acquaint the Observer with the weight of the Atmosphere in the Climate where he is, and that upon the Sea: which may give some welcome Information to the Cu∣riosity of Speculative Naturalists, and perhaps prove either more directly or in its consequences of some use to Navigators them∣selves, as by enabling them by its suddain changes to foretell the end of the Calme. Besides that, having one of these Instruments ready at hand, where ever they set foot on shore, though it be but upon a small Island, or a Rock, they can presently and easily take notice of the Gravity of the Atmosphere in that place; which whether or no, if compared with other Observations, it may in time prove not altogether useless to the Guessing whereabouts they are, and the foreseeing some aproaching changes of Wea∣ther, I leave to future Experience, if it shall be thought worth the making, to determine.

Besides the ordinary Baroscope, and this Travailing one, I have imployed 2 or 3 other Instruments of quite differing kinds, to discover the varying Gravities of the Atmosphere; but though they have hitherto succeeded well (for the main,) yet being wil∣ling to make further Observations about them, I reserve one of them for another opportunity, and think fit to leave the other in a Tract it belongs to.