of the tub should be sunk below the surface of water contained in proper cisterns. The excrements are soon dissolved in water, and so carried off, every time the privy is washed, which should be as often as it is used.

In towns the stench of the common sewers is sometimes very offensive. This may be prevented by interrupting the current of air through them by means of stink-traps; the construction and utility of which, are of late years well known in London. As sand or other filth may be apt to lodge in the deepened place, it should be so contrived, as to be easily come at, in order to clear away every obstruction.

Let me add here to the method of correcting bad water, proposed by Dr. Munro, in his Essay on the means of preserving the health of soldiers, the following easy method of keeping water clear and sweet, ascertained by several experiments, made some years ago by the Society for the Encouragement of Arts, &c. in London. The method is to mix clay with the water in such quantities, that when the clay is dissolved, the hand immersed under the surface of the water shall not be seen. The clay subsiding, carries down with it all the impurities, and, in a manner burying them, prevents their communicating any bad taste or smell to the water, which thereby continues long clear and sweet. Clay may probably correct stagnant water, and thereby preserve it clear and good in dry seasons, and may thus become very useful, where there is no running water. If any bad taste or smell remains after the use of the clay, it may be carried off by one of the ventilators recommended for that purpose by the Rev. Dr. Hales. The clear water may be drawn off by a siphon or a cock, placed high enough not to touch the clay. A. S.

ON RAIN.

Extract of a Letter to Dr. Percival, 1771.

"On my return to London I found your favor of the 16th May, (1771.) I wish I could, as you desire, give you a better explanation of the phenomenon in question, since you seem not quite satisfied with your own; but I think we want more and a greater variety of experiments in different circumstances, to enable us to form a thoroughly satisfactory hypothesis. Not that I make the least doubt of the facts already related, as I know both Lord Charles Cavendish and Dr. Heberden to be very accurate experimenters: but I wish to know the event of

the trials proposed in your six queries; and also, whether in the same place where the lower vessel receives nearly twice the quantity of water that is received by the upper, a third vessel placed at half the height will receive a quantity proportionable. I will, however, endeavor to explain to you what occurred to me, when I first heard of the fact.

I suppose it will be generally allowed, on a little consideration of the subject, that scarce any drop of water was, when it began to fall from the clouds, of a magnitude equal to that it has acquired when it arrives at the earth: the same of the several pieces of hail; because they are often so large and weighty, that we cannot conceive a possibility of their being suspended in the air, and remaining at rest there for any time, how small soever; nor do we conceive any means of forming them so large, before they set out to fall. It seems then, that each beginning drop, and particle of hail, receives continual addition in its progress downwards. This may be several ways: by the union of numbers in their course, so that what was at first only a descending mist, becomes a shower; or by each particle in its descent through air that contains a great quantity of dissolved water, striking against, attaching to itself, and carrying down with it, such particles of that dissolved water as happen to be in its way; or attracting to itself such as do not lie directly in its course, by its different state with regard either to common or electric fire; or by all these causes united.

In the first place, by the uniting of numbers, larger drops might be made, but the quantity falling in the same space would be the same at all heights; unless, as you mention, the whole should be contracted in falling, the lines described by all the drops converging, so that what set out to fall from a cloud of many thousand acres, should reach the earth in perhaps a third of that extent, of which I somewhat doubt. In the other cases we have two experiments.

1. A dry glass bottle, filled with very cold water, in a warm day, will presently collect from the seemingly dry air that surrounds it, a quantity of water that shall cover its surface and run down its sides; which perhaps is done by the power wherewith the cold water attracts the fluid: common fire that had been united with the dissolved water in the air, and drawing that fire through the glass into itself, leaves the water on the outside.

2. An electrified body left in a room for some time, will be more covered with dust than other bodies in the same room not electrified; which dust seems to be attracted from the circumambient air.

Now we know that rain, even in our hottest days, comes from a very cold

region. Its falling sometimes in the form of ice shows this clearly; and perhaps even the rain is snow or ice when it first moves downwards, though thawed in falling: and we know that the drops of rain are often electrified. But those causes of addition to each drop of water, or piece of hail, one would think could not long continue to produce the same effect; since the air through which the drops must fall, must soon be stripped of its previously dissolved water, so as to be no longer capable of augmenting them. Indeed very heavy showers of either are never of long continuance, but moderate rains often continue so long as to puzzle this hypothesis: so that upon the whole I think, as I intimated before, that we are yet hardly ripe for making one.

REPORT ON LIGHTNING CONDUCTORS FOR THE POWDER MAGAZINES AT PURFLEET.

TO THE PRESIDENT AND COUNCIL OF THE ROYAL SOCIETY.

GENTLEMEN,

(Drawn up by B. Franklin Aug. 21, 1772.)

The Society being consulted by the Board of Ordnance, on the propriety of fixing conductors for securing the Powder Magazines at Purfleet from lightning, and, having thereupon doneus the honor of appointing us a committee to consider the same and report our opinion, we have accordingly visited those buildings, and examined with care and attention their situation, construction, and circumstances, which we find as follows:

They are five in number, each about 150 feet long, about 52 feet wide, built of brick, arched under the roof, which in one of them is slated with a coping of lead 22 inches wide on the ridge, from end to end; and the others, we were informed, are soon to be covered in the same manner. They stand parallel to each other, at about 57 feet distance, and are founded on a chalk rock about 100 feet from the river, which rises, in high tides, within a few inches of the level of the ground, its brackish water also soaking through to the wells that are dug near the buildings.

: The barrels of powder, when the magazines are full, lie piled on each other up to the spring of the arches; and there are four copper hoops on each barrel, which, with a number of perpendicular iron bars (that come down through the

arches to support a long grooved piece of timber wherein the crane was usually moved and guided to any part where it was wanted), formed broken conductors, within the building, the more dangerous from their being incomplete; as the explosion from hoop to hoop in the passage of lightning drawn down through the bars among the barrels, might easily happen to fire the powder contained in them: but the workmen were removing all those iron bars (by the advice of some members of the society who had been previously consulted); a measure we very much approve of.

On an elevated ground, nearly equal in height with the tops of the magazines, and 150 yards from them, is the house wherein the Board usually meet: it is a lofty building, with a pointed hip roof, the copings of lead down to the gutters; whence leaden pipes descend at each end of the building, into the water of two wells 40 feet deep, for the purpose of conveying water forced up by engines to a cistern in the roof.

There is also a proof-house adjoining to the end of one of the magazines; and a clock-house at the distance of feet from them, which has a weathercock on an iron spindle, and probably some incomplete conductors within, such as the wire usually extending up from a clock to its hammer, the clock, pendulum, rod, &c.

The blowing-up of a magazine of gunpowder by lightning within a few years past at Brescia in Italy, which demolished a considerable part of the town with the loss of many lives, does in our opinion strongly urge the propriety of guarding, such magazines from that kind of danger. And since it is now well known from many observations, that metals have the property of conducting, and a method has been discovered of using that property for the security of buildings, by so disposing and fixing iron rods, as to receive and convey safely away such lightning as might otherwise have damaged them, which method has been practised near 20 years in many places, and attended with success in all the instances that have come to our knowledge, we cannot therefore but think it advisable to provide conductors of that kind for the magazines in question.

In common cases it has been judged sufficient, if the lower part of the conductor were sunk three or four feet into the ground till it came to moist earth; but this being a case of the greatest importance, we are of opinion that greater precaution should be taken. Therefore we would advise, that at each end of each magazine a well should be dug in or through the chalk, so deep as to have in it at least 4 feet of standing water. From the bottom of this water

should rise a piece of leaden pipe to or near the surface of the ground, where it should be strongly joined to the end of an upright iron bar, an inch and a half in diameter, fastened to the wall by leaden straps, and extending ten feet above the ridge of the building, tapering from the ridge upwards to a sharp point; the upper 12 inches to be copper, the iron to be painted.

We mention lead for the under-ground part of the conductor, as less liable to rust in water and moist places, in the form of a pipe, as giving greater stiffness for the substance; and iron for the part above ground as stronger and less likely to be cut away. The pieces of which the bar may be composed should be screwed strongly into each other by a close joint, with a thin plate of lead between the shoulders, to make the joining or continuation of metal more perfect. Each rod, in passing above the ridge, should be strongly and closely connected by iron or lead, or both, with the leaden coping of the roof, whereby a communication of metal will be made between the two bars of each building, for a more free and easy conducting of the lightning into the earth.

We also advise, in consideration of the great length of the buildings, that two wells of the same depth with the others, should be dug within 12 feet of the doors of the two outside magazines; that is to say, one of them on the north side of the north building, the other on the south side of the south building; from the bottoms of which wells similar conductors should be carried up to the eaves, there joining well with a plate of lead extending on the roof up to the leaden coping of the ridge, the said plate of lead being of equal substance with that of the coping.

We are further of opinion, that it will be right to form a communication of lead from the top of the chimney of the proof-house to the lead on its ridge, and thence to the lead on the ridge of the corridor, and thence to the iron conductor of the adjacent end of the magazine; and also to fix a conductor from the bottom of the weathercock spindle of the clock-house down on the outside of that building into the moist earth.

As to the board-house, we think it already well furnished with conductors by the several leaden communications above mentioned, from the point of the roof down into the water, and that by its height and proximity it may be some security to the buildings below it: we therefore propose no other conductor for that building, and only advise erecting a pointed rod on the summit, similar to those before described, and communicating with those conductors.

To these directions we would add a caution, that in all future alterations or