An account of a useful discovery to distill double the usual quantity of sea-water, by blowing showers of air up through the distilling liquor

by Stephen Hales

75. In order to bring the Method of procuring Plenty of good Fresh-water at Sea, into practice, several previous Trials were made, at the laudable Motion and Desire of Peter Wyche, Esq; in a Still containing 24 Gallons of Water, at Messrs. Steel and Stephens’s Copper Workhouse, near the Falkon-Stairs, Southwark. The first thing to be done, was to find out the proper Size of the Diameter of the Copper Air-box. The Diameter of the Still, near its Bottom, being 19 Inches, the Diameter of the first Air-box was 18 + ½ Inches. On three several Trials, a Winchester Quart of Water was distilled in five Minutes by the common way of Distillation; and by Ventilation a Quart was, in 3 or 4 Trials, distilled in 2 Minutes; but at the Distillation of the ninth Quart, the Water was so much cooled by the ventilating Showers of ascending Air, as to require 5 Minutes and 44 Seconds. Now this great Decrease of the Quantities distilled, was plainly owing to the too great Breadth of the Air-box, which hindered the Fire from heating and keeping hot the Water which was above it, to such a Degree, as to have no Distillation in the common way, for 8 or 10 Minutes after the Ventilations; for the half Inch Circle of Water which was not covered by the Air-box, was but one nineteenth Part of the whole. The next Trial, in the same Still, was made with an Air-box, whose Diameter was 13 Inches, its Area therefore 127 square Inches; which deducted from 271 square Inches, the whole Area or Breadth of that part of the Still, there remains 144 square Inches, for the Area of the three Inch broad annular Circle round the Air-box, viz. 17 square Inches more than the Area of the Air-box. The Event was, that in seven Distillations in the common way, a Quart was distilled in 5 Minutes, sometimes in a little less, and sometimes in a longer Time. And by Ventilation 14 different Quarts were distilled in different Lengths of Time, from a Quart in 2 Minutes and 6 Seconds, to 3 or 4 Minutes.

76. I then repeated the like Trials with my small three Gallon Still, distilling in the common way a Pint in 10 + ½ and 9 Minutes; and seven separate Pints were distilled by Ventilation in 3 + ½ to 4 + ½ Minutes. Hence we see that Ventilation has a more steddy and constant good Effect where the Air-box is less in Proportion to the Still, than it was in the other two Distillations: For in this lesser Still, only 27 out of 99 square Inches were ventilated, 72 square Inches being unventilated. It will therefore probably be advisable to have the Air-boxes of other Stills to be nearly in the like Proportion to the respective Stills, the Diameter of this Still being 11 + ½ Inches, of its Air-box 6 Inches.

77. As by repeated Trials a Quart of Water was distilled in the common way in five Minutes, in the above-mentioned twenty Gallon Still, so an 120, or two Tuns and 14 Gallons, may be distilled by Ventilation in twenty Hours.

78. Mr. Wyche, who was present at the Trials near the Falcon-Stairs, observing the Water in the upper Part of the Worm-tub to be reeking hot, tho’ the Water below it was cool, very rightly proposed to have the hot Water run from the upper part of the Worm-tub, thro’ a small Copper Pipe, into the upper Part of the Still, as fast as it was distilled off, the running Quantity of Water to be adjusted by a Turn-cock in the Pipe. This I tried in my little Still, by conveying the upper warm Water of the Worm-tub into the Still thro’ a small Pipe Which passed thro’ a Hole in the Head of the Still, almost down to the Bottom of the Water in the Still: My Reasons for doing of which were, viz. lest the less hot Water, by running on the Surface of the boiling Water, might considerably check the Ascent of the rising Vapours; whereas, being mixed and blended with the lower Water, it will soon acquire a due Heat. Another Reason why I chose to lead the entering Rill of Water so slow, was, that it might be intimately mixed with the Chalk, which is there in an agitated State by reason of the Action of the Fire. The Event was as follows, viz. that six separate Pints of Water were distilled by Ventilation during the running in of the warm Water, some in 3½ Minutes, others in 4 Minutes, 4 + ½, and 4 Minutes, 50 Seconds, which was the case of the second Pint.

79. Hence we see the great Benefit of this ingenious Improvement of Mr. Wyche’s in distilling Sea-water, where the Still will be filled with the same Liquor as the Worm-tub; but this Method cannot so well be put in Practice in other Distillations. The Benefit of thus gently and incessantly keeping the Still full, will save about one-fourth of the Time and Fuel, which it would otherwise require to refill with cold Liquor, and bring to a distilling Heat. This great Advantage, added to that of distilling about double the Quantity by Ventilation, must needs both together be of so great Improvement in distilling three Parts in four more than usual, as will be of the greatest Benefit to Navigation in several respects. By this means the Still need be emptied and cleansed from the Chalk, and very salt Water, only at the end of each daily Distillation.

80. And whereas if all the Chalk requisite for a whole Day’s Distillation were put into the Still at once, it might be suspected that such a Quantity laying at the Bottom of the Still, might hinder the boiling of the Water: I put into a broadly flat-bottomed sauce-pan, half an Inch Depth of powdered Chalk, and two Quarts of Water boiled in ten Minutes: The Water mingling freely, especially in the time of boiling, among the thin pappy Chalk, was freely acted on by the Fire. But as in boiling it frothed much, it may be proper to put at first into the Still only as many half Ounces of Chalk as there are Gallons of Water; and afterwards from time to time in proportion to what additional Water shall have run into the Still, at a Hole purposely made in the Head of the Still, taking care to ventilate while the Chalk is putting in, which will cause it to mix intimately with the Water, and thereby have the better Effect on it. And perhaps a less Proportion of Chalk than half an Ounce to a Gallon of Sea-water may suffice.

81. The Degree of Fulness or Emptiness of the Still, may be known by putting a small floating metalline Vessel, in at the Chalk Hole, with a long slender Wire to it.

82. Mr. Wyche thought of other Means the better to promote Distillation, viz. by conveying the Air from the Bellows through a Pipe with several spiral Coiles, thereby in passing thro’ boiling Water in the Still, to make the ascending Showers of the Air the hotter, and so the less refrigerating. Also to have the warm Water pass from the Worm-tub, in a Pipe, with several the like spiral Coiles, in the Head of the Still; thereby the more to heat the entering Water. But we find, by the above-mentioned Experiments, that there is no occasion for either of them; but if wanted may be used.

83. As to the Benefit of Ventilators in preserving the Health and Lives of People in Slave and other Transport Ships, I have received farther Confirmation of their great Benefit, in a Letter from Dr. Demainbray, who has shewn Courses of Experimental Philosophy to his Royal Highness the Prince of Wales and Prince Edward, viz. “That in the Year 1753 Ventilators were put into the Vessels in the Slave Trade at Bordeaux, and in other Ports of France; the happy Effect of which was, that instead of the Loss of one-fourth of those valuable Cargoes, in long Passages from Africa to the French Plantations, the Loss seldom exceeded a twentieth. And since my Return to England, I have been informed of a French Vessel, which by this self-evidently reasonable Precaution, saved 308 out of 312 Slaves, spite of most tedious Calms and a long Passage.” And Dr. Garden, in his Letter to me, dated Charles-Town, South-Carolina, March 24th, 1756, says, viz. “It is indeed very wonderful, that the Slave Merchants do not come into the Use of Ventilators. There are few Ships come here from Africa, (even though they call for fresh Provisions and Water at the Islands) but have had many of their Cargoe thrown overboard; some one-fourth, some one-third, some lose half; and I have seen some that have lost two-thirds of their Slaves. I have often gone to visit these Vessels on their first Arrival, in order to make a Report of their State of Health to the Governor and Council; but I never yet was on board one, that did not smell most offensive and noisome. What from Filth, putrid Air, putrid Dysenteries (which is their common Disorder) it is a wonder any escape with Life.”

84. A probable Means lately occurred to my Thoughts, tho’ not perfectly to cure, yet much to abate the great Degrees of stinking of the Bilge-water in the Well of Ships, viz. by laying at the Bottom of the Bilge-water, round the Main-mast, Copper Pipes full of very small Holes, not one twentieth of an Inch in Diameter; and blowing Showers of Air up thro’ them from small Black-smith’s Bellows, fixed out of the way, within the Well. Such Bellows may be worked with great Ease, for one, two, or three quarters of an Hour, in every 24 Hours, more or less, as shall be found requisite by Experience: But with this Precaution, that whenever the Water stinks much, first to pump it out of the Ship, and after letting in sweet Water, then to ventilate it a little now and then, as shall be found needful; but care must be taken not to ventilate very stinking Water, because it may increase the Unwholsomeness of the foul Air in Ships. By this means Ships will probably be made something less unhealthy; for the putrid Vapours which arise from stinking Water, must needs greatly contribute to increase the Putridness, and consequently the Noxiousness of the foul Air in Ships. This Precaution therefore, with Ventilators frequently to convey off the close foul Air, will be effectual Means to make Ships much more healthy; as will the changing the foul Air in Goals, Hospitals, and sick Rooms, with proper Precautions, make them more wholsome: And it is with pleasure that I observe, that these salutary Means are coming more in use in Hospitals, &c. either by means of Ventilators, or by admitting fresh Air in small Quantities, and those spread in thin Sheets, so as not to have the entering Air blow directly on, and incommode the Patients. Where it can be had, a thorough Air entering on one side, and passing out on the other side of Wards or Rooms, will be best; because the Change will be almost constant, and may be as gradual and gentle as we please. I shall give a more particular Account of this in my second Volume of Ventilators.