If I ask too great a favor of you to forward the inclosed Letter to the Sieur Lomonosow at Petersburg, I leave it intirely with you to suppress it. I have taken the Liberty, as you see, of asking an Answer thro’ your hands; if I make too free a use of your name and Friendship, you have it in your power to prevent the Abuse. At least however give me Leave to ask from yourself an Account of the Discoveries of the Polar Voyage, if such an one should be effected. I suppose your Petersburg Correspondance is with Aepinus or Braunius. If the Baltic Voyages should continue to be prosecuted from America as they are begun, I should be glad of an epistolary Connexion at Petersburg. Your Residence in London, and even in this World, will not probably continue many years; I had thought to have availed my self of your Friendship so far as to have asked your introduction to a Correspondence in the Philosophic way with some Gentleman in London; but this I leave also to your Humanity.

When you read the inclosed which had I Time I would have recopied more correctly; you may consider it addressed to yourself as well as to the Sieur Lomonozow, particularly as to the Thermometrical Observations here of 1764; which your beneficence has enabled me to make. I have published a Request in the prints, that Gentlemen of Curiosity would furnish themselves with Barometers and Thermometers, and publish like Observations in the respective provinces, that we may particularly be enabled to collect, for 1766, annual accounts of the Mercurial Altitudes in several parts of each of the Sixteen continental provinces for four of which we have them already. But I fear this will fail of Success chiefly for Want of Thermometers; unless the Royal Society should condescend to dispense Thermometers over America, as they did over Europe and Asia about 30 years ago. Twenty Thermometers judiciously distributed, beside those we already have here, would answer the End.

Upon reading in the annual Register of 1762, an account of the congelation of Mercury by Mr. Braunius with artificial Cold at Petersburg in Dec. 1759: I ventured yesterday on some of his Experiments of Refrigeration, with common Salt, Aqua fortis, and Spirit Sal. Marin. affused upon Snow; tho’ with no view of pursuing them to the fixation of Mercury, least I should break my Thermometer. From Braunius I expected to find the Temperature of the Snow and the external Air the same; but I found it otherwise. On the 19th. Inst. i.e, yesterday, from Xh. to IIh. PM the Therm. in open Air abroad was from 37 to 43 and the Snow dissolving apace; during this Time for four hours, I repeatedly set the Tube or rather the Bulb in snow covered with it to 10 to 20 40 degrees, when on every Trial it fell to 32, and then became stationary. And when by artificial cold I had reduced it far below the freezing point, upon removing the Tube to pure fresh unmit snow, the [mercury] constantly arose and became stationary at 32; yet removing the snow it ascended to 37 and beyond. Bringing the Th[ermometer] into the house it rose to 50, when the bulb being placed in a Glass of Snow brought from Abroad, the instantly descended from 50 and above to 32 and there was stationary. From thence I concluded that, altho’ below the freezing point both Snow and Air might be of the same Temperature; yet when the natural Temperature of the Air is above freezing, it differs from that of snow, which in this case is always stationary at or about 32.

This Morning being down to 25 abroad, I applied Snow to the Bulb expecting no Alteration, yet it fell half a degree; and at noon there was near a degree’s difference the Snow a degree colder than the Air, tho the air held the cold of 28½. But perhaps after 20 or 30 degrees descent below the freezing point, there may be a Coincidence of Temperature, as it happened at the Braunian Experiment of congealing Mercury, when he found both Snow and Air at 210 of Deslile corresponding to 40 below 0 of Fahrenheit, or 72 below his freezing point. This little Occurrence is new to me, who see few other recent physical Discoveries than what are exhib[it]ed in Magazines &c. Had we duly the periodical publications of the several Academies of Sciences in Europe, or those of the London Royal Society only (of which we have here only the first 10 Vol. abr.) I might perhaps save you the Trouble of this Account.

Mr. Watson in his Account of the Braunian Experiment tells us the Mercury fixt at the several Descent of 470, 530, 650, 680, 700, and 800; and yet in another Experiment the fixity did not take place till the amazing Descent of above 1500 (equal I think to 1620 below Farenheit, freezing point for Rain Water): sundry of these Fixations were in the Bulb, and it does not appear by Mr. Watson (in the Abstract I saw) that any were not in the Bulb; And yet he speaks of 650 as the mean Term of mercurial congelation. I should be glad to know if ever a Tube has been used of Sufficient Length to admit a Dereliction of part of the Thread when the Mercury is totally and centrally congealed; and the Line or degree which the summit of the mercurial Thread should mark, just at the moment when it shall appear the central fixity is effected. the Points of initial and final Congelation I should be glad to see determined whether they coincide or not. It appears to me that the altitude of fixity will be found various, the Diameter of the Thread remaining the same and the Quantity of Mercury in the Bulb is increased or diminished. The finer the Thread in Proportion to the Diameter of the Bulb, the lower the descent and the longer must be the Tube. And if these be of the Diameters only of either of Thermometers mentioned to have been used in the Braunian Experiment, and the Bulb and Quantity of the Mercury be increased indefinitely, I see not but that, instead of the 1500th, the Descent might proceed to the 15 Thousand or indefinitely. Had the Thread of Braunius’ Thermometer been larger in proportion to the Bulb, the central Fixation might have been effected, before the whole Thread had been retracted into the Ball; in which case its summit had marked the true degree of central intire Congelation—a point, which perhaps may be found to be as invariably the same in Mercury as in any other fluids and Metals. When it is found how great a Diminution of Bulk is sustained in retracting the dilatation from the boiling to the point of fixity for purified Mercury, whether a Tenth or an Eighth of the Bulk; it will then be found that a columnal Thread, a very little greater than this difference, will not be intirely absorbed in the sphere of the Bulb, but leave an Extremity to mark the freezing point. But while portion of Thread, intercepted between the boiling and freezing points, is less in Bulk, than the difference above said, (as I suppose was the Case in Mr. Braun’s Thermometers) in freezing, it will be constantly retracted within the Bulbs:—and in this Case the Thermometers may be of different Lengths, and yet in all the mercury be even centrally congealed, at points a thousand or ten Thousand degrees apart, without indicating the true thermometrical degree of Fixation. Such must be the construction that at the Moment of fixing, the Mercury shall more than fill the Bulb. And this may be done with a very short as well as a very long Thermometer. I think I may expect to see Thermometers of a foot or even six Inches long, distinctly and well comprehending both the boiling and freezing or fixing points. For this, I am not certain there is need of any Bulk at all—the Expansion and contraction of the Columnal Thread only may be sufficient. I have sometime thought that sufficient Attention has not been paid to apportioning the Bulk of the Thread, under certain determinable Limits, to the Magnitude and Contents of the Bulb—and that this is necessary to the perfection of Thermometers. I am confirmed in it by the Braunian Experiment: in which I Judge he rather sought for lengthened Tubes than diminished Bulbs. Had he diminished the Bulbs, the thread remaining the same, so as that the Bulk of the Thread would have exceeded the freezing diminution of the whole Body of Mercury be might have found the Fixation at different degrees from those he did: unless I should say he found, no degrees; he fixed but at what degree I know not. I certainly think the several degrees for the Congelation he mentions are none of them the points or point of fixing Mercury—in all the variety indeed which he experienced, with this difference that the Ratio would have been determinable from any of the last, but could have been from none of the first: and in fact Mr. Watson left it a problem. He doubtless very justly assumes the boiling point for mercury 414 degrees of Delisle above that of boiling water; but I am at a loss to understand “a mean Term of Congelation” at all or that 650 should be the mean between 470 and above 1500. If we could distinguish the degree the summit of the Thread touches when the exterior congelation of the bulbular mercury commences, from that it touches when the central finishes, the Medium of these may be the mean Term of Congelation. But even in Thermometers so constructed that there would be a portion of Thread remaining at the Moment of central fixity, these points would be very different according to the Diameters of the Bulbs. The degree at the initial Congelation is the true point to be found—this continued will fix any given bulk, and neither rises nor falls. If therefore after a congelation comences the cold be augmented it ceases to be that of mercurial freezing—nor in fixation can any augmented Cold diminish the bulk or retract the Thread lower than it would descend if left to be operated upon by the Cold of its initial Congelation: The different, surprizingly different fixing descents of Braunius’s Mercury are not to be ascribed to the different augmentation of Cold: caet par. with the same degree of Cold fixes, and with a greater it can do no more—nor in congealing can it indicate more than one degree of cold and that invariably the same. All this is said upon Supposition that Mr. Braun always found the Congelation in the Bulbs—otherwise it is to no purpose. However give me Leave to add on this supposition, that the Rapidity of the last stage, for perhaps a third of the space from natural cold, is easily accounted for; a phaenomenon which might not be exhibited on a Thread too long or large to be swallowed up in the Bulb.

We have had a severe Winter in New England: Janry 27th. at Sunrise my Th[ermometer] stood at five below Cypher; I am told the same morning at Boston it was nine below 0. I hope you will not forget to recomend that ingenious and learned Gentleman Mr. Professor Winthrop to the Honors of the Royal Society, now that you are in London. I am Sir your most obedient servant