BURNING-GLASS. 



»rt of mar, and a? none of tlicfe fiibftanccs have been found 

 ptrfeftly vitrilkd, the concliifiun is, that the volcano mull 

 cxcfcd thtin in its powtr. 



The following obftivation madt by major Gardner, lie ap- 

 plies to all the experiments made by him during a period of 

 many months; we will ufc his own words : 



" That no fub'lance can rcma-'n any length of time in its 

 focns luitufed or unvitrified, nnlefs it be externally white or 

 diaphanous; but in a number of inllaiiccs even thefe properties 

 do not prevent their beinjr fuled or vitrified : when they do, 

 it appears, in the firlt ca(e, tliat the rays were in ])a't re- 

 fiefted (perhaps before they came in contaft with the in- 

 tcnftly white fubilance) ; and from this circumftance the 

 fnn's rays are prevented from exerting their full power upon 

 them." 



Some experiments were made in the year 1802, with 

 Mr. Parker's lens, with the view of alcertaining whether 

 the moon communicated any heat to the earth, in common 

 with the reflected light ironi which we derive fo much ad- 

 Fantage. 



This ex'periment was attended by fir Jofeph Banks, with 

 feveral members of the Royal Society, together with 

 Dr. Crawford, who provided the moll lenfible thermome- 

 ters ; but after applying them to the luminous focus, fo far 

 from a perceptible increale of heat, it was thought there was 

 perceived rather a diminution thereof; but this fufpioion did 

 not lead them to a fair ii.velligation of the futl. 



Since this period fonie experiments have been made that 

 evince the power of comniun'cating cold by rcfledlion ; but 

 as this faft has not yet been explauied confillcntly with the 

 prefent received theory, we (hall content ourfelves with 

 taking notice of the experiment made by M. Pidct. Two 

 concave mirrors being placed at the dillancc of lo^ feet from 

 each other, a very delicate air thermometer »vas put into one 

 of the foci, and a glafs matrafs full of fnow in the other. The 

 thermometer funk feveral degrees, and rofe again when the 

 matrafs was removed. When nitric acid was poured upon 

 the fnow (which increafcd the cold) the thermometer funk 

 5° or 6° lower. Here cold feems to have been emitted by 

 the fnow, and reflefted by the mirrors to the thermometer, 

 which it is thought could not happen unlefs cold were a 

 fubilance. 



It has been found that upon an admixture of equal quantities 

 of fnow, which is always at 32°, and of water heated to 172°, 

 the relult is that the compound only retains the lowell heat 

 of 32^, fo that i.^o" of heat or caloric difappears. 



Much has been faid refpefting the point or degree at 

 which the thermometer fhonld indicate the prefcnce of heat. 

 The experiments of Dr. Crawford feem to place it at 1268" 

 below the prefent o. Mr. Kirwan places it at 1048°. 

 MelTrs. Lavoifier and La Place at 27;6° ; and by a mixture 

 of four parts of fulphuvic acid with three pints of water, it 

 feems that it Ihould be placed at 5803'' below o. 



Experiments of this kind may be made ad injirii/itm, and in 

 time it may poflibly be afcertained that cold is a real fub- 

 ilance ; but for the purpofe of getting an anfwer to the 

 prefent queflion, we will accommodate the fcale of Fah- 

 renheit, by adding 1 08'^ thereto fo as to make the o cor- 

 refpond with the caloric imbibed by fnow or ice before it 

 can melt. The average temperature of the air, mentioned 

 under the article Burning Gltifs, of 70° will be now con- 

 fidered as 178 of the new fcale. 



The fuperficies of fpherical bodies are to each other as the 

 fquares of their refpedlive diameters. The diameter of the 

 moon is confidered to be 2180 miles, and its mean dillance 

 from the earth 240,000 ; from which it follows, on the 

 fuppofitioii that all the folar rays received by the moon 



6 



were refleiSed back, and that the earth was abfoluttfy 

 without heat, that the cftcCi of this rtflcdtion would be 

 found to be .00,367 of a degree (for 240.COOX2': 178':; 

 2 180' : .00J67) ; which multiplied into 1051^.25, and this 

 fum iiicreafed four times for the incrcafed power of tht fe- 

 cund lens, would give 15.51234° as the heat of the focus ; 

 92.2S766" below the prefent O, or 124.28766° bilow ttic 

 treeziiig point. 



This dinertation is interilling in another point of view, 

 for this calculation afcertaiiis tli^t the light afforded by the 

 moon, when compared with that by tlie fun, abtlrafting all 

 imptdimeiits in both cafes, is only as I to 4848c. 



A fubfi ription was prcpolcd for raifing the fum of- 700 

 guineas, towards indcninitying the charges of the inventor, 

 and retaining the very curious and ulcful machine above 

 delcnbed in our own country ; but from the failure of 

 the fubfcription, and feme oilier concurring circumllances, 

 Mr. Parker was induced to difpofe of it to capt. Mack- 

 intofli, who acccmpanicd lord Macartney in the tmbafly to 

 that country ; and it wr.s left, much to the regiet of philofo- 

 phers in Europe, at Pikin ; where it remains in the hand? 

 ol perfons, who moll probably know neither its value nor 

 ufe. 



Burning of Heath, in Agricuhure, a prailice employed 

 in fome dill rifts for clearing ground covertd with this fub- 

 ilance, in order to procure grafs and herbage for cattle. 

 The moll proper time for this biifmefs is towards the latter 

 end of the lummer, when the plants are withered: care 

 (hould be taken that the fire extends no farther than is in- 

 tended, by clearing away all the grafs, and other dry vege- 

 table matters, on the fide which is to be prefervcd from the 

 flames, to a diilance futhcient to prevent all communication; 

 the grafs and other lubllances which are cut down, bein^ 

 fpread upon the part intended to be burnt, may ferve for 

 kindling the fire after they are become dry, and in a Hate 

 fit for combuftion. 



For this operation a fair calm day fliould likewife be cho- 

 fen ; when, by kindling the fire on the fide the wind blows- 

 from, the danger of its fpreading too cxtenfively is more- 

 fully guarded againll. If, however, notwithllanding thefc- 

 precautiors it Ihould fpread to places intended to be pre- 

 ferved, and where there is no water, the moll effeftual way 

 of Hopping the progrefs of it is to dig a trench ; as, by 

 throwing up the earth on the fide where the fire is, the grafs 

 is covered and the flames thereby hindered from extending- 

 farther. 



Burning of Lime, the procefs of converting hard or- 

 llrong calcareous iubllauces into lime by means of fire. See 



Burning of Straw, a wafteful difiipating praftice em- 

 ployed in fome dillrifts tor the purpofe of converting it inttx 

 alhes for manuring land. It is obferved by Mr. Young, in. 

 the Report of Lincolnfliire, that " the moll fingular praftice 

 he ever met with 111 manuring fubfills on the Wolds ; it is 

 that of fpreading dry Hraw on the land and burning it. At 

 lord Yarborough's, he fays, he tirll heard of this cullom. 

 His lordlhip's tenant, Mr. Richardfon, a very good and in- 

 telligcnt farmer, gave him the account, having long prattifed 

 it with fuccefs. The quantity is about five tons an acre. 

 At Great Lumber hefraiv-iurnt a piece in the middle of a 

 field preparing for turnips, and on each fide of it manured 

 with ten loads an acre of yard-dung, and the burned part 

 was vifibly fuperior in the crop. In another piece the fame 

 comparative trial was made, in 1796, for turnips ; and now, 

 in 1797, the barley is equally fuperioK On another farm he 

 bad at Wold-Newton he did it for turnips, then barley, and 



laid. 



