HAS 



RAT 



tip; the wings are large, reticulated, 

 and semi-transparent. Ii is a native of 

 North America. R. rnantispa, a small 

 species that has the habits of the genus 

 Mantis, and is supposed by some to be- 

 K.I-.LT to tli.it genus 



RAREFACTION, in physics, is the 

 nruiki. g a body to expand or occupy more 

 room or space, without the accession of 

 now matter. It is by rarefaction that 

 gunpowder takes effect ; and to the same 

 pritxiple also we owe eolipiles, theimo- 

 nu-\ rs, &e. The degree to which air is 

 rurefiable exceeds all imagination ; per- 

 haj>t indeed its degree of expansion is 

 absolute ly beyond all limits. Upon the 

 rarefaction of ti>e air is founded the me- 

 thod of measuring altitudes by the baro- 

 meter ; in aii cases of which the rarity of 

 the air is found to be inversely as the force 

 that compresses it, or inversely as the 

 weight of all the air above it at any place. 



The open air, in which we breathe, 

 says Sir Isaac Newton, is 8 or SOO times 

 lighter than water, and by consequence 

 8 or 900 times rarer. And since the air 

 is compressed by the weight of the in- 

 cumbent atmosphere, and the density of 

 the air is proportionable to the compress- 

 ing force, it follows by computation, that 

 at the height of about seven Engligh miles 

 from the earth, the air is four times rarer 

 than at the surface of the earth ; and at 

 the height of 14 miles, it is 16 times rarer 

 than at the surface of the earth ; and at 

 the height of 21, 28, or 35 miles, it is res- 

 pectively 64, 256, or 1024 times rarer, or 

 thereabouts ; and at the height of 70, 140, 

 ajid 210 miles, it is about 1,000,000, 

 1,000,000,000,000, or 1,000,000,000,00 J, 

 000, &c. 



Mr. Cotes has found, from experiments 

 made with a thermometer, that linseed- 

 oil is rarefied in the proportion of 40 to 

 39 in the heat of the human body ; in that 

 of 15 to 14, in that degree' of heat 

 wherein water is made to boil ; in the 

 proportion of 15 to 13, in that degree of 

 heat wherein melted tin begins to har- 

 den ; and finally, in the proportion of 23 

 to 20, in that degree wherein melted tin 

 arrives at a perfect solidity. The same 

 author discovered, that the' rarefaction of 

 the air, in the same degree of heat, is ten 

 times greater than that of the linseed-oil ; 

 and the rarefaction of the oil, about fif- 

 teen times greater than that of the spirit 

 of wine. 



RASAXT, or RAZAJTT, in fortification. 

 Kasant-flank, or line, is that part of the 

 curtain or flank, whence the shot explod- 



ed rase, or glance, along the surface of 

 the opposite bastion. 



RAT. See MLS. 



HATCH, or RASH, in clock-work, a sort; 

 of wheel having twelve fangs, which 

 serve to lift up the detents every hour, 

 and make the clock strike. 



RATCHETS, in a watch, are the small 

 teeth at the bottom of the fusy, or barrel, 

 which stops it in winding up. 



RATE, a standard or proportion, by 

 which either the quantity or value of a 

 thing is adjusted. 



RATE of a ship of ivar is its order, de- 

 gree, or distinction, as to magnitude, bur- 

 den, &c. The rate is usually accounted 

 by the length and breadth of the gun- 

 deck, the number of tons, and the num- 

 ber of men and guns the vessel carries. Of 

 these there are six rates. A first-rate 

 man of war has its gun-deck from 159 to 

 174 feet in length, and from 44 to 50 feet 

 broad ; it contains from 1313 to 1882 tons, 

 has from 706 to 800 men, and carries 

 from 96 to 100 guns. Second rate ships 

 have their gun-decks from 153 to 165 

 feet long, and from 41 to 46 feet broad ; 

 they contain from 1086 to 1482 tons, and 

 carry from 524 to 640 men, and from 84- 

 to 90 guns. Third rates have their gun- 

 decks from 140 to 158 feet in length, 

 from 37 to 42 feet broad ; they contain 

 from 871 to 1262 tons ; carry from 389 to 

 476 men, and from 64 to 80 guns. Fourth 

 rates are in length on the gun-decks from 

 118 to 146 feet, and from 29 to 38 broad; 

 they contain from 448 to 915 tons ; carry 

 from 226 to 346 men, and from 48 to 60 

 guns. Fifth rates have their gun-decks 

 from 100 to 120 feet long, and from 24 to 

 31 broad; they contain 'from 259 to 542 

 tons, and carry from 145 to 190 men, and 

 from 26 to 44 guns. Sixth rates have 

 their gun-decks from 87 to 95 feet long, 

 and from 22 to 25 feet broad ; they con- 

 tain from 152 to 256 tons, carry from 50 

 to 110 men, and from 16 to 24 guns. 



It is to be observed, that the new-built 

 ships are much larger, as well as better, 

 than the old ones of the same rate; 

 whence they double numbers all along; 

 the larger of which express the propor- 

 tions of the new-built ships, as the less 

 those of the old ones. 



RATIO, in mathematics, is the relation 

 which one quantity bears to another in 

 respect of magnitude, the comparison be- 

 ing made by considering 1 how often one 

 contains, or is contained by the other. 

 Thus, in comparing 6 with 3, we observe 

 that it has a certain magnitude with res 



