PHYSICAL SCIKNCK. 



xliii 



the thirtieth part of the logarithmic difference to 

 denote fathoms. 



Discrepancies still existed, which were first 

 accounted for by Deluc, who showed that they 

 were owing to the influence of heat on the den- 

 sity of the air. He corrected the defects in the 

 construction of the barometer and thermometer, 

 and determined the amount of the expansion 

 which air undergoes when heated. The rules 

 which he thence deduced were published in 1772, 

 in his w r ork entitled Rtcherches sur les Modifica- 

 tions de ^Atmosphere. In 1774 they were re- 

 duced to English measures, and simplified by 

 Maskelyne and Horsley. General Roy com- 

 puted and observed the heights of mountains in 

 Wales and Scotland; and Sir George Shuck- 

 borough, at the same time, and for the same pur- 

 pose, ascended the Alps. Their conclusions very 

 nearly agreed, making the difference between 

 the logarithms of the mercurial columns to ex- 

 press the elevation in fathoms at a temperature 

 only a fraction of a degree below the freezing 

 points. 



Very little addition has been made to the data 

 since 1777. A simple proportion given by the 

 late professor Leslie of Edinburgh, however, de- 

 serves to be stated, because it is easily remem- 

 bered, and will enable us to calculate the heights 

 even when we have not in our possession a set of 

 logarithmic tables. It is this : As the sum of 

 the mercurial columns is to their difference, so 

 is the constant number 52,000 to the height 

 This number is more easily remembered, from 

 the division of the year into fifty-two weeks. 



Something has been done of late years respect- 

 ing capillary attraction, especially by Mr Ivory, 

 who has unraveled the intricate formula of La- 

 place, by disjoining it into two separate portions ; 

 the one depending on the adhesion of the watery 

 film to the inside of the tube, and the other re- 

 sulting from half the cohesion of the particles of 

 the liquid to each other. We refer the reader 

 who wishes for information on this subject, to 

 Mr Ivory's masterly treatise on the elevation of 

 lluids, published in the supplement to the fourth 

 edition of the Encyclopasdia Britannica. 



ELECTRICITY. 



THIS branch of science has been chiefly culti- 

 vated by experiment It is, in consequence, full 

 of interest, and exhibits many splendid and 

 striking phenomena. The term is borrowed from 

 the Greek name for amber, and was applied to a 

 property which amber acquired when rubbed. 

 It attracted light bodies, and afterwards repelled 

 them. To this alternate attraction and repul- 

 sion the name electricity was given. 



The first person \\lioiuade any striking pro- 

 gress in the investigation of electricity was 

 Stephen Gray, a pensioner of the Charterhouse. 

 His first paper appeared in the Philosophical 

 Transactions for 1720, and he continued to pro- 

 secute the subject till the year 1736, when he 

 died. His great discovery was, that electricity is 

 capable of passing, with great rapidity through 

 metals, animal bodies, green wood, and water; 

 but not through those bodies which may be ex- 

 cited by friction, as glass, hair, silk, feathers, &c. 

 Hence he divided all bodies, as far as electricity 

 is concerned, into conductors and non-conductors ; 

 the former he could not excite by friction, the 

 latter he could excite without difficulty. He 

 found likewise, that when a conductor is sur- 

 rounded by non-conductors, or insulated, as it is 

 termed, electricity passes through it to any dis- 

 tance; but when it is in contact with conductors, 

 that electricity is dissipated. 



Ill the year 1733, M. Dufay, whose youth had 

 been spent in the army, turned his attention to 

 electricity, and speedily distinguished himself by 

 three important discoveries. 1. He found that 

 conductors may be excited, as well as non-con- 

 ductors, by friction, provided we use the precau- 

 tion to insulate them before we begin to rub. 

 It was from not attending to this necessary pre- 

 caution, that Mr Gray had failed in exciting 

 these bodies. 2. Electrical bodies, when excited, 

 attract those that are not excited, communicate 

 electricity to them, and repel them. 3. There 

 are two kinds of electricity, which Dufay dis- 

 tinguished by the names of vitreous and resinous. 

 The terms positive and negative are now usually 

 applied to them. Bodies having the same kind 

 of electricity repel t those having different electri- 

 cities attract each other. The reason why 

 excited bodies attract light substances to them- 

 selves, is that they produce in them the contrary 

 electricity from what they themselves possess. 

 Hence they are attracted in consequence of the 

 general law, that bodies having opposite electri- 

 cities attract each other. Dufay died of the 

 small pox at the early age of forty-one. 



In the year 1746, Cuneus, in company with 

 Muschenbroeck and Allamand at Leyden, made 

 an accidental discovery, which drew the attention 

 and astonishment of the whole scientific world. 

 Observing that electrified bodies, when exposed 

 to the common atmosphere, soon lost their elec- 

 tricity, and were capable of retaining but a small 

 quantity, these gentlemen imagined that if an 

 excited body were terminated on all sides by 

 electrics, it might be capable of receiving a 

 stronger charge, and of retaining it a longer 

 time. A gun barrel was suspended on silken 

 strings, having one of its ends very near a glass 

 globe, which was turned rapidly, while electricity 

 r e 2 



