150 ANNUAL OF SCIENTIFIC DISCOVERY. 



illustration of its condition ; the force of gravity or weight of the ball 

 standing in place of the centrifugal force in the case of the earth, and 

 the tension of the curd representing the restraining iorce of the sun's 

 attraction which at each instant holds the earth to the place in its or.iit 

 which it occupies, but as this attraction of the sun diminishes rapidly 

 with an increase of distance it is plain that it cannot hold all parts of 

 the earth alike or equally, the nearest part being about b,(K)0 miles less 

 distant than the most remote, while of course it holds the whole as a 

 mass as it would hold it were it at the mean distance of ail the parts, 

 that is, at the distance of the center. Consequently on the nearest part 

 or side towards the sun the attractive force being greater than the mean, 

 there will be a small excess over what is sufficient to hold this part to 

 its place in the orbit, and this excess acting upon the surface waters 

 there in opposition to the force of gravity renders them specifically 

 lighter and the exact equilibrium before described is immediately dis- 

 turbed : these waters will therefore rise somewhat while those that are 

 so situated as to be unaiiected by tins disturbing influence will sink 

 simply from the giving way of those which having become lighter yield 

 to their superior pressure. Again on the opposite side of the earth or 

 that most remote from the sun, the attractive or restraining force will 

 be less than the mean and therefore not quite equal to the centrifugal 

 force, and here accordingly there will be an excess of this latter force : 

 but on this side it is this centrifugal force that acts in a direction oppo- 

 site to that of gravity, and this excess of it will consequently disturb the 

 equilibrium of the surface waters here in precisely the same manner as 

 in the other case. 



Such is a statement of the general principle upon which the attraction 

 of the sun (or of the moon) tends to produce a tidal elevation on two 

 opposite sides of the earth, with intermediate depressions. 



ON A NEW FORMULA FOR CALCULATING THE INITIAL PRESSURE 



OF STEAM. 



In a communication on the above subject, made to the British 

 Association, lfeC4, by Mr. R. A. Peacock, the author stated that some 

 years ago he had occasion to attempt to calculate the probable pres- 

 sure of steam at the highest known temperatures, and found, amongst 

 other Illinois, that between the pressures of 2o Ib. per square inch and 

 iJUO Ib. to the square inch, the latter being the highest pressure to 

 whkh trii.-.tw( Tthy experiments had been carried, the law of increase 

 was, approximately: That the temperature of high-pressure steam of 

 say 2'5 11). to the square inch and upwards, increases as the 4 root 

 of the pressure ; and that, conversely, the pressure of the steam of 

 say 25 ib. to the square inch and upwards, increases as the 4^. power 

 of the temperature. At lower pressures than about 2'5 Ib. per square 

 inch, a dihcirnt law pn vails. As it is necessary to verily the new 

 formula by comparison with some well-known Ibrmulte and experi- 

 ments, the author has attempted to do so in a very voluminous table, 

 and graphically in a vt ry carefully-executed diagram. What is to be 

 gathered Iroin these is, that the new lormula agrees with Dr. Fair- 

 bairn's experiments, from about 40 Ib. to GO Ib., and very nearly with 

 RegnaulYs, between 220 Ib. and ooG Ib. 



