148 ANNUAL OF SCIENTIFIC DISCOVERY. 



From Oh. to 6k. the air has a forward motion greater than that of 

 the earth, so that it tends to lly away ; its pressure is therefore dimin- 

 ished, and the mercury falls. From 6h..to 12h. the earth's motion is 

 greatest ; it therefore presses against the lagging air, and the barom- 

 eter rises. From 12h. to ISh. the earth moves away from the air, 

 and the barometer falls ; while from ISh. to 24h. the increasing 

 velocity of the air urges it against the earth, and the barometer rises. 



From a relation of these forces to the power of gravitation, &c. 

 Mr. Chase calculates what should be the daily changes in the bight of 

 the barometer, and the results are found to correspond very closely 

 with the changes in the barometer at St. Helena, the point nearest the 

 equator where a long series of barometric observations have been 

 made. From these changes in the barometric pressure he also com- 

 putes the distance of the sun from the earth, and obtains results agree- 

 ing pretty nearly with those obtained by the most approved of other 

 methods. 



His conclusions also suggest that the revolution of the sun around 

 the great Central Sun must also cause barometric fluctuations that 

 may possibly be measured by delicate instruments and long and 

 patient observation. The Toricellian column may thus become a 

 valuable auxiliary in verifying or rectifying our estimates of the dis- 

 tances and masses of the principal heavenly bodies. 



POPULAR EXPLANATION OF THE THEORY OF THE TIDES. 



Mr. Willi mi Dennis, of Philadelphia, contributes to S Hitman's Journal, 

 March, 1864, an interesting article on the best method of presenting in 

 a popular form the " Theory of the Tides ; " from which we make the 

 following extracts : 



After alluding to the difficulty which many persons experience, in 

 understanding how the tides are produced, the author says : If a learner 

 be told that the waters of the ocean are ra'sed by the moon's attraction, 

 his first idea, in many cases, will be that they are lifted up by main 

 strength, as it were, the force of gravity being overcome, and having 

 nowhere observed any similar effect of the moon's attraction, he cannot 

 conceive how this can be. Nor will it tend in any degree to le sen his 

 perplexity if he shall sec it stated (as he may) that, according to 

 Newton's calculations, the disturbing power of the moon's attraction on 

 the surface of the earth is less than a ten mill! Jiith part of the force of 

 gravity, and that of the sun's attraction not even half as great as it. It 

 is therefore important to show, by a preliminary explanation, tint the 

 waters of the ocean, in their general figure and outline, arc in a >t;te of 

 perfect equilibrium or perfectly balanced, s that, in vif-.v of this. ;n;d of 

 their vast extent and perfect freedom of motion, they m >y lie compared 

 to a scale-beam in perfect equipoise, suspended in t.ie ;no>t dei'c te 

 manner, and several thousand miles in Icnyth. To omit this would be 

 much the same as if one should state, in proof a::d ilk* t ration of the 

 attraction of gravitation, that a weight or ball at the side of a mountain 

 had been observed (referring to the Schehallien experiment) to be 

 drawn towards the mountain by its attraction, leaving the learner to 

 suppose that the weight w.s pl.ced on a table or other level surface 

 instead of being suspended by a long thread or wire. 



In explaining this condition of equilibrium the most obvious course 



