liv INTRODUCTION TO ASTRONOMY. 



passing across the sun's disk : this is called a transit of the planet. It 

 was by the last transit of Venus that astronomers were enabled to calcu- 

 late with some degree of accuracy the distance of the earth from the sun, 

 and the dimensions of the latter. 



The tides are produced by the attraction of the moon. The cohesion of 

 fluids being much less than that of solid bodies, they more easily yield to 

 the power of gravity ; in consequence of which the waters immediately 

 below the moon are drawn up in a protuberance, producing a full tide, or 

 what is commonly called high-water, at the spot where it happens. 

 According to this theory, you would imagine that we should have full tide 

 only once in twenty-four hours that is, every time that we were below 

 the moon while we find that we have two tides in the course of twenty- 

 four hours, and that it is high-water with us and with our antipodes at 

 the same time. 



This opposite tide is rather more difficult to explain than that which is 

 drawn up beneath the moon. In order to render the question more sim- 

 ple, let us suppose the earth to be everywhere covered by the ocean, as 

 injfig. IV. M is the moon, ABCD the earth. Now the waters on the 

 surface of the earth about A, being more strongly attracted than in any 



Fig. 19. 



other part, will be elevated, the attraction of the moon at B and C being 

 less ; but still it will be greater there than at D, which is the part most 

 distant from the moon. The body of the earth will therefore be drawn 

 away from the waters at D, leaving a protuberance similar to that at A : 

 so that the tide A is produced by the waters receding from the earth, and 

 the tide D by the earth receding from the waters. 



The influence of the sun on the tides is less than that of the moon ; for 

 observe, that the tides rise in consequence of the moon attracting one 

 part of the waters more forcibly than another part : it is this inequality of 

 attraction which produces full and ebb tides. Now the distance of the 

 sun is so great, that the whole globe of the earth is comparatively but as 

 a point, and the difference of its attraction for that part of the waters 

 most under its influence, and that part least subject to it, is but trifling ; 

 no part of the waters will be much elevated above, or much depressed 

 below their general surface by its action. The sun has, however, a con- 

 siderable effect on the tides, and increases or diminishes them as it acts 

 in conjunction with, or in opposition to, the moon. 



The moon is a month in going round the earth ; twice during that 

 time, therefore, at full and at change, she is in the same direction as 

 the sun. Both then act in conjunction on the earth, and produce very 

 great tides, called spring-tides, as represented inj%. 20, at A and B ; but 

 when the moon is at the intermediate parts of her orbit, the sun, instead 

 of affording assistance, weakens her power by acting in opposition to it ; 

 and. smaller tides are produced, called neap-tides, as represented in fig. 21. 



