xiv THIRD REPORT — \SSo. 



tion, some further delicacy of observation, some further accu- 

 mulation of exact facts, are requisite ; though in every branch 

 of the subject the labour of calculation, the delicacy of obser- 

 vation, and the accumulation of exact facts, have already gone 

 so far that the mere statement of what has been done can 

 hardly be made credible or conceivable to a person unfamiliar 

 with the study. 



*' One article, indeed, in his list of recommendations to future 

 labourers, read at the last Meeting of the Association, may ap- 

 pear capable of being accomplished by more limited labour than 

 the rest, — the determination of the mass of Jupiter by obser- 

 vations of the elongations of his satellites. And undoubtedly, 

 many persons were surprised when they found that on this, so 

 obvious a subject of interest, no measures had been obtained 

 since those which Pound took at the request of Newton. Yet 

 in this case, if an accuracy and certainty worthy of the present 

 condition of Astronomy were to be aimed at, the requisite ob- 

 servations could not be few nor the calculation easy, when it 

 is considered in how complex a manner the satellites disturb 

 each other's motions. But the Meeting will learn with pleasure 

 that the task which he thus pointed out to others, he has him- 

 self in the intervening time executed in the most complete 

 manner. He has weighed the mass of Jupiter in the way he 

 thus recommended ; and it may show the wonderful perfection 

 of such astronomical measures to state, that he has proved with 

 certainty, that this mass is more than 322 and less than 323 

 times the mass of the terrestrial globe on which we stand. 



" Such is Astronomy : but in proceeding to other sciences, 

 our condition and our task are of a far different kind. Instead 

 of developing our theories, we have to establish them ; instead 

 of determining our data and rules with the last accui'acy, we 

 have to obtain first approximations to them. This, indeed, 

 may be asserted of the next subject on the list, though that 

 is, in its principles, a branch of Physical Astronomy ; for that 

 alone of all the branches of Physical Astronomy had been al- 

 most or altogether neglected by men of science. I speak of 

 the science of the Tides. Mr. Lubbock terminated his Report 

 on this subject, by lamenting in Laplace's words this unmerited 

 neglect. He himself in England, and Laplace in France, were 

 indeed the only mathematicians who had applied themselves 

 to do some portion of what was to be done with respect to this 

 subject. Since our Meeting last year, Mr. Dessiou has, under 

 Mr. Lubbock's direction, compared the tides of London, Sheer- 

 ness, Portsmouth, Plymouth, Brest, and St. Helena ; and the 

 comparison has brought to light very remarkable agreements. 



