Dr. Pearson's Introduction to Practical Astronomy. 1 33 



grounds, but especially from his own quotation of the St. Bernard observa- 

 tions, where, a8 the mean temperature is much above 0° cent, in summer 

 and below it in winter, the oscillation should be distinctly positive in the 

 former case, and negative in the latter. This I have shown to be precisely 

 the reverse of the fact. 



" 5th, Availing myself of M. Bouvard's excellent Table, with such addi- 

 tions as I could make to it, I proceeded to investigate, from observations 

 made near the level of the sea alone, the influence of latitude in modifying 

 the oscillation ; and from a careful combination of the best results, by 

 reducing the squares of the errors to a minimum, I obtained the following 

 equation, which represents wonderfully well the existing observations : 



* =3-031 cost — -381 



for millimetres, z being the oscillation in latitude 0. This gives for the 

 equatorial oscillation 2 mm- 650, and for the poles — *38l. The latitude 

 where the oscillation changes its sign, or is = 0, is 64° 8'. 



" 6th, In the course of this investigation, having selected the observations 

 atCumana and Toulouse, (both being places where the oscillation is positive,) 

 for obtaining approximate values of the constants in the formula, I found 

 to my surprise and satisfaction, that from these observations alone, we might 

 have inferred, a priori, not merely a negative oscillation in the arctic circle, 

 but one not differing sensibly in amount from the actual observation of 

 Captain Parry *. 



" 7th, I have determined from the formula the mean atmospheric tide 

 from the equator to the pole to be equivalent to the weight of a stratum of 

 air 1(H metres in thickness ; and the mean for the whole surface of the earth 

 to be 16 metres, the air being considered under the usual pressure and tem- 

 perature." 



[To be continued.] 



Dr. Pearson's Introduction to Practical Astronomy. 4to. 2 vols. 

 [Concluded from vol.i. p. 457.] 



The First Section of the Second Volume of this work consists of 

 " Preliminary Remarks" on the objects of Astronomical Science, 

 and on the interest of those objects as manifestations of Divine wis- 

 dom, power, and benevolence. 



After these remarks our author commences §. II. with that which 

 is properly the subject of his work, viz. the best means of making 

 correct astronomical observations, the first requisite for which is a 

 suitable and well situated observatory. 



In §. III. we have an account of two constructions of a rotative 

 dome. This part, in the construction of the roof of an observatory, 

 is essential, as it is the only convenient mode of making an equa- 

 torial or altitude and azimuth instrument fully effective for the 

 purposes for which they are designed. It is desirable therefore 

 that this dome should revolve without difficulty; which it cannot be 

 expected to do, when there is much friction arising from a warping 

 of the parts. The construction of Smeaton, though very ingenious, 

 is not free from this defect. But the last dome, described by our 

 author as having been constructed by himself at the suggestion of 

 Troughton, appears to be free from this defect j as the ebony balls 

 * See Art. 15. of the paper. 



