Notices respecting New Buo/cs. 131 



motion ; and we should remember that an impulse once given 

 will maintain itself long with little support. Virgil says of 

 the dove: 



" Celeres neque commovet alas." 



Having enumerated such suggestions as do not seem utterly 

 groundless, and which may separately or in combination sup- 

 j)ly materials for the speculations of others, I will add a hope 

 that the Society of Arts may draw the attention of scientific 

 persons to a full investigation of the question. Youi-s &c. 



QUERIES. Septimus. 



1. Does not the velocity of an aerial current generally di- 

 minish very rapidly as it approaches the earth, and should 

 not an aerostatic machine be kept low in the air, except when 

 the current is favourable? — Vide Principia, book ii. last sect. 



2. Since the resistance varies nearly as av^— bv, v being 

 the velocity ; and since the power acting upon the air may 

 move with much greater velocity than that produced in the 

 balloon's motion, — will not power be gained by acting in the 

 lower and denser strata of the atmosphere ? 



3. Must not the line of direction of any force, to be applied 

 with a maximum effect, pass through the centre of pressure 

 of the system ? And what must be the position of a plane 

 connecting two balloons, that a force acting in the direction of 

 the plane may pass through the centre of pressure of the whole 

 system, one balloon being supposed to precede ? 



4. Let a fan be wound spirally about a rod, like an Archi- 

 medes' screw, and let the rod turn in the air on its axis with 

 a uniform velocity, — will the rod acquire any other motion in 

 addition to its motion of rotation, and in what direction ? 



XXII. Notices respecting New Books, 



PROPOSALS have been issued for publishing by sub- 

 scription, in about fifty quarterly parts. Species Conchy- 

 limum: or, Descriptions of all the known species of Recent 

 Shells. By G. B. Sowerby, F.L.S. Illustrated with coloured 

 plates, by J. D. C. Sowerby, F.L.S.— The temporary posses- 

 sion of the celebrated Tankerville Collection, Messrs. Sowerby 

 observe, will enable them to secure drawings and descriptions 

 of many shells that could not otherwise be easily obtained : 

 this, in addition to their own private collections, and the im- 

 mense number of species contained in the collection late the 

 property of Mr. George Humphrey, with the access they have 

 to the cabinets of many of their friends, will enable them to 

 render this work by fur tlie most complete of its kind. 



R 2 Recently 



