Mr. Macgillivray's Account of the Outer Hebrides. 95 



four inches high, should maintain two hundred black cattle, a hun- 

 dred horses, and four or five hundred sheep, and yet this has been 

 the case with the island of Pabbay, in the sound of Harris. The 

 cattle of the small tenants are indeed stunted and ill-favoured ; 

 but this is caused by overstocking, and the degeneracy arising from 



E reserving the breed unmixed ; and where a judicious management 

 as been adopted, the cattle of the Outer Hebrides have proved 

 inferior to none in the neighbouring districts. 



In conclusion, it may be observed that the great mass of the ve- 

 getation is the same as that which occurs in other parts of the 

 country, although the absence of wood, furze, and broom, the di- 

 versified tints of the sandy pastures on one side of the range, and 

 the general heathy vegetation of the eastern side and interior, 

 make up together a picture very different from that exhibited by 

 most parts of the mainland of Scotland. 



ART. III. Description of a new Torricellian Air-Pumpy with a 

 Plate. By K. T. Kemp, Esq. Member of the Royal Physical 

 Society, &c. 



The air-pump, as an instrument for philosophical investigation, 

 yields to none either in the variety or importance of the discoveries 

 to which it has led. By means of it we have become better ac- 

 quainted with many of the phenomena of nature, — such as the 

 pressure of the atmosphere, — the cause of evaporation, — the nature 

 of sound, &c. It likewise constitutes an important part of the 

 steam-engine itself. 



Since it was first discovered, it has undergone many modifica- 

 tions, and has been greatly improved : at present, when constructed 

 in the best manner, it may be said comparatively to be a very per- 

 fect instrument. 



In every construction the end to be attained is the same, viz. 

 the removal of as great a portion of air as possible from within an 

 inclosed vessel or receiver. The more perfect the pump is, the 

 better vnM this be efl^ected. 



In the air-pimip of the common construction, the barrel and pis- 

 ton both contain a valve opening upwards, which depend entirely 

 for their action on the elasticity of the air within the receiver. If 

 we suppose the piston to be at the top of the barrel, and the air 

 within the barrel and the receiver to be of the same density, when 

 the piston is made to descend, the density of the air in the barrel 

 is increased, and the air, by this means, forces open the valve in 

 the piston, and escapes into the atmosphere. When the piston is 

 again raised, its valve is shut by the external pressure of the atmo- 

 sphere, and the air in the receiver, by its elastic force, opens the 

 valve, and flows into the barrel, until an equilibrium is restored. 

 This exhaustion goes on until the elasticity of the air within the 



