Neiv Species of Cypselid(^. 355 



It is not only the fact that such limits exist, as here shown ; 

 but it is also the fact that the limit to economical expansion is 

 reached at a low value of the ratio of expansion for ordinary 

 engines — sometimes probably as low as 3 or 4. 



Thus, in the condensing unjacketed engine of moderate speed, 

 as in the United States steamer "Michigan," it will be found 

 that, whatever steam-pressure is attained, there exists a limit 

 to economical expansion at some point near r=3, and it can 

 never, in such a case, be economical to "cut-off" within one- 

 third stroke unless a better curve of efficiency is obtained. 



In well-designed engines of more economical types, the limit 

 is found at higher values of r, but may still occur within the 

 range of expansion often met with in practice. 



HoBOKEN, N. J., April, 1882. 



XX. — Description of a New Species of Bird of the Family 



CypselidcB. 



BY GEOEGE N. LAWRENCE. 



Read October 2d, 1882. 



Hemiprocnc minor. 



Above, the plumage is of a lustrous black ; the upper tail-coverts and 

 tail are smoky blackish-brown ; the wings are black ; the quills, with the 

 exception of the outer three, are narrowly margined with grayish-white at 

 their ends ; the chin and throat are fuliginous-brown ; the breast, abdomen 

 and under tail-coverts are smoky brownish-black ; a white collar encircles 

 the neck, behind it is rather narrow and well-defined, in front it is not 

 so clearly defined, and widens out on the breast, where the feathers have 

 their centres mottled with black ; the collar on the hind neck is one-quarter 

 of an inch in width ; on the breast, at the widest part, it is three-quarters 

 of an inch ; bill black. 



Length (skin), 7 inches ; wing, 7 ; middle tail-feathers, 2^ ; outer tail- 

 feathers, 3|. 



