STR 



STR 



resistance which bodies oppose to a 

 force acting upon them. It is obvi- 

 ously a matter of much importance 

 to be able to estimate with tolerable 

 accuracy the efforts which an animal 

 of the average strength employed in 

 labour is capable of exerting, and, ac- 

 cordingly, very numerous observa- 

 tions have been made on the subject ; i 

 but this species of force is subject to 

 variation from so great a number of 

 circumstances, both physical and me- 

 chanical, that the results given by 

 different authors present very little 

 agreement with each other, though 

 they are of great value as affording 

 data for determining the modes in 

 Avhich animal labour is most advan- 

 tageously empUiyed. 



" Of all animals employed as first \ 

 movers, the horse is, beyond ques- 

 tion, the most useful, and that whose ; 

 labour is suscejjtible of the most nu- j 

 merous and varied applications. For 

 the purpose of determining his mus- 

 cular power, the dynamometer may 

 be conveniently used ; but as the ac- 

 tion of the-animal is very quickly re- 

 duced by continued exertion, it is 

 more usual to estimate it according 

 to the amount of daily labour perform- 

 ed. Desaguliers and Smeaton esti- 

 mate the strength of a horse as equiv- 

 alent to that of five men ; the French 

 authors have commonly stated it as 

 equal to seven men ; and Schulze 

 makes it equal to that of fourteen 

 men, in drawing horizontally. Ac- 

 cording to Desaguliers, a horse's pow- 

 er is equal to 44-000 lbs. raised one 

 foot high in one minute. Smeaton 

 makes this number 22 916, Hachett 

 28000, and Watt 33 000. The last 

 estimate is commonly understood by 

 the term horse pmccr as applied to 

 steam-engines. Tlie quantity of ac- 

 tion which a horse can e.xert dimin- 

 ishes as the duration of the labour is 

 prolonged. The following table, by 



Tredgold, shows the average maxi- 

 mimi velocity witii which a horse un- 

 loaded can travel, according to the 

 number of hours per day. 



" The useful effect a liorse is capa- 

 ble of producing depends much upon 

 the manner in which his strength is 

 applied. 



'•Strength of Materials. — There are 

 four difl'erent ways in wiiich the 

 strength of a solid body may be ex- 

 erted : first, in resisting a longitudin- 

 al tension, or force lending to tear it 

 asunder ; secondly, in resisting a 

 force tending to break the body by a 

 transverse strain ; thirdly, in resist- 

 ing compression, or a force tending 

 to crush the body ; and, fourthly, in 

 resisting a force tending to rend it 

 asunder by torsion. Mr. Hodgkinson 

 gives the following results of his ex- 

 periments on the resistance of a 

 crushing force on short pillars of some 

 of the most common descriptions of 

 wood, the force being applied in the 

 direction of the fibres : 



S s s 2 



— {Brande's Diet, of Science ; Barlow^ s 

 Treatise on the Strenslh of Timber.) 



STREPSIPTERANS. An order 

 of insects possessing rudimentary 

 elytra in the form of scales. 



S T R I A T E. Grooved, coloured 

 with narrow lines or streaks. 



JSTKlGID.i:. The raptorial birds 

 of the owl kind. 



STROBILE. The conical fruit of 

 the pines, firs, hop, <SiC. 



761 



