The force with which a horse acts is compounded of his weight and muscular strength. If, then, the weight of one horse exceed that of another to which it is inferior in muscular strength, the weaker and heavier horse will overcome a resistance which the stronger and lighter horse cannot, provided the excess of his weight in the smallest degree exceeds his deficiency in strength. When a horse draws in a mill or machine of any kind, care should be taken that the horse-walk, or circle in which he moves, be large enough in diameter, otherwise he can only exert a part of his strength ; for, in a small circle, the tangent in which he draws deviates more from the circle in which he is obliged to go than in a larger circle. The diameter of the walk for full-sized horses ought not to be less than forty feet; but if such a space cannot be obtained, and the circle be reduced, it is advisable to procure horses of similarly reduced proportions; for it has been found that the same horse loses two-thirds of his effective strength in being removed from a walk of 40 feet in diameter to one of only 19 feet.

In drawing a carriage, a horse works to the best advantage when the line of draught inclines a little upwards to his breast, making a small angle with the horizontal plane.

With respect to the quantity of power a horse of average strength can thus exert, experimentalists have materially differed, owing probably to the limited extent of their trials, with horses of different degrees of strength, and under different circumstances; for much will depend upon the nature of the ground, the proper shoeing, the angle of draught, the fitting of the collar, etc. As a variation in these points will fatigue or cramp the full exertion of a horse, a great difference in the amount of a whole day's work must result; but if we take the average of the data thus furnished to us by the different authors and experimentalists on the subject, we shall find it to amount to 160 pounds' weight, raised at the velocity of 21/2 miles per hour, the correctness of which has been most satisfactorily shown by some experiments on a very extensive scale by Mr. Bevan, that were recently communicated to the editors of the Philosophical Magazine. Mr. Bevan states, " In the period from 1803 to 1809 l had the opportunity of ascertaining correctly the mean force exerted by good horses in drawing a plough, having had the superintendance of the experiments on that head at the various ploughing matches, both at Woburn and Ashridge, under the patronage of the Duke of Bedford and the Earl of Bridgewater. l find among my memoranda the result of eight ploughing matches, at which there were seldom fewer than seven teams as competitors for the various prizes.

Horse Power 664

The first result is from the mean force of each horse in six teams of two horses each team, upon light sandy soil . . •

lbs.

156

The second result is from seven teams of two horses each team, upon loamy ground, near Berkhempstead......

15

The third result is from six teams, of four horses each team,

lbs.

127

The fourth result is from seven teams, of four horses each team,

167

The fifth result is from seven teams, of four horses each team, upon strong stony land (old Hertfordshire ploughs)

193

The sixth result is from seven teams, of two horses each team,

177

The seventh result is from five teams, of two horses each team,

170

The eighth result is from seven teams, of two horses each team,

160

The mean force exerted by each horse, from 52 teams or 144 horses, is equal to 163 lbs. each horse; and although the speed was not particularly entered, it could not be less than at a rate of 21/2 miles per hour. As these experiments were fairly made, and by horses of the common breed used by farmers, and upon ploughs from various counties, the numbers may be considered as a pretty accurate measure of the force actually exerted by horses at the plough, and which they are enabled to do without injury for many weeks; but it should be remembered, that if these horses had been put out of their usual pace, the result would have been very different. The mean power of the draught horse, deduced from the above-mentioned formula, exceeds the calculated power from the highest formula of Mr. Lesslie, and, we may add, that of Mr. Tredgold. The latter gentleman has, however, furnished us with a most valuable table, showing the maximum quantity of labour that a horse of average strength is capable of performing, at different velocities, in drawing boats on canals, and carriages on railways and turnpike roads.

Duration of the day's work at the preceding velocity.

Useful effect of one horse working one day, in tons, drawn one mile.

Velocity in miles per hour.

Force of traction in pounds.

On a canal.

On a level, railway.

On a good level turnpike road

Miles.

Hours;

Lbs.

Tons.

Tons.

Tons.

21/2

111/2

831/3

520

115

14

3

8

831/3

243

92

12

31/2

59/10

831/3

153

82

10

4

41/2

831/3

102

72

9

5

29/10

831/3

52

57

72

6

2

831/3

30

48

6.0

7

l1/2

831/3

19

41

5.1

8

11/8

831/3

12. 8

36

4.5

9

09/10

831/3

9.0

32

4.0

10

o3/4

83

6.6

28 8

3.6

It will be seen by the foregoing, that the loss of effect is very considerable upon increasing the velocity of a horse beyond that of 21/2 miles per hour; this speed has, indeed, been considered by all writers on the subject, as the fittest for obtaining the greatest quantity of power from horses, as they can exert it without being overstrained, throughout a whole day's work. At the highest speed of 10 miles per hour, which is that of many of our stage-coach horses, it will be observed that they cannot keep up this pace for more than three quarters of an hour; that is, 71/2 miles per day is as much as they can perform without being soon exhausted. The amount of useful effect assigned by Mr. Tredgold for this day's work of only three quarters of an hour, is 3.6 tons drawn one mile, which is equivalent to nearly half a ton drawn 71/2 miles. Row the weight of an ordinary stage-coach and its load is about four times the last mentioned quantity, or two tons; consequently we perceive the necessity of four horses being employed, as is customary, to do the work at that high velocity which one horse would easily perform at the velocity of 21/2 miles per hour.

In stating the power of steam engines, it is usual to say the number of horses' power it exerts, each horse power being estimated as equivalent to raising 33,000 lbs. one foot high per minute. See the article Steam.