376 REPORt— igOB. 



uncertainty both as to the formulje to be applied, and the vaUtes of the 

 coefBcients which appear in them. 

 The best known formula is 



B = KSY^ 



in which R equals resistance in kilogrammes. 



S equals projected area in square metres of total surface of vehicle on 

 a plane normal to direction of motion. 



V equals velocity in metres per second. 



K a numerical co-efficient which varies between very wide limits 

 according to the form and the speed of the vehicle. 



The formula by M. Desdouits, R = KV, is sometimes preferred, as it 

 is more correct for high speeds. 



The different values given to K in the first formula may be due to the 

 varied conditions under which the experiments were made. 



Signor Canovetti had made some experiments at Zossen to determine 

 the value of K. He had a copper Avire, 380 metres long, stretched 

 between the summit of the fortifications at Brescia and a point in the 

 plain, about 70 metres below. Along this wire different surfaces were 

 allowed to descend freely, A circle, with a surface of "073 square metres, 

 moving with a velocity of 12 metres per second, gave a resistance of 

 84 grammes. The same circle, having a spherical cap in front, offered a 

 resistance of only 21 grammes. When this hemisphere was followed by 

 a cone, whose height was five times its diameter, the resistance fell to 

 13 grammes, or one-sixth of that of the plane circle. With this same 

 solid, turned the other way about — that is, with the apex of the cone 

 towards the direction of motion — the resistance rose to 18 grammes. 



Signor Canovetti has recognised tliat a rectangular surface, placed 

 with its long sides horizontal, offers a sensibly greater resistance to the 

 air than when its short sides are hoi'izontal. His experiments seem to 

 show that the coefficient K diminishes somewhat as the speed increases, 

 'but investigations carried out at Zossen point to the conclusion that the 

 resistance may increase tenfold when the velocity is only tripled. It is 

 thus clear that air resistance is a matter of no small importance when 

 speeds up to GO or 80 miles an hour are attained. At 85 kilometres 

 (53 miles) per hour, the energy required to overcome the air resistance on 

 a vehicle, with an opposing surface of 1 square metre (1,550 square 

 inches), may bo 7, 11, or 20 horse-power, according to the coefficient K 

 given as 0-0288, 0-0648, or 0-116 by MM. Forestier, Bourlet, or 

 Thibault. 



The question then arises, What is the best shape for a car 1 The 

 answer depends on several things — as, for example, the necessity of placing 

 the radiator in such a position that it may be efficiently cooled by the air 

 rushing through it. Only general principles may be laid down. The 

 front of the car ought to taper, and the back be more pointed still, like 

 the form of a fish : transverse rectangular surfaces that cannot be dis- 

 pensed with, should, as far as possible, have the longer sides vertical ; and 

 it is well to have doors on the car to prevent the air from rushing in 

 between the dashboard and the seat. 



These conditions are quite neglected in most of the present-day cars. 

 Particularly is this the case in the ' Coffin Head,' that unlovely affair so 

 much in vogue — a flat surface directly opposed to the air pressure. With 

 a radiator of the honeycomb type, a transverse position is necessary for 



