In summing up to the jury the learned trial judge gave only the following direction on the expert's evidence. "According to the evidence of Mr. Foster Joy, if you accept it, that sort of vehicle" (an articulated vehicle) "has particular characteristics which are not associated with the ordinary motor car and one of the characteristics, according to his evidence, if you accept it, is on taking a bend an articulated vehicle such as this we are concerned with has a tendency to swing out; in other words, there is a time lag between the alteration in the direction of the prime-mover on the one hand and the trailer portion of the vehicle on the other". Continuing, the learned judge said this: "You are entitled to consider all the circumstances of the case. What might be a perfectly proper speed to drive any particular vehicle on a straight road might not be a proper speed to drive that same vehicle at, be it semi-trailer or not, when one is negotiating a bend in the road. What might be a perfectly proper speed for any sort of a vehicle on a dry road might not be a proper speed on a wet road. I am mentioning these matters merely as being matters that you are entitled to take into consideration in determining whether or not reasonable care was or was not exercised by the defendant, Mr. Clark, in relation to this particular vehicle on this particular day under the particular circumstances where this accident occurred". In the Full Court, Owen J., who gave the judgment for the Court, dealt with the ground of appeal founded upon the objections to Mr. Joy's evidence at the trial as follows: "In the course of the plaintiff's case, a man named Joy was called, he being a person who is not unknown as a witness in these courts, and who, not infrequently, is called to give so-called scientific evidence in motor-car collision cases. The objection made to his evidence was that it was not shown that he was sufficiently qualified as an expert to give the evidence which he in fact gave. His evidence was to the effect that, where a vehicle such as a semi-trailer is travelling downhill and the prime-mover is turned to its left, centrifugal force tends to cause the rear part of the vehicle to swing out to its right. He said also that where, as here, such a vehicle was travelling at thirty-five miles per hour downhill that tendency would operate, and expressed the opinion that that was probably the cause of the accident. This evidence was objected to on the ground that the witness was not sufficiently qualified to give it. I feel considerable doubt whether it needs such expert knowledge to be qualified to express an opinion of that kind. The matter is one on which I would have thought any intelligent person could speak with some authority but, in any event, I think the witness was sufficiently qualified. He said that he had had considerable experience in the investigation, on behalf of insurance companies and others, of road accidents and their causes, and I think it is impossible to say that the learned judge fell into error in allowing the evidence to ge given, although, for my part, I would not have thought that the weight to be attached to it would be very great". This passage contains an accurate statement of the substance of the evidence given by Mr. Joy which the defendant contends was inadmissible. That evidence was clearly admissible if Mr. Joy was sufficiently qualified to give it. At the outset of his evidence he gave some evidence of his qualifications and then he gave considerable evidence without objection. It included this evidence: "Q. You have heard evidence as to the semi-trailer's movement along the road? A. Yes. Q. With a vehicle such as that, an articulated vehicle, it depends for its movements upon traction from the prime mover, in front? A. Yes, and in this case it was approaching down a grade - I saw the scene of the accident and I viewed the road approaching the scene, and from the northern end and the southern end, in other words, from the Gosford end and the Sydney end, the road has a bitumen surface approximately twenty to twenty-two feet wide with gravel shoulders eight feet wide on the one side and four feet wide on the other side. They both approach a distance of flat, reasonably level road. They both approach down the grade towards the bridge over Piles Creek and a distance of flat and reasonably level road in two directions - by two directions I mean level longitudinally in the direction of travel and level transversely - there is no bank on the road at that point. Approaching from the Gosford end it is a long grade approximately one in three - I am wrong, one in thirty-three - and it winds slightly but it is approximately - you can see at least a third of a mile of it and it is a downgrade, a gentle well made downgrade all the way. Q. That is from - . A. From the Gosford end coming towards Sydney; coming from the north and proceeding south. On the other side of the bridge there is also a long well-formed downgrade, slightly steeper, about one in twenty-five and of the same width - twenty to twenty-two feet, of bitumen with gravel shoulders eight feet and four feet. That approaches the level portion at the bottom near the bridge. The two form nearly a right-angle, not quite a right-angle, the junction being the bridge and the flat portion of the highway, thereabouts. Q. The flat portion is south of the bridge? A. Yes. Visibility for drivers, of both motor vehicles, that is approaching from the southern end or the northern end, would be good. In this case, I was shown a position on the roadway where the Standard Vanguard " That was the plaintiff's panel-van. The objection to the witness giving expert evidence was made when the plaintiff's counsel, having summarized to the witness the material facts proved by the evidence led for the plaintiff describing how the accident happened, asked Mr. Joy this question: "Assuming that happened, what in your opinion was the cause of that occurring?" In rejecting that question the learned trial judge said that if the witness was sufficiently qualified he could tell the Court what can cause vehicles to move in certain ways, but that is not the same thing as asking him to express an opinion as to what caused the defendant's vehicle to move in the way described in the evidence led for the plaintiff. Then plaintiff's counsel asked a question framed in accordance with the judge's ruling. Counsel for the defendant objected to the question and challenged the qualifications of the witness to express an opinion on "the behaviour of motor vehicles on roadways, and particularly the behaviour of semi-trailers on roadways". Counsel for the plaintiff elicited from the witness that he had investigated accidents in which semi-trailers were involved on a number of occasions, that the accidents had happened on curves and grades and that in cases of some of the accidents a semi-trailer had moved from its side of the road on to the other side and the impact was with the front end of the trailer. The witness further said that over many years he had been considering such problems and had made reports to persons engaging him to consider them. Counsel for the defendant then asked leave of the Court to ask the witness questions on the voir dire. The evidence which the witness gave at the outset before his qualifications were called into question was as follows. He was a professional engineer and had been in practice for thirty-six years and had fifty years experience of engineering problems in Australia and abroad. In the course of his practice he had investigated road accidents for insurance companies and aircraft accidents for Lloyds Aviation Group; he was Lloyds' aviation surveyor for twenty-five years: during the war he was an engineer officer in the Royal Australian Air Force: he is a past Associate Member of the Institute of Engineers and past Associate of the Institute of Engineer Surveyors. As I have already stated, his evidence proved that he investigated many accidents in which semi-trailers were involved, on curves and grades and where, as in the present case, the surface of the road was bitumen. On the voir dire counsel for the defendant asked the witness questions as to his investigation of accidents in which semi-trailers were involved. The witness gave a number of specific instances. Some were cases in which the witness said that he "personally accompanied the driver on a reconstruction of an accident with regard to the behaviour of semi-trailers attached to prime-movers". Asked whether he had done any automobile engineering course, the witness said he had not, adding that he had been trained in places where automobile engines have been built. His present practice, he said, is particularly "in engineering surveying; surveying machines"; and that it also involves "automobile engineering work of various kinds where an opinion is required or an investigation or report with regard to causes of failure or causes of something else happening on roads is concerned, and traffic engineering". In order to carry out such work he said: "You have to have general knowledge of the vehicle and its power, and its weight, its length and its steering, manoeuvring and its braking and the rate at which it travels grade both ways, if there are grades both ways". The semi-trailer in the present case was an "International". In answer to questions the witness said that he had in the course of his practice examined the engines of many of such vehicles and personally inspected and supervised their dismantling and repair. The enquiry into the qualifications of the witness went back to his apprenticeship in the Clyde Engineering Co. at Granville and his studies at the Sydney Technical College where the subjects which he studied were mathematics, applied mechanics, drawing and physics. There is much more in the appeal book which was elicited in the course of the enquiry on the voir dire as to the qualifications of the witness to be called to give evidence as an expert in this case. It is sufficient to say that it strengthens the proofs of his qualifications which have been detailed, if indeed that is necessary. In the case of Reg. v. Silverlock [1] , Lord Russell C.J. discussed the question whether a witness was an expert on handwriting. He said that to be qualified to give evidence as an expert a witness need not become peritus in any definite way. "The question is," said Lord Russell, "is he peritus? Is he skilled? Has he an adequate knowledge? Looking at the matter practically, if a witness is not skilled the judge will tell the jury to disregard his evidence When once it is determined that the evidence is admissible, the rest is merely a question of its value or weight, and this is entirely a question for the jury, who will attach more or less weight to it according as they believe the witness to be peritus " [2] . In my opinion, there was evidence on which the learned trial judge was justified in holding that Mr. Joy had sufficient knowledge and experience on the subject of the jack-knifing of semi-trailers to warrant the opinion which he expressed as to what are the probable causes of jack-knifing, to be left to the jury, and that the direction which the learned judge gave as to that evidence was correct. I think that the reasons of the Full Court for its judgment on that question are right. In my opinion, the appeal should be dismissed.